Hydrography and phytoplankton distribution in the Amundsen and

Hydrography and phytoplankton distribution in the Amundsen and Ross Seas

_______________

A Thesis

Presented to

The Faculty of the School of Marine Science

The College of William and Mary in Virginia

In Partial Fulfillment

of the Requirements for the Degree of

Master of Science

_______________

by

Glaucia M. Fragoso

2009

ii

APPROVAL SHEET

This thesis is submitted in partial fulfillment of

the requirements for the degree of

Master of Science

_____________________ Glaucia M. Fragoso

Approved by the Committee, November 2009

______________________ Walker O. Smith, Ph.D. Committee Chairman/Advisor

______________________ Deborah A. Bronk, Ph.D.

______________________ Deborah K. Steinberg, Ph.D.

______________________ Kam W. Tang, Ph.D.

iii

DEDICATION

I dedicate this work to my parents, Claudio and Vera Lucia Fragoso, and family for their encouragement,

guidance and unconditional love.

iv

TABLE OF CONTENTS

ACKNOWLEDGMENTS.................................................................................................... vi

LIST OF TABLES.............................................................................................................. vii

LIST OF FIGURES ........................................................................................................... viii

LIST OF APPENDICES………………………………………………………………………… x

ABSTRACT ....................................................................................................................... xii

INTRODUCTION ............................................................................................................... 2

Phytoplankton and the Southern Ocean……………………………………………… 2

The Southern Ocean and the climate change……………………………………….. 6

The Ross and Amundsen Seas hydrography……………………………………….. 7

Amundsen and Ross Sea phytoplankton…………………………………………….. 9

MATERIALS AND METHODS…………………………………………………………………. 16

Study area…………………………………………………………………………......... 16

Collection and analysis of water samples………………………………………….... 19

Remote sensing………………………………………………………………………… 20

Bloom definition………………………………………………………………………… 21

Modes of taxonomic discrimination………………………………………………...... 21

Phytoplankton blooms and mixed layer depth……………………………………… 23

P. antarctica and the MCDW…………………………………………………………. 23

Pulse amplitude modulation (PAM) and maximum photosynthetic quantum

yield………………………………………………………………………………………. 24

RESULTS………………………………………………………………………………………… 25

Remote sensing of the study regions………………………………………………… 25

Hydrographic data……………………………………………………………………… 31

Chlorophyll a distributions………………………………………………………...…… 38

Phytoplankton bloom heterogeneity………………………………………………….. 40

Approaches of taxonomic discrimination…………………………………….. 40

Silicoflagellates distributions………………………………………………….. 45

Blooms distributions……………………………………………………………. 46

Diatoms and mixed layer depths………………………………………………………. 48

v

MCDW and Phaeocystis antarctica distribution……………….…………….………. 53

DISCUSSION…………………………………………………………………………………….. 60

Phaeocystis antarctica and diatoms blooms distribution………..………………….. 60

Mixed layer depth………..…………………………………………………………….... 61

Algal seeding…………………………………………………………………………….. 64

Zooplankton grazing…………………………………………………………………….. 64

Fe...……………………………………………………………………………………...... 65

SUMMARY………………………………………………………………………………………... 70

LITERATURE CITED…………………………………………………………………………….. 71

APPENDICES..………………………………………………………………………………….... 87

VITA………………………………………………………………………………………………… 99

vi

ACKNOWLEDGMENTS

I would like to thank my advisor Walker O. Smith for his support and guidance

through my degree. To my committee Kam Tang, Debbie Steinberg and Debbie

Bronk, I would like to show my gratitude to for their advice and expertise throughout

the preparation of this Master’s thesis. Additionally, I would like to thank Amy

Leventer for kindly sharing her data and helping to improve the quality of this work. I

am grateful to Patrick Dickhudt, Xiao Liu, Yicheng Teng and Scott Polk for technical

support. Special thanks go to my labmates and friends Jennifer Dreyer, Xiao Liu,

Xiaodong Wang and Sasha Tozzi, for their help and entertainment. To Iris Anderson,

Debbie Bronk, Mike Newman and Liz Canuel, I owe my deepest gratitude for their

incentives and advice. Special thanks go to the scientists, captains and crew

members of OSO-2007 cruise and LMG09-01 for their companionship during best

months of my life. To Debbie Steinberg, I owe many thanks for the opportunity given,

instruction and friendship. Additional thanks are extended to the National Science

Foundation and Virginia Institute of Marine Science. This research was supported by

NSF grants ANT-0741380 and ANT-0836112 to W. Smith, and grant ANT-04-40775

to S. Jacobs (Columbia University).

It is a pleasure to thank all my friends that constantly encourage me through

the completion of my work: Althea Moore, Christopher Lins, Liza Hernandez,

Catarina Wor, Kate Ruck, Priscila dos Reis, Viviane Scumparin, Carolina Funkey,

Amy Then, Gabrielle Saluta, Ana Verissimo and Theresa Davenport. Because of

them, life as a graduate student was more amusing and friendly. My deepest

gratitude goes to Althea Moore for being a true friend, which I will remember through

my entire life. I would like to also thank my friends from Brazil, that even distant, have

also been constantly present in my life. My deeply admiration goes to my best friends

Vanessa Felix and Clara-Luz da Aurora for their loyalty and guardianship through my

entire life.

This thesis would not have been possible without the love and support of my

family. They always believe in myself and always were present through the rough

times. To them, I express my endless love, consideration and respect.

vii

LIST OF TABLES

Table Page

Table 1. Mean ± standard deviations, minimum and maximum Zmix and number of

stations ………………..…………………………………………………………………………… 36

viii

LIST OF FIGURES

Figure Page

Figure 1 Location of stations and transects from NBP07-02 and OSO-200………... 17

Figure 2 Ice coverage and distributions ……………………………..…………………. 26

Figure 3 Weekly chlorophyll distributions derived from remote sensing.……………. 27

Figure 4 Monthly climatology of remote sensing chlorophyll distribution……………. 29

Figure 5 Maximum chlorophyll from SeaWiFS………………………..……………….. 30

Figure 6 θ-S diagram with depth in the Z axis ………………………..………….……. 32

Figure 7 Distribution of water masses along Ross Ice Shelf……..…………….…….. 33

Figure 8 Distribution of water masses in the Amundsen Sea..………………….….... 34

Figure 9 Distribution of mixed layer depth in the Ross and Amundsen Sea.………. 37

Figure 10 Plot of the surface chlorophyll a versus mixed layer depth ...……………... 38

Figure 11 Surface chlorophyll a concentrations………………………………………… 39

Figure 12 Relationship of nitrate + nitrite and PO4……………………..………………. 41

Figure 13 Distribution of genus/species of phytoplankton based on microscopy…… 43

Figure 14 Picture of Dictyocha speculum……….……………………………………….. 44

Figure 15 Relationship between 19’-butanoyloxyfucoxanthin and Dictyocha speculum

…………………………………………………………………………………… 46

Figure 16 Distribution of P. antarctica, Dictyocha speculum and diatoms blooms….. 47

Figure 17 Distribution of blooms in relation to mixed layer depth ……………………. 48

Figure 18 Relationship between fucoxanthin/(fucoxanthin+19’-hexanoyloxyfucoxanthin)

and mixed layer depth…………………………………………………………. 50


and salinity……………………………………………………………………… 52

ix


and salinity………………………………………………………………….…… 53

Figure 21 Temperature versus salinity plot from the upper 200 m from the Ross Sea

…………………………………………………………………………………… 54

Figure 22 Relationship between surface chlorophyll a and 19’- hexanoyloxyfucoxanthin

from Ross Sea samples……………………………………………………….. 56

Figure 23 Relationship of 19’-hexanoyloxyfucoxanthin to chlorophyll c3 ratio and

chlorophyll a…………………………………………………………………….. 57

Figure 24 Relationship between Fv/Fm and chlorophyll a…………………………..….. 59

x

LIST OF APPENDICES

Appendix 1. Surface chlorophyll a, mixed layer depth, abundant assemblages based on

microscopic observations and average salinity of the upper 50 m for each station of the

NBP07-02

cruise…………………………………………………………….………………………..……….. 71

Appendix 2. Surface chlorophyll a, mixed layer depth, abundant assemblages based on

microscopic observations and average salinity of the upper 50 m for each station of the

OSO-2007

cruise………………………………………………………..……………………….…….………. 72

Appendix 3. Integrated chlorophyll c3, 19’-butanoyloxyfucoxanthin, 19’-

hexanoyloxyfucoxanthin, fucoxanthin, pigment ratios and assemblage dominance based on

pigment ratios for each station of the NBP07-02 cruise……………………………..……….. 73

Appendix 4. Integrated chlorophyll c3, 19’-butanoyloxyfucoxanthin, 19’-

hexanoyloxyfucoxanthin, fucoxanthin, pigment ratios and assemblage dominance based on

pigment ratios for each station of the OSO-2007 cruise…………………............................. 74

Appendix 5. Integrated phosphate, silicate, nitrate and nitrite drawdown, nutrient drawdown

ratios and assemblage dominance based on nutrient ratios for stations from NBP07-02

cruise………………………………………………..……………………………………………… 75

Appendix 6. Integrated phosphate, silicate, nitrate and nitrite drawdown, nutrient drawdown

ratios and assemblage dominance based on nutrient ratios for stations from OSO-2007

cruise…………………………………………………………..…………………………………… 76

Appendix 7. Silicoflagellates abundance (ind mL-1) and average 19’-butanoyloxyfucoxanthin

(19’-but) of the upper 50 m for each station of the NBP07-02 cruise…...…………….…….. 77

Appendix 8. Fv/Fm and phytoplankton dominance based on pigments……………..……….. 79

xi

Appendix 9. Fluorometric-based chlorophyll a concentrations (µg L-1) in the Amundsen and

Ross Sea during OSO-2007 cruise………………………………………………..……….…… 82

xii

ABSTRACT

The phytoplankton of the Ross Sea have been intensively studied during the

last decade, as opposed to the Amundsen Sea, where virtually nothing is known

about phytoplankton taxonomy and distribution. Blooms in the Ross Sea are usually

composed of diatoms and the prymnesiophyte Phaeocystis antarctica; diatoms are

often dominant in strongly stratified waters during the summer, whereas P. antarctica

usually dominates in less stratified waters in the south-central polynya during spring.

This study focused on understanding the environmental variables that influence the

spatial patterns of phytoplankton assemblages during late summer and early fall,

2007, and late spring and early summer, 2008 in the Amundsen and Ross Seas.

Large differences in the distribution of phytoplankton were noted in space and time.

In general, P. antarctica dominated the blooms in southwestern part of the Ross Sea.

The silicoflagellate Dictyocha speculum was present in relatively high abundance and

co-dominated with P. antarctica and diatoms in the eastern part of the Ross Sea

during February. In the Amundsen Sea P. antarctica co-dominated with diatoms, and

diatom blooms were more common than in the Ross Sea. Shallow mixed layer

depths supported the growth of diatoms in the Ross and Amundsen Seas, but it was

not the only factor required for diatom bloom development. Blooms dominated or co-

dominated by diatoms were also more frequent in relatively fresher waters of the

Amundsen Sea than in the Ross Sea in late summer and early fall. Modified

Circumpolar Deep Water (MCDW), a water mass that can potentially be a source of

Fe to phytoplankton, intruded the upper waters (from 80-120 m) near the Ross Ice

Shelf. I hypothesized that this water mass intrusion may have favored P. antarctica

blooms, releasing them from Fe limitation because they occurred in waters where the

MCDW was shallow. However, pigment and quantum yield data show that P.

antarctica blooms were approaching Fe stress in waters where the MCDW was

observed, suggesting that intrusions of MCDW strengthen stratification and restrict

Fe inputs into the surface. Because the Ross and the Amundsen Sea have a wide

range of environmental and climatic conditions, understanding the factors that

xiii

influence phytoplankton distribution in these areas will provide information of how

phytoplankton respond to a changing climate.

Hydrography and phytoplankton distribution in the

Amundsen and Ross Seas

2

INTRODUCTION

1- Phytoplankton and the Southern Ocean

The Southern Ocean (all waters south of the Antarctic Polar Front) plays a

critical role in the marine carbon cycle. It contributes to 20% of the global ocean CO2

uptake (Takahashi et al., 2009), although it comprises only 10% of the global ocean

area (El-Sayed, 1978). Both physical and biological processes explain the significant

absorption of CO2 from the atmosphere in the Southern Ocean. The physical

processes are the cooling of poleward-moving warm waters from the tropics, which

increases the flux of CO2 into the surface water as they cool (Takahashi et al., 2002),

and deep vertical convection caused by strong wind stress coupled with air-sea

exchange (Russell et al., 2006). The biological processes are mediated by

phytoplankton, which through photosynthesis decrease the pCO2 in the euphotic

layer during austral summer (Falkowski et al., 2000, Takahashi et al., 2002).

Phytoplankton utilization of CO2 accounts for the major biological regulation of the

global sea-air flux of CO2 in Antarctic waters because of pronounced austral spring

and summer blooms occurring in certain areas (Smith and Comiso, 2008; Arrigo et

al., 2008).

The phytoplankton of the Southern Ocean have the ability to regulate the

global climate over broad time scales (Falkowski et al., 1998, Anderson et al., 2002,

2008). Significant abundance of Phaeocystis antarctica (an important phytoplankton

species in some Antarctic waters) may regulate the albedo and cloud cover, given

that this species produces DMS (dimethylsulfide) from the enzymatic conversion of

DMSP (dimethylsulfoniopropionate), and whose oxidation products contribute to

cloud condensation nuclei (Stefels et al., 1995). Moreover, phytoplankton of the

Southern Ocean incorporate CO2 into organic matter during photosynthesis, which

can be exported to the deep ocean through the biological pump (Ducklow et al.,

2001). The processes by which dissolved and particulate organic carbon are

transported to depth are the settling of particulate organic matter, physical mixing of

3

particulate and dissolved organic carbon, and active flux due to zooplankton diel

vertical migration (Longhurst and Harrison, 1989; Ducklow et al., 2001). The

efficiency of the biological pump is a function of the degree of organic matter

regeneration (Buesseler et al., 2007), sinking velocity (Huisman and Sommeijer,

2002), formation and sinking of aggregates (Alldredge, 2000), and incorporation of

carbon into fecal pellets (Lampitt et al., 1990).

In addition to the substantial phytoplankton blooms, the Southern Ocean can

be an important region of the world ocean for carbon export because of the distinct

features of the continental margin (Smith et al., 2008). Narrow continental margins,

deep canyons and steep continental slopes, along with saline, dense, deep water

formed during the winter promotes downward advection of dense waters

(Bergamasco et al., 2004, Gordon et al., 2004, Jacobs, 2004). The lateral transport of

dense waters off the shelf during winter may contribute to the export of particulate

organic carbon produced during the summer, if advection of the biogenic matter to

the deep ocean occurs prior to complete remineralization (Smith et al., 2008).

However, movement of water across the continental shelf break and the flux of

particles is limited by the bottom topography at the continental margin (Dinniman et

al., 2003), the direction, intensity and duration of the currents (Shapiro and Hill, 1997,

Jacobs, 2004), and interannual variability of phytoplankton production and location of

the bloom.

Primary production in the Southern Ocean is low compared to other regions,

particularly on an annual basis, but isolated regions of high biomass and productivity

occur during the austral summer. It has been proposed that light is the major limiting

factor during the austral spring (Smith and Gordon, 1997), whereas Fe limits primary

production during the summer (Sedwick and DiTullio, 1997) in macronutrient-rich

waters of the Southern Ocean. However, it is possible that co-limitation occurs

(Tremblay and Smith, 2007), since Fe and light limitation interact with each other

(Sunda and Huntsman, 1997; Sedwick et al., 2007). The regions of highest

productivity in the Southern Ocean are generally located in coastal polynyas (Arrigo

and van Djiken, 2004, Tremblay and Smith, 2007) and other coastal zones (Smith

and Nelson, 1985, Smith and Comiso, 2008).

4

Polynyas are areas of reduced ice cover surrounded by consolidated ice

(Tremblay and Smith, 2007). They can be formed either by strong katabatic winds

during winter that continually remove old or newly formed frazil ice from a region

(latent heat polynya), or by upwelling of relatively warm water from depth (sensible

heat polynya), or by both processes (mixed polynyas). Polynyas have pronounced

blooms because ice removal, usually starting early in spring, provides a well

illuminated environment for phytoplankton growth. Irradiance, in conjunction with

water column stability, is the physical factor that triggers spring phytoplankton growth.

The upwelling of CDW and/or the intrusion of the warm MCDW is also assumed to be

a source of bioavailable Fe (Peloquin and Smith, 2007), which sustains

phytoplankton growth until its depletion. However, the strength, frequency and

duration of these intrusions are poorly constrained.

Ice melting releases algae from the ice to the water and provides meltwater-

induced stability of the water column, and an optimal environment for phytoplankton

growth. The sharp discontinuity between the low- and high-salinity layers impede

vertical mixing, allowing phytoplankton to grow in a well-illuminated environment

(Smith, 1987). Moreover, reduced wind stress during spring and summer further

decreases turbulence and shoals the depth of the mixed layer. Input of bioavailable

Fe from melting ice (Sedwick et al., 1997), resuspended shelf sediment (Fitzwater et

al., 2000), and intrusion of the Modified Circumpolar Deep Water (MCDW) onto the

shelf in some specific regions (Peloquin and Smith, 2007, Prézelin et al., 2000, 2004,

Hiscock, 2004) may also stimulate the phytoplankton growth.

The Ross Sea is the most productive in the Southern Ocean (Smith and

Comiso, 2008; Arrigo et al., 2008) and one of the best studied regions in the Antarctic

regarding phytoplankton ecology. It experiences substantial interannual variability of

phytoplankton biomass and primary production induced primarily by changes in ice

cover (Smith et al., 2000, 2006). However, stronger and more persistent katabatic

winds, followed by changes in water mass circulation, may also induce some

temporal and spatial interannual variability in phytoplankton as well (Arrigo et al.,

1998; Smith et al., in press). Growth usually starts in late October (Smith and Gordon,

1997; Arrigo et al., 1998) in the southern Ross Sea polynya; biomass reaches

5

maximal concentrations in mid- to late December (Smith et al., 2000), and declines

rapidly in January and February (Smith et al., 2000; Peloquin and Smith, 2007).

Blooms in the Ross Sea generally are comprised of diatoms and the

prymnesiophyte Phaeocystis antarctica. These blooms often have substantial spatial

and temporal variations during spring and summer. The bloom of Phaeocystis

antarctica usually occurs in the south-central polynya during spring - an area

characterized by mixed layers from 25 to 50 m (Arrigo et al., 1999). Diatoms

generally form blooms in waters with greater vertical stability and shallower mixed

layers (Zmix < 20 m; Arrigo et al., 1999), such as close to ice-edge, near the coast of

Victoria Land, and in the eastern portion of the polynya (Goffart et al., 2000). One of

these functional groups usually dominates; however, they can be co-dominant in

surface waters as well (Smith and Asper, 2001, Garrison et al., 2003b).

Changes in the environment influence phytoplankton composition, which in

turn alters the biogenic matter that is being produced in the euphotic zone that can

be transported to depth. Fe limitation can affect the ratio of removed silicate to nitrate

in diatoms because the lack of Fe restricts nitrate uptake, resulting in high BSi:PON

(biogenic silica : particulate organic nitrogen) ratios and consequently in “heavier”

diatoms, which can sink faster (Hutchins and Bruland, 1998, Takeda, 1998). The

mechanisms of vertical export within these assemblages differ as well. Diatoms can

be transported to depth through passive sinking, aggregate formation, or fecal pellet

formation. Phaeocystis antarctica, which may have relatively higher carbon content

than diatoms (on a per cell basis) due to the colonial mucilage, can be transported by

the same mechanisms, although aggregate formation seem to be the most common

(Smith and Dunbar, 1998, Asper and Smith, 1999). Grazing on colonial Phaeocystis

appears to be substantially reduced (Haberman et al., 2003, Karnovsky et al., 2007).

Because of its polymorphic life history, however, P. antarctica may shift from colonies

to single cells, which do not sink appreciably and are more likely grazed by

microzooplankton (Smith et al., 2003). Furthermore, as colonies sink in the water

column, solitary cells can be liberated, thus altering the composition of the sinking

material (Wassmann et al., 2005, Reigstad and Wassmann, 2007). Changes in the

phytoplankton distribution potentially influence shifts in the food web, since diatoms

6

are preferentially grazed over P. antarctica (Haberman et al., 2003, Karnovsky et al.,

2007). However, Shields et al. (2007) observed ingestion of colonial and single cells

of P. antarctica by herbivorous microzooplankton that were found inside the colonial

matrix.

2 -The Southern Ocean and climate change

The Antarctic sea ice extent has shown great interannual variability during the

last decades (Cullather et al., 1996; Watkins and Simmonds, 2000; Kwok and

Comiso, 2002, Cavalieri and Parkinson, 2008). The decline in sea ice extent of the

Bellingshausen-Amundsen Sea sector occurred coincident with sea ice expansion in

the western Ross Sea sector (Kwok and Comiso, 2002; Cavalieri and Parkinson,

2008). Cavalieri and Parkinson (2008) found a yearly Antarctic sea ice retreat of 5.2 ±

2.1% per decade in the Amundsen-Bellingshausen sector from 1979 to 2006, along

with an increase in air temperature of about 0.5˚C per decade, since the mid-1940’s

in the Antarctica Peninsula (Vaughan et al., 2003). The trends in ice cover, which are

a response to the annual sea ice duration (shorter in the West Antarctica Peninsula

(WAP) and longer in the western Ross Sea), are modulated by ice-atmospheric

interactions associated with two climate modes - the El Nino Southern Oscillation

(ENSO) and the Southern Annular Mode (SAM) (Stammerjohn et al, 2008).

In the Pacific sector of the Southern Hemisphere, ENSO alters the equatorial-

to-polar thermal gradient, which induces changes in jet stream/storm distribution.

During La Niña events, the tropical cooling strengthens the polar front jet (centered

near 60˚S) in the South Pacific and leads to warmer conditions in the southern

Bellingshausen and Weddell Sea and colder temperatures in the Amundsen and

Ross Seas (Yuan, 2004). The opposite trend occurs in El Niño conditions. The

poleward shift of the westerlies during the last two decades, which is generally

described as a trend of SAM toward to its positive phase since 1990s (Chen and

Held, 2007), brings surface warm air from the tropics to the West Antarctic Peninsula

and decreases ice extent in the Amundsen and Bellingshausen Seas (Jacobs and

Comiso, 1993). Although no significant trend in mean ice extent has been shown for

7

the entire Southern Ocean, increases in greenhouse gases and subsequent global

warming may induce rapid modifications in the physical parameters of the Southern

Ocean and impact phytoplankton distribution and biogeochemistry.

The positive SAM trend observed during the late 20th century has been

attributed to photochemical ozone losses (Thompson and Solomon, 2002), whereas

the continuous positive SAM trend for this century predicted by climate models is

explained by increased level in greenhouse gases (Chen and Held, 2007, Fyfe and

Saenko, 2007, Le Quére et al., 2007, 2008, Zickfeld et al., 2008). Models predict that

poleward-intensified westerly winds will result in a better alignment of these winds

and strengthen the deep and vigorous Antarctic Circumpolar Current (ACC), as well

as vertical mixing within it (Russell et al., 2006). This will consequently promote

outgassing of CO2, acting as positive feedback on atmospheric CO2 levels (Le Quére

et al., 2007, Anderson et al., 2009). It is also likely that strong wind stress will deepen

the mixed layer, enhance brine rejection, and intensify the upwelling of the MCDW in

the Ross Sea continental shelf and ice ablation in the Amundsen Ice Shelf. The

Amundsen Sea, which is historically known as a region of heavy ice, is undergoing

sea ice recession within the last decades (Jacobs and Comiso, 1993), and extensive

phytoplankton blooms near the coast have been observed (Smith and Comiso,

2008). Yet, the Amundsen Sea is one of the least studied regions in the entire

Southern Ocean, and an analysis of the phytoplankton taxonomy and distribution in

this area would contribute toward a more comprehensive understanding of climate

effects on Southern Ocean phytoplankton.

3 - Ross and Amundsen Sea Hydrography

The Ross Sea is located over the continental shelf from Cape Adare at 170˚E

to Cape Colbeck at 158˚W (Dinniman et al., 2003). The general circulation consists of

a strong westward current that flows along the shelf break and intrudes onto the

shelf. The currents on the Ross Sea continental shelf are sluggish and dominated by

a wind-driven gyre (Dinniman et al., 2003). Flow is influenced by three submarine

ridges situated in the southwest-northeast direction (Locarnini, 1997, Dinniman et al.,

8

2003). The physical processes that determine the cross-shelf exchanges of the Ross

Sea polynya are sea ice formation, advection and melting. Cold air temperatures

drive sea ice formation, and the continual formation and advection of sea ice by

katabatic winds enhances brine rejection, which increases the density of the water.

The water masses formed in this process are the High Salinity Shelf Water (HSSW)

and Ice Shelf Waters (ISW). HSSW is characterized by a salinity maximum due to

continuous freezing and ice formation. ISW is fresher and less dense and derived

from the interaction of the HSSW with waters from the basal Ross Ice Shelf (Budillon

et al., 2003). The movement of these water masses off the continental shelf promotes

the formation of the Antarctic Bottom Water (AABW) that expands throughout the

bottom of the southern Pacific Ocean. Deep-water formation creates a convective

and cross-shelf overturn of the warm Circumpolar Deep Water (CDW). The CDW,

which is carried by the Antarctic Circumpolar Current (ACC) and introduced onto the

continental shelf, mixes with Antarctic Surface waters to form Modified Circumpolar

Deep Water (MCDW). The intrusions of MCDW onto the shelf along several deep

troughs (Dinniman et al., 2003) and promote suspension of fine sediments, which

may be a source of Fe for surface phytoplankton blooms (Peloquin and Smith, 2007).

The Amundsen Sea is located at 71˚S along the Marie Byrd Land between

100 and 135˚W. It has been historically known for persistent summer sea ice that

made it the most remote and least known continental shelf region of the Southern

Ocean (Jones, 1982). The Amundsen Sea was relatively unexplored until the late

1980s, and it is still considered an area of difficult access because of perennial ice,

despite the development of sophisticated icebreakers. However, the Amundsen Sea

has received much attention in recent years because of the observation of sea ice

retreat (Jacobs and Comiso, 1997) and the noticeable melting of the Pine Island

Glacier (PIG) (Jenkins, 1997) located east of the Amundsen Sea. The PIG has

experienced a melt rate exceeding 40 m yr-1 near to its grounding line (Rignot and

Jacobs, 2002). Much of this melting is presumably to be driven by the warm CDW

that floods into deeper troughs of the Amundsen Sea continental shelf and cause

basal floating ice ablation.

9

The main, deep trough (depth from 1000 to 1500 m) located at 114˚W cuts the

continental shelf and bifurcates south, in which the Pine Island Bay is located east

and the Bear Peninsula west. Similar to the Bellingshausen Sea and the West

Antarctic Peninsula, the southern boundary of the ACC (SBACC) reaches the

continental margin and brings the CDW close to the shelf break. Because the glacial

trough acts as a channel that permits the intrusion of the relative warm and salty

CDW toward the inner shelf, the CDW easily crosses the continental shelf and

remains relatively undiluted (Walker et al., 2007). The CDW induces melting deep

beneath the regional ice shelves and glacial tongues and promotes upwelling of less

dense melt water along the coastline (Jacobs et al., 2002). In the Amundsen Sea the

CDW is located deep in the water column (ca. 400 m) and is unlikely to be a source

of Fe for phytoplankton growth. However, there is the possibility that Fe may be

derived from the basal melt waters, and is possibly augmented by the accelerating

glacial meltwater in this region. Cold, dense shelf water formation is practically

absent in the Amundsen Sea. Brine rejection is limited by melt-driven upwelling and

perennial ice and snow cover. The AASW varies seasonally and is not dense enough

to completely overturn the water column and mix with the AABW formed elsewhere

(Jacobs et al., 2002).

4 - Amundsen and Ross Sea phytoplankton

The Ross and the Amundsen Seas are the most productive areas of the

Southern Ocean, with annual primary production reaching up to 160 g C m-2 and

chlorophyll concentrations exceeding >10 µg L-3 (Arrigo and van Djiken, 2003). The

high productivity in these regions is attributed to the three most productive post-

polynyas. The Ross Sea polynya is the largest coastal polynya (397,000 km2 in

summer) and has the largest seasonal phytoplankton bloom in the Southern Ocean,

with spatial coverage of ca. 187,000 km2 (Sullivan et al., 1993). The Pine Island Bay

polynya and Amundsen Sea polynya, located in the eastern and western portions of

the Amundsen Sea, respectively, contribute appreciably to primary production (ca.

310 g C m-2 per year), despite their small size (16,800 and 38,000 km2; Arrigo and

10

van Djiken, 2003). These polynyas are confined to a wide continental shelf, in which

physical processes, including vertical mixing, stratification and irradiance, are the

primary factors that influence phytoplankton growth and distribution during spring and

summer (Smith et al., 2000).

Blooms of the Ross Sea generally are dominated by two distinct assemblages

(Smith and Asper, 2001): diatoms and the prymnesiophyte Phaeocystis antarctica,

although small blooms of cryptophytes may also occur in some particular areas

(Arrigo et al., 1999). P. antarctica is endemic in the Southern Ocean and usually

develops early in the spring and forms extensive blooms in the south central region of

the Ross Sea (El-Sayed et al., 1983; Arrigo et al., 1999). Diatoms usually dominate

during mid- and late summer in stratified waters along the coast of Victoria Land

(Smith and Nelson, 1985; Arrigo et al., 1999). Cryptophytes also can occur during

mid- and late summer in the eastern Ross Sea (Arrigo et al., 1999). The Amundsen

Sea is far less studied than the Ross Sea, despite being one of the most productive

areas of the Southern Ocean. Diatoms and P. antarctica are also part of the

phytoplankton in this region, with diatoms dominating at the ice edge, and P.

antarctica co-dominating with diatoms off the shelf in the summer (Stambler, 2003).

Dinoflagellates are also present, although in low abundances (Stambler, 2003).

Phaeocystis is distributed widely, from polar and sub-polar regions, to

temperate seas and tropical systems (Lancelot et al., 1998). It has a complex and

poorly understood life cycle (Schoemann et al., 2005), and the taxonomy of some

species is still uncertain. P. antarctica is one of the three colony-forming Phaeocystis

species and can occur in three different morphotypes: two flagellated, single-celled

form and one non-flagellated, colonial form (Rousseau et al., 2007). One type of

flagellated P. antarctica has scales and produces filaments and stars, which are

attaching structures located at the outer face of the flagella membrane, whereas the

other type lacks them. During the colonial stage P. antarctica cells are embedded in a

thin, organic matrix, which separates the internal fluid (sap) from the external

environment. Actively growing colonies are generally spherical, and their interiors are

cell-free. P. antarctica usually forms colonies that are ca. 2 mm in diameter (Caron et

11

al., 2000, Smetacek et al., 2004); however, colonies up to 9 mm have been reported

(Baumann et al., 1994).

Diatoms are generally considered to be the most common taxa in polar

regions, including the Southern Ocean. They can be both centric and pennate and

found both in sea ice and the water column. Diatoms often form aggregates, which

increase their sinking rate (Brzezinski et al., 1997). Thus, turbulence is an important

physical process that allows diatoms to be retained in the euphotic zone (Patel et al.,

2004). Diatoms also exhibit many mechanisms to overcome sinking, such as

exchange of heavier with lighter ions, high cell to vacuole volume ratios, presence of

lipids, and protuberances in their cell wall that enhances friction with the water by

increasing the surface:volume ratio (Smetacek, 1985). They are often dominant in

strongly stratified waters of the Ross Sea (Arrigo et al., 1999; Smith and Asper,

2001). They are generally represented by the Nitzschia group (Pseudonitzschia and

Fragilariopsis spp.), which has been reported as abundant in the pack ice and water

near the ice edge (Andreoli et al., 1995). These diatoms are generally elongated,

small pennates; however, large centric diatoms such as Thalassiosira spp.,

Eucampia, Corethron, Chaetoceros and Rhizosolenia/Proboscia also occur, but in

lower abundance. Large variability of diatom genera is observed within a growing

season (Dennett et al., 2001).

Photosynthetic pigments (chlorophylls and carotenoids) are commonly used to

identify taxonomic and functional groups. In general, eight algal categories can be

described by their pigment content: diatoms, dinofagellates, cryptophytes,

prasinophytes, chlorophytes, cyanobacteria, and two categories of prymnesiophytes

(haptophytes) - Hapto3’s (e.g., coccolithophorids) and Hapto4’s (e.g., Phaeocystis

antarctica, Parmales and other chrysophytes) (Wright and van Enden, 2000).

Chemotaxonomy is a practical technique for the separation of phytoplankton by

chemical means, and is often far less time consuming and advantageous when only

broad, functional group classification is needed. Some phytoplankton assemblages

have unique pigment markers; for instance, peridinin is only found in dinoflagellates,

and alloxanthin in cryptophytes. However, interpreting the pigments is not always

straightforward, since some pigments are common to some different phytoplankton

12

assemblages. Fucoxanthin (fuco), for example, is a dominant pigment in diatoms, but

can also be found in prymnesiophytes, chrysophytes, and some dinoflagellates. On

the other hand, derivative esters of fuco, such as 19’-hexanoyloxyfucoxanthin (19’-

hex) and 19’-butanoyloxyfucoxanthin (19’-but) are present in some, but not in all

prymnesiophyte and chrysophyte groups (Wright and van Enden, 2000). In addition,

environmental factors, including Fe concentration and light, may change the

concentrations and ratios of these pigments. Small variations in Fe concentrations

(from nM to µm) induce interconversion of 19’-hex and 19’-but to fuco (van Leeuwe

and Stefels, 1998, 2007). More recently, DiTullio et al. (2007) showed that nanomolar

variations in Fe concentrations (up to 2.2 nM) significantly influences pigment ratios

of P. antarctica. Reduced light conditions may increase the cellular content of light

harvesting pigments, such as chlorophyll c and fucoxanthin, and high light may

increase the content of photoprotective pigments (Morales, 1994). However, the

interconversion of xanthophylls is more sensitive to changes in Fe concentration

when compared to the effects of light (van Lewee and Stefels, 1998, 2007, DiTullio et

al., 2007), although Fe demand for phytoplankton growth varies according to light

availability.

Despite these caveats, chemotaxonomic analysis has successfully been

applied in the Ross Sea (DiTullio and Smith, 1996; Smith and Asper, 2001). Although

there are potential errors due to alterations of pigments by variable Fe

concentrations, Fe concentration in the Ross Sea rarely exceeds 1 nM, and is on

average 0.3 and 0.2 nM in the spring and summer, respectively (Sedwick et al.,

submitted). These Fe concentrations are too low to influence the levels of fuco, 19’-

hex and 19’-but, at least as indicated in laboratory experiments. Indeed, DiTullio et al.

(2003) showed a strong correlation between P. antarctica cell numbers estimated by

microscopy and its marker pigment, 19’-hex. Besides 19’-hex, P. antarctica has

pigments found in prymnesiophyte Type 4, such as chlorophyll c3 (chl c3), 19’-but

and fuco. However, 19’-but is found in low concentrations in P. antarctica blooms in

the Ross Sea (DiTullio et al., 2003). Because Fe limitation induces the synthesis in

19’-hex in P. antarctica relative to fucoxanthin, the ratio of 19’-hex to chl c3 (a

pigment only found in prymnesiophytes) also indicates the Fe-nutritional status of this

13

species (van Leewue and Stefels, 1998, DiTullio et al. 2007). Diatoms contain fuco,

but not the other three pigments found in P. antarctica.

Diatom and P. antarctica blooms vary spatially and temporally in the Ross Sea.

One of these taxa usually dominates; however, they can be co-dominant in surface

waters as well (Smith and Asper, 2001, Garrison et al., 2003b). The mechanisms

behind this spatial heterogeneity are still unclear. Depth of the mixed layer is often

considered to be the major factor that controls these blooms and their distribution.

Arrigo et al. (1999) argued that P. antarctica grows well in deeper mixed layers due to

its photosynthetic plasticity, but van Hilst and Smith (2002) could not clearly

differentiate the photosynthetic characteristics of the two groups and their observed

distribution. Furthermore, a recent study found that P. antarctica occurred in highly

stratified waters near melting glacial ice in February (Arrigo, pers. comm.),

suggesting that mixed layer depths may not drive the distinct distribution of P.

antarctica and diatoms as previously thought. However, it is also important to discern

temporal variability within spatial patterns of phytoplankton assemblages. Moreover,

the degree of interaction between environmental factors and their control of

taxonomic distributions may vary temporally as well.

It has also been suggested that salinity (Fonda Umani et al., 2005),

temperature (Wright and Enden, 2000, Shields, 2007), Fe (Peloquin and Smith,

2007) and light (Goffart et al., 2000) may impact phytoplankton distribution. Loss

processes, including grazing (DiTullio and Smith, 1996) and aggregate formation

(Asper and Smith, 1999), also regulate surface biomass and composition. A complex

set of interactions of all factors, rather than a single variable, is likely to control the

distribution of diatoms and P. antarctica in the Ross Sea (van Hilst and Smith, 2002).

This thesis focuses on understanding the environmental parameters that are

strongly related to the spatial patterns of P. antarctica and diatoms in two highly

productive regions, the Amundsen and Ross Seas. I choose a distinct region of the

Southern Ocean – the eastern Ross Sea and the Amundsen Sea areas – with a wide

range of environmental and climatic conditions over a longitudinal gradient. This will

offer the opportunity to understand the factors that influence phytoplankton of the

Southern Ocean and provide information that can be used to predict the future

14

changes of phytoplankton under a changing climate. Moreover, the Amundsen Sea is

one of the least studied regions in the entire Southern Ocean, and an analysis of the

phytoplankton taxonomy and distribution in this area would contribute toward a more

comprehensive understanding of Southern Ocean phytoplankton.

Despite the lack of some types of data, such as Fe concentration and rates of

zooplankton grazing, I used hydrographic parameters that are intrinsically related to

light and Fe availability, such as the depth of the mixed layer and the occurrence of

MCDW. Moreover, zooplankton abundance in the Ross Sea is often assumed to be

low compared to other regions (Tagliabue and Arrigo, 2004). Unfortunately, little is

known about the zooplankton in the Amundsen Sea. Therefore, this chapter is an

initial effort to describe the hydrography of the two areas and to understand the

factors that influence the patterns of phytoplankton distribution.

15

OBJECTIVES

My objective is to define the environmental factors, with emphasis on mixed layer

depth and water mass distributions that influence the heterogeneous distribution of

phytoplankton in the Amundsen Sea and eastern Ross Sea.

Hypothesis to be addressed in this thesis:

H1: Diatoms will be dominant in areas where the mixed layer depth is shallow and the

water column more strongly stratified.

H2: The abundance of P. antarctica will be greatest in waters where the MCDW is in

the upper 200 m in the Ross Sea.

H3: In P. antarctica blooms, Fe is not limiting where the MCDW is shallower than 200

m.

16

MATERIALS AND METHODS

1 - Study area

Data for this project were collected as part of the RVIB Nathaniel B. Palmer

NBP07 - 02 and Oden Cruise 2007 (OSO-2007) to the Amundsen (100 - 130˚W) and

eastern Ross Seas (Fig. 1). In the first cruise transects were sampled along the Ross

Ice Shelf (NBP0702-T1) and at the Amundsen Sea shelf break (~700 m), as well as

in deep troughs on the Amundsen Shelf (NBP0702-T2 to NBP0702-T6) during

February and March of 2007 (Fig. 1a,c). NBP0702-T2 crossed the entire Amundsen

continental shelf and sampled in the Amundsen Sea polynya region, and along the

deep trough starting at Bear Peninsula to the Dotson Ice Shelf (Fig. 1c). NBP0702-T3

was near Pine Island in a deep trough across Bear Peninsula and Crosson Ice Shelf

(Fig. 1c). NBP0702-T4 was a long-shelf transect that cut from Getz Ice shelf near

Bear Peninsula to Carney Island (Fig. 1c). NBP0702-T5 was also a long-shelf

transect that started at the west side of Hobbs coast at Marie Byrd Land and finished

at Siple Island (Fig. 1c). NBP0702-T6 was the last transect, located along continental

shelf break of the entire Amundsen Sea (Fig. 1c). Other stations that were not

included within the transects were also sampled (Fig. 1a,c). In the second cruise

during December, 2007 and January, 2008, transects were sampled approximately at

the same locations (ODEN07-T1 to ODEN07-T3) as the first cruise (Fig. 2b,d).

17

Figure 1. Location of stations and transects from NBP07-02 and OSO-2007. (a) Stations from NBP07-02 (from stations 2 to 40) in the Ross Sea and (b) in the Amundsen Sea (stations 41 to 167). (c) Stations from OSO-2007 in the Ross Sea (stations 22 to 32) and (d) in the Amundsen Sea (stations 4 to 21).

a

b

18

Figure 1. Location of stations and transects from NBP07-02 and OSO-2007. (a) Stations from NBP07-02 (from stations 2 to 40) in the Ross Sea and (b) in the Amundsen Sea (stations 41 to 167). (c) Stations from OSO-2007 in the Ross Sea (stations 22 to 32) and (d) in the Amundsen Sea (stations 4 to 21).

c

d

19

2- Collection and Analysis of Water Samples

Vertical profiles of temperature and salinity (and derived density) were

measured with two independent CTD/rosette systems: a standard SeaBird 911+ CTD

system, with an epoxy-coated rosette with 24 10-L Niskin bottles equipped with

epoxy-coated springs, and a “trace-metal clean” CTD system, with a Teflon-coated

rosette fitted with 8 30-L Go-Flo bottles (Hunter et al., 1996). All sensors were

calibrated before and after each cruise, and sensor drift corrected using discrete

salinity samples analyzed at sea. Vertical profiles of fluorescence were collected with

a Chelsea fluorometer attached to the rosette. Water samples were collected from

both CTD-systems in the upper 200 m. Mixed layer depths (Zmix) were calculated

from σT vertical distribution and defined as depth where σT changes by 0.01 unit from

a stable surface value (Smith et al., 2000).

MCDW in the Ross Sea was defined as shelf waters with neutral density (γn)

between 28.0 and 28.27 kg m-3 and temperatures > -1.85˚C (Orsi and Wiederwohl,

2009). AASW in the Ross Sea are the surface waters found on the continental slope

(> 700 m) and shelf (< 700 m) where γn is < 28.0 and temperatures > -1.85˚C (Orsi

and Wiederwohl, 2009). CDW in the Amundsen Sea is characterized by waters

where the temperature is 3˚C warmer than the surface freezing point, which often

exceeds 1˚C over the in situ temperature (Shoosmith and Jenkins, 2006).

Water samples were collected for chlorophyll a, photosynthetic pigments and

biogenic silica (BSi). Chlorophyll a samples were filtered under low (1/3 atm) vacuum

through either 25 mm polycarbonate (20 μm; Poretics) or Whatman GF/F filters,

placed in 7 mL 90% acetone, and extracted for at least 24 h in cold (~-10˚C) and dark

conditions. The filters were removed and the samples read on a Turner Designs

Model 700 fluorometer. The fluorometer was calibrated before and after the cruise

using commercially purified chlorophyll (Sigma), which was quantified via high

performance liquid chromatography. Biogenic silica samples were filtered through 25

mm 0.6-μm Poretics polycarbonate filters, dried in plastic Petri dishes at 60°C, and

returned to the laboratory for analysis. The samples were then digested in NaOH at

100°C for 40 min, neutralized, and quantified colorimetrically for reactive silicate on a

20

dual-beam spectrophotometer (Brzezinski and Nelson, 1989). Samples for pigment

analysis by high performance liquid chromatography (HPLC) were collected and

filtered under low pressure through GF/F filters, wrapped in aluminum foil and quickly

frozen at -80ºC. Samples were returned to the laboratory frozen and processed on

Water Millenium high performance liquid chromatograph equipped with dual-beam

photocells and a fluorescence detector. The solvent gradients used were 80:20 (v/v)

methanol: ammonium acetate (0.5 M, pH 7.2), 87.5:12.5 (v/v) acetonitrile:distilled

water, and 100% ethyl acetate (Bidigare and Ondrusek, 1996). The HPLC was

calibrated with known standards that were either commercially prepared or extracted

from unialgal cultures (Jeffrey et al., 1997). Nutrients were collected and run at sea

using automated techniques.

3 – Remote Sensing

Remote sensing analyses from the Sea-viewing Wide Field of View Sensor

(SeaWiFS) weekly mean chlorophyll concentrations from November and December

were provided by NASA/Goddard Earth Science (GES)/Distributed Active Archive

Center (DAAC). Level 3 global Standard Mapped Images at 9 km resolution were

used with the OC4v4 algorithm, which is the latest algorithm developed for SeaWiFS.

The Wimsoft program was used to manipulate and extract numerical data from the

images. It is well known that the default algorithm used by SeaWiFS (OC4v4) can

provide poor estimates of surface chlorophyll a in some areas of the Southern Ocean,

but in the absence of regional algorithms, these estimations were used to compare

chlorophyll a between the Amundsen and Ross Seas. Ice concentrations were

obtained from the University of Bremen. The data was obtained using AMSR-E

(Advanced Microwave Scanning Radiometer), and ARTIST Sea Ice (ASI) algorithm

was used to quantify ice concentrations (www.seaice.de). The remote sensing data

allow for the determination of trends in ice cover and chlorophyll distribution on larger

space and time scales that cannot be ascertained by discrete water samples and

observations from ships.

21

4 – Bloom definition

To distinguish blooms dominated or co-dominated by P. antarctica and

diatoms, I defined bloom stations as those sites where surface chlorophyll a (chl a)

exceeds a concentration of 1 μg L-1. In the Southern Ocean this value is commonly

applied for a definition of a bloom, independent of the chl a derived from remotely

sensed estimates (Comiso et al., 1993) or fluorometric in situ measurements (Mitchell

and Holm-Hansen, 1991, De Baar et al., 1997). In this study chl a was determined by

fluorometric methods in the OSO-2007 cruise (December and January 2008) and by

high-performance liquid chromatography (HPLC) during both cruises. HPLC- and

fluorometrically-derived chl a may diverge from each other at times (Gall et al., 2001,

Hiscock et al., 2003), and this also occurred in this study. HPLC-derived chl a uses

the sum of chl a and its derivates: chl a allomer and epimer. Fluorometrically-derived

measurements include not only chl a, but potentially other fluorescent pigments

unresolved by HPLC, as well as chlorophyll precursors that are generally at low

concentrations (Hiscock et al., 2003). Therefore, fluorometrically-derived

measurements can overestimate the chl a compared to HPLC measurements.

Although the pair of HPLC and fluorometric measurements approached the predicted

values in a regression analysis (r2 = 0.90, a = 0.430, b = 0.059, n = 80), fluorometric

measurements overestimated chl a when concentrations were < 1 µg L-1. Therefore,

for consistency I used the cut off of surface chl a > 1 μg L-1 as a bloom, independent

of whether chl a was based on HPLC or fluorometry.

4.1 – Modes of taxonomic discrimination

Pigment ratios of fucoxanthin and 19’-hexanoylfucoxanthin were used to

quantify the dominance of diatoms or P. antarctica. Because both taxa potentially

contain fucoxanthin, stations dominated by diatoms were considered to be those

where the integrated euphotic zone ratio of fuco:19’-hex is > 1, whereas those

stations considered dominated by P. antarctica had an integrated euphotic zone ratio

of fuco:19’-hex < 0.3 (Smith and Asper, 2001; van Hilst and Smith, 2002). However,

22

another phytoplankton species co-occurred with diatoms and P. antarctica in this

study: the silicoflagellate Dictyocha speculum. This chrysophyte incorporates silica

like diatoms and synthesizes the same pigments used to distinguish type 4

haptophytes, including P. antarctica (Daugberj and Henriksen, 2001). Therefore, a

criterion that reasonably corresponded with the qualitative microscopic analysis was

used to discriminate Dictyocha from the other groups. That is, when fuco:19’-hex

ranges from 0.3 to 1, and when 19’-but:19’-hex is > 0.04, P. antarctica was

considered to be co-dominant with D. speculum, otherwise if 19’but:19’hex is < 0.04,

P. antarctica was considered to co-dominate with diatoms (see results). In cases

where blooms have fuco:19’-hex > 1, it is assumed that diatoms are dominant;

however, if 19’-but:19’hex is > 0.2, the station would be considered diatom and D.

speculum co-dominated.

Although the use of pigment ratio method seems to be a robust discriminator

of P. antarctica and D. speculum in the Ross Sea (DiTullio et al., 2003), I used an

additional proxy to separate diatoms and P. antarctica blooms: the Si(OH)4 :

(NO2+NO3) drawdown ratio (ΔSi:ΔN). Si(OH)4 and (NO2 + NO3) drawdowns were

calculated as the difference of integrated nutrient values during the sampling period

and the integrated winter values (Smith et al., 2006) from the upper 200 m. The

winter values were set as the highest NO2 + NO3 and Si(OH)4 concentration found

within 200-500 m at each station. NH4 is a regenerated source of nitrogen and,

therefore, was not included in this proxy. NO2 concentration is exceedingly low in

Antarctica waters (average 0.037 ± 0.025 μM; Smith and Asper, 2000), but was

nevertheless included in the calculation. In waters where diatoms are dominant, the

drawdown of silicate is higher because of the Si requirement of diatoms. Therefore,

the ΔSi:ΔN will tend to be higher in waters dominated by diatoms than those stations

dominated by P. antarctica. With this criterion P. antarctica would be dominant if

ΔSi:ΔN is < 0.9, whereas diatoms would be dominant if ΔSi:ΔN is > 2.15 (Dunbar et

al., 2003). When ΔSi:ΔN varies from 0.9 to 2.15, the phytoplankton assemblage

would be considered mixed. Because little is known about silicoflagellate Si

requirements in the Southern Ocean, I decided to apply the nutrient mode of

taxonomic discrimination only to those stations that ΔSi:ΔN is < 0.9 or ΔSi:ΔN is >

23

2.15. To assess whether the nutrient and pigment based modes differ from each

other in blooms dominated by diatoms or P. antarctica, the slopes of the linear

regression of nitrate + nitrate versus phosphate concentration in the water were

compared. The third proxy of taxonomic determination of blooms from the Amundsen

and Ross Seas is a qualitative description based on microscopy. Although the

estimate is only qualitative, it provides sufficient information to confirm the pigment

and nutrient mode of taxonomic discrimination.

Although accuracy increases when several approaches are used to determine

assemblage dominance, it is the possible that these results may differ between

approaches. The nutrient approach, in some instances, differs from the pigments and

microscopic approach because the nutrient levels during late summer and early fall

(February and March, 2007) do not exclusively reflect the phytoplankton dominant at

that time, but rather the sum of all groups that occurred during spring and summer.

As such, I used the nutrient mode as a confirmation of the pigment and microscopic

approaches. For consistency I used the pigment criterion to determine taxonomic

discrimination within the blooms because these data are more available and

generally match microscopic observations.

5 – Phytoplankton blooms and mixed layer depth

To test the hypotheses that diatoms dominate in areas where the Zmix is

reduced, analysis of variance (ANOVA) was used to compare the means of Zmix in

different blooms dominated by different assemblages during different seasons.

6 – Phaeocystis antarctica and the MCDW

To investigate the Fe-status of P. antarctica, I used the ratio of 19’-hex:chl c3

in blooms dominated by this species. Because Fe limitation induces the synthesis of

19’-hex in P. antarctica (van Leeuwe and Stefels, 1998), a ratio of 19’-hex:chl c3 > 3

indicates that P. antarctica is under Fe stress (DiTullio et al., 2007).

24

7 – Pulse Amplitude Modulation (PAM) and Maximum Photosynthetic Quantum Yield

Water samples were also collected to evaluate the maximum photosynthetic

quantum yield by fluorometry. This is a powerful analysis of the photosynthetic

potential of algae and a proxy for the physiological status of the cell. The maximum

potential quantum yield from PSII, when all reaction centers are open, is defined as:

Fv/Fm = [(Fm -F0)/ Fm]

where Fm is the maximum fluorescence when all the reaction centers are closed, F0

is the initial fluorescence, and Fv, the variable fluorescence term, is the difference

between Fm and F0. Because a depression in Fv/Fm can result from various

environmental stressors, including light, limitation of micro- or macronutrients, and

changes in temperature and salinity, water samples were kept near ambient

temperatures in the dark for at least 30 min before fluorescence measurements were

made to minimize the effect of non-photochemical quenching (Parkhill et al., 2001),

which is a photoprotective mechanism by which the excess of excitation is dissipated

as heat. The discrete fluorescence measurements of fluorescence were completed

using a Pulse Amplitude Modulation (PAM) fluorometer (Gademann Instruments;

Peloquin and Smith, 2007). Four pseudo-replicates of the saturation pulse every 30

seconds were completed. Analysis of variance was used to determine the

significance between the mean of these measurements at each depth of each station

(α value set a priori at p < 0.05).

25

RESULTS

1 - Remote Sensing of the Study Regions

Ice coverage varied both in time and space. During early February in the Ross

Sea, open water area reached a maximum, and ice is nearly absent along the Ross

Ice Shelf and Victoria Land (Fig. 2a - 2c). At the end of February and the beginning of

March, ice coverage noticeably increased in the Amundsen Sea, covering most of the

continental shelf (Fig. 2d - 2f). During end of November and beginning of December

of 2007, sea ice covered most of the Amundsen Sea shelf, although the Pine Island

and Amundsen Sea Polynyas were expanding (Fig. 2g - 2i). At the end of December

of 2007 and beginning of January of 2008, the central Ross Sea was ice free,

whereas ice concentrations near Victoria Land decreased more gradually (Fig. 2j - 2l).

SeaWiFS estimates of maximum chlorophyll concentrations also showed

spatial and temporal variations (Fig. 3 and 4). Maximum chlorophyll concentrations

were, in general, similar between the Amundsen and Ross Seas during February and

March of 2007 (Fig. 3a - 3f, 5a). Chlorophyll concentrations increased gradually

during February of 2007 and achieved its maximum (ca. 20 μg L-1) in the middle and

end of the month in the Ross and Amundsen Seas, respectively (Fig. 3a – 3c, 5a).

During March 2007 chlorophyll concentrations declined substantially (Fig. 3d – 3f, 5a),

and at the end of the month, chlorophyll was only high (ca. 5 μg L-1) off of the

continental shelf of the Amundsen and Ross Seas (Fig. 3f). In general, chlorophyll

concentrations were greater in the Ross than Amundsen Sea during November and

December, especially in the southwestern Ross Sea (Fig. 3g - 3k). At the end of

November and early December of 2007, chlorophyll concentrations were elevated

close to Pine Island Bay and Dotson and Getz Ice Shelves in the Amundsen Sea (ca.

10 μg L-1) and in the southwest Ross Sea (ca. 30 μg L-1) (Fig. 3g and 3h, 5b).

Towards the end of December, chlorophyll concentrations increased in the both

areas, reaching a maximum of 60 μg L-1 in the southwestern Ross Sea and 40 μg L-1

at Pine Island Bay (Fig. 3i, 3j, 5b).

26

Figure 2. Ice coverage (%) in February (a-d), March (e, f), November (g) and December (h to k) of 2007 and January of 2008 (l) in the Ross and Amundsen Seas. Data obtained from www.seaice.de.

3 Feb 2007 10 Feb 2007 17 Feb 2007

a b

24 Feb 2007 6 Mar 2007 17 Mar 2007

d e f

30 Nov 2007 10 Dec 2007 15 Dec 2007

g h i

22 Dec 2007 29 Dec 2007 5 Jan 2008

j k l

c

27

Figure 3. Weekly remote sensing chlorophyll distributions in (a-d) February, (e, f) March, November (g) and December of 2007 (h-k) in the Ross and Amundsen Seas derived from SeaWiFS database. The regions in white indicate absence of data.

02 Feb - 09 Feb 2007 10 Feb - 17 Feb 2007

18 Feb - 25 Feb 2007 26 Feb - 05 Mar 2007

06 Mar - 13 Mar 2007 14 Mar - 21 Mar 2007

a b

c d

e f

28

Figure 3. Weekly remote sensing chlorophyll distributions in (a-d) February, (e, f) March, November (g) and December of 2007 (h-k) in the Ross and Amundsen Seas derived from SeaWiFS database. The regions in white indicate absence of data.

27 Nov - 02 Dec 2007 03 Dec - 10 Dec 2007

11 Dec - 18 Dec 2007 19 Dec - 26 Dec 2007

g h

i j

k

27 Dec - 31 Dec 2007

29

Figure 4. Monthly climatology of remote sensing chlorophyll distributions (mg/m3) during (a) December (1997-2008), (b) January (1998-2009), (c) February (1998-2007) and March (1998-2007) in the Ross and Amundsen Seas derived from SeaWiFS database. The regions in white indicate absence of data.

a b

c d

30

0

5

10

15

20

25

02-08

Feb

10-17

Feb

18-25

Feb

26Fe

b-02M

ar

06-13

Mar

14-25

Mar

Max

imum

Chl

a co

ncen

tratio

n (μ

g.l-1

)

0

10

20

30

40

50

60

27Nov

-02Dec

03-10

Dec

11-18

Dec

19-26

Dec

27-31

Dec

Max

imum

Chl

a co

ncen

tratio

n (μ

g.L

-1)

Figure 5. Maximum chlorophyll (mg m-3) from SeaWiFS for 104˚W (dark blue), 114˚W (pink), 128˚W (orange), 133˚W (light blue) in the Amundsen Sea and for 175˚W (green) and 175˚E (black) in the Ross Sea during (a) February and March, 2007 and (b) November and December, 2007.

a

b

31

2 - Hydrographic data

Potential temperature (θ) versus salinity (S) diagrams show that the

Amundsen and the Ross Seas present different oceanographic conditions. According

to the θ-S plots (Fig. 6a, b), the Ross Sea has five distinct water masses, whereas

the Amundsen Sea has only two: CDW and AASW. In the Ross Sea the distribution

of these water masses of the upper 500 m varied spatially along the Ross Ice Shelf

(Fig. 7a, b). In general, the Antarctic Surface Water occupied a larger volume at the

western part of the Ross Ice Shelf, and decreased in thickness as it approached

Ross Island. Modified Circumpolar Deep Water consists of a mixture of AASW with

Circumpolar Deep Water and was shallower in the west, where it occurs less than

200 m. Within the transects along the Ross Ice Shelf during both seasons, MCDW

was found below the AASW and above the denser water masses, such as the

Modified Shelf water (MSW) and Shelf Water. During February 2007, when

stratification was stronger throughout the entire transect, the AASW was present

throughout the upper 100 m, being shallower from stations 6 to 15 (Fig. 7b). The

depth of MCDW ranged from 100 to 200 m in the east to below 400 m in the west.

Conversely, MCDW intruded above 100 m in the east at stations 31 and 32 (Fig. 7a)

in January 2008. MCDW was also present from 200 to 300 m at station 28 and 29

during the same period (Fig. 7a).

32

Figure 6. θ-S diagram with depth in the Z axis (maximum 1000 m) for the (a) Amundsen and (b) Ross Seas. Solid lines show the neutral density surfaces (28.00 and 28.27 kg m-3) that discriminates the CDW (Circumpolar Deep water) and MCDW (Modified Circumpolar Deep water) from the AASW (Antarctic Surface Water), MSW (Modified Shelf Water) and SW (Shelf Water).

Depth (m

) D

epth (m)

a

b

33

Figure 7. Distribution of water masses AASW (purple), MCDW (green), MSW and SW (orange and red) along the Ross Ice Shelf during (a) February, 2007 and (b) January, 2008. Numbers represent stations.

As mentioned previously, only two water masses were found in the upper 800

m in the Amundsen Sea: AASW and the CDW. Along transects 2 and 3, where most

of phytoplankton blooms were most intense, the CDW was present at 600 m across

the wide continental shelf (Fig. 8). Along transect 2, although the CDW was found on

the continental shelf, it did not approach Dodson Ice Shelf (at least the upper 800 m),

since it was found north of 72.5˚S (in the middle of the continental shelf, stations 45

and 20, NBP07-02 and OSO-2007, respectively) (Fig. 8 a,b). However, along

transect 3 in the Amundsen Sea, the CDW cut across the entire continental shelf and

a

b

Dep

th (m

) D

epth

(m)

34

approached Crosson Ice Shelf (as for south as 74˚S, stations 92 and 14, NBP07-02

and OSO-2007, respectively) (Fig. 8c, d). This suggests that basal melting may occur

at this site.

Figure 8. Distribution of water masses AASW (purple) and CDW (red), along transect 2 in the Amundsen Sea during (a) February, 2007 and (b) January, 2008 and along transect 3 during (c) February, 2007 and (d) January, 2008. Numbers represent stations. The regions in white indicate absence of data.

a

b

Dep

th (m

) D

epth

(m)

35

Figure 8. Distribution of water masses AASW (purple) and CDW (red), along transect 2 in the Amundsen Sea during (a) February, 2007 and (b) January, 2008 and along transect 3 during (c) February, 2007 and (d) January, 2008. Numbers represent stations. The regions in white indicate absence of data.

c

d

Dep

th (m

) D

epth

(m)

36

Zmix was significantly different between February and March of 2007 and

December and January of 2008 (p = 0.0001, ANOVA), but not between the

Amundsen and Ross Seas (p = 0.797, ANOVA) (Table 1). Zmix was deeper during

February and March of 2007, with an average of 27.4 m and maximum of 168 m

close to Getz Ice Shelf (Fig. 9a), whereas during the late spring and early fall, Zmix

averages 11 m, being deeper (ca. 25 m) near the western part of the Ross Ice Shelf

and off the Amundsen continental shelf (Fig. 9b).

Table 1. Mean ± standard deviations, minimum and maximum mixed layer depth (m)

and number of stations (N) in each cruise.

Cruise Mean ± std dev Min Max N

NBP07-02 27 ± 20 7 168 137

OSO-2007 11 ± 5 4 25 28

Ross Sea 25 ± 22 6 71 46

Amundsen Sea 24 ± 17 4 168 119

37

Figure 9. Distribution of Zmix (m) in the Ross and Amundsen Seas during (a) February and March, 2007 and (b) December, 2007 and January, 2008. The regions in white indicate absence of data.

Mitchell and Holm-Hansen (1991) argued that mixed layer depths of 40 m are

the threshold for positive oxygen evolution in the Southern Ocean; therefore, at those

stations where Zmix were > 40 m, low light levels were likely impeding net

phytoplankton growth. Chl a concentrations of > 1 µg L-1 were commonly found

where Zmix was < 40 m during February and March, 2007. In general, stations where

the Zmix > 80 m, chl a was < 0.5 µg L-1 (Fig. 10).

Depth (m

) D

epth (m)

a

b

38

Figure 10. Plot of the surface chl a (µg L-1) and Zmix (m) during February and March, 2007 (red squares) and December, 2007 and January, 2008 (black dots). 3 - Chlorophyll a distributions

Chl a concentrations varied temporally and spatially. In general, surface chl a

was elevated during the beginning of austral summer (December 2007) and near the

ice shelves in the Amundsen and Ross Seas, and at a minimum far from the coast

(~< 1 µg L-1) and during the end of the summer and beginning of fall (February and

March, 2007; Fig. 11 a,b). Chl a was low between the Ross and Amundsen Seas

(~140˚W) during both seasons (Fig. 11 a,b). During February and March 2007, chl a

was maximal at stations 9 and 33 located southwest and southeast of Ross Sea

(2.48 and 3.16 µg L-1) and at stations 94 and 89 close to Crosson and Dodson Ice

Shelves in the Amundsen Sea (Fig. 11a). During December and early January, 2007,

chl a was maximal at stations 11, 15 and 16 near Pine Island Bay (10.4, 9.11, and

8.16 µg L-1; Fig. 11b). In the Ross Sea chl a was elevated from the mid-Ross Ice

Zmix (m)

S

urfa

ce C

hl a

(µg

L-1)

39

Shelf at station 28 to McMurdo Sound at station 32, ranging from 1.63 to 6.83 µg L-1

(Fig. 11b).

Figure 11. Surface chlorophyll a (µg L-1) concentrations during (a) February and March and (b) December of 2007 and January of 2008.

a

b

Chl a (µg L

-1) C

hl a(µg L

-1)

40

4 - Phytoplankton bloom heterogeneity

4.1 – Approaches of taxonomic discrimination

Several approaches were applied to accurately determine phytoplankton

taxonomic dominance. The quantitative approaches were based only in pigment and

nutrient information, whereas the qualitative mode was based in microscopic

observation of the major groups. The slopes of the regression line of (NO3 + NO2)

versus PO4 concentration were 13.9 and 12.7 in diatom blooms determined by

pigments and ΔSi:ΔN, respectively. In blooms dominated by P. antarctica, the slopes

of (NO3 + NO2) versus PO4 were 15.4 and 15.6, when these blooms were separated

by pigments and ΔSi:ΔN, respectively (Fig. 12). Because these slopes are not

significantly different (ANCOVA, p > 0.05) when the nutrient and pigments modes

were used for each assemblage, it is likely that both approaches agree, which

supports their use as proxies to discriminate blooms dominated by either functional

group.

Given that both taxonomic approaches present similar results in determining

blooms dominated by P. antarctica and diatoms, I used the pigment criterion as a

taxonomic discriminator because these data are more frequently available and

generally better match microscopic observations (Fig. 13). Furthermore, other

phytoplankton groups (e.g., silicoflagellates) were present in relatively high

abundance; and because this group also assimilates Si, the pigment approach is

better capable of separating this group from the others.

41

Diatom dominated (pigments)

y = 13.863x + 2.4192R-sq = 0.7866

n = 63p < 0.0001

0

5

10

15

20

25

30

35

40

0 0.5 1 1.5 2 2.5

PO4 (µM)

N +

N (µ

M)

Diatom dominated (nutrients)

y = 12.661x + 4.8845R-sq = 0.6079

n= 41p < 0.0001

05

10152025303540

0 0.5 1 1.5 2 2.5

PO4 (µM)

N +

N (µ

M)

P. antarctica dominated (pigments)

y = 15.406x - 3.1365R-sq = 0.9499

n = 33p < 0.0001

0

5

10

15

20

25

30

35

0 0.5 1 1.5 2 2.5

PO4 (µM)

N +

N (µ

M)

P. antarctica dominated (nutrients)

y = 15.562x - 1.8839R-sq = 0.8983

n = 76p < 0.0001

0

510

1520

2530

35

40

0 0.5 1 1.5 2 2.5

PO4 (µM)

N +

N (µ

M)

Figure 12. Relationship of phosphate (PO4) (μM) and nitrate plus nitrite (NO3 + NO2) (μM) in diatoms and P. antarctica blooms separated by nutrients and pigments.

42

In general, pigment data match the qualitative microscopic information. As

expected, P. antarctica, both in the solitary or colonial form, was the only

Prymnesiophyte observed in the Amundsen and Ross Seas. The diatoms observed

in the Ross Sea were, in general, species from the genus Fragilariopsis along the

Ross Ice Shelf; however, the genera Chaetoceros and Corethron were also present.

Thalassiosira was present in the Ross Sea at station 2, close to Drygalski Ice Tongue,

during late summer. In the Amundsen Sea the diatoms observed were those taxa

also found in the Ross Sea; however, centric diatoms, such as Thalassiosira and

Chaetoceros, were found close to melting glaciers (Fig. 13 a–d). A third

phytoplankton species was found in relatively high abundance in the Ross Sea

during February, 2007: the silicoflagellate Dictyocha speculum.

43

Figure. 13. Distribution of taxa of phytoplankton based on microscopy in the (a) Ross Sea during February, 2007 and (b) March, 2007 and in the Amundsen Sea during (c) December, 2007 and (d) January, 2008 (Leventer, unpublished). Stations in red are considered bloom stations (chl a > 1 µg L-1). Boxes in the Amundsen Sea map refers to 1- Getz Glacier, 2- Dodson Glacier, 3– Crosson Glacier, 4- Thwaites Glacier, 5- Pine Island Ice Shelf 6 – Abbot Ice Shelf.

a

b

44

Figure. 13. Distribution of taxa of phytoplankton based on microscopy in the (a) Ross Sea during February, 2007 and (b) March, 2007 and in the Amundsen Sea during (c) December, 2007 and (d) January, 2008 (Leventer, unpublished). Stations in red are considered bloom stations (chl a > 1 µg L-1). Boxes in the Amundsen Sea map refers to 1- Getz Glacier, 2- Dodson Glacier, 3– Crosson Glacier, 4- Thwaites Glacier, 5- Pine Island Ice Shelf 6 – Abbot Ice Shelf.

c

d

45

4.2 – Silicoflagellate distribution

Blooms in the Amundsen and Ross

Seas were not only dominated or co-

dominated by diatoms and P. antarctica.

Another phytoplankton assemblage

surprisingly co-dominated with diatoms and

P. antarctica in this study, the

silicoflagellate Dictyocha speculum (Fig. 14).

Although Dictyocha speculum presumbably

has a similar accessory pigment

composition to that of Haptophyte “Type 4”

species, such as P. antarctica, a conspicuous pattern in the ratio of 19’-but:19’-hex

was observed. In stations where silicoflagellates were appreciably abundant under

microscopic observations, the 19’-but:19’-hex ratio was higher (Tables 4 and 5; also

see Appendices 3,4,7,10). Although I expected to find substantial concentrations of

these two pigments in blooms dominated or co-dominated by D. speculum and P.

antarctica, the discrepancy of the ratio occurs because P. antarctica cells from the

Ross Sea have greater concentrations of 19’-hex than D. speculum. Conversely, D.

speculum has higher concentrations of 19’-but. As a result, the low contribution of

19’-hex from D. speculum would increase both fuco:19’-hex and 19’-but:19’-hex in

blooms where this species co-dominates with diatoms (Table 4 and 5; also see

Appendices 3,4,7,10). Furthermore, the linear regression between the average 19’-

but concentration (from the upper 50 m) and D. speculum microscopic counts

provides evidence that most of the 19’-but is derived from this species (r2 = 0.80, a =

0.0004, b = 0.0091, n = 130) (Fig. 15) (Table 8; Appendices 3,4,7,10).

Figure 14 – Dictyocha speculum, 400X magnification. Cell size = 12 μM. Photo by Amy Leventer.

46

Figure 15. Relationship between 19’-butanoyloxyfucoxanthin (µg L-1) and D. speculum (ind mL-1) during February and March 2007 in the Ross and Amundsen Seas.

4.3 – Bloom distribution

Blooms were defined as regions where fluorometric and HPLC-derived surface

chl a exceeded 1 µg L-1, and taxonomic dominance was based on pigment ratios

(fuco:19’-hex and 19’-but:19’-hex). During the study large differences in assemblage

distribution were noted in space and time (Fig. 16). In general, P. antarctica

dominated the blooms in the western part of the Ross Sea (Fig. 16 a,c). The

distribution of these blooms expanded to the east during February, 2007 when

compared to January, 2008 (Fig. 16 a,c). In the Ross Sea diatoms bloomed at

stations 27 and 29 (located east of the Ross Ice Shelf) during February, 2007 and at

stations 28 and 29 during January, 2008. The silicoflagellate D. speculum was

present in relatively high abundance in the eastern part of the Ross Sea (150 and

170˚W) during February, 2007 (Fig. 16 a). D. speculum co- dominated with P.

Ave

rage

19’

-but

(µg

L-1)

Dictyocha speculum (ind. mL-1)

47

antarctica (stations 22, 25, 27 and 30) and diatoms (stations 32, 33 and 39) during

February, 2007 (Fig. 16 a).

Blooms dominated by diatoms were more common in the Amundsen Sea than

in the Ross Sea (Fig. 16 a-d). During March 2007, diatoms blooms occurred along

Crosson (stations 88, 89, 90 and 91) and Dodson Ice Shelves (stations 66, 93 and

95), except at station 94 where diatoms co-dominated with P. antarctica (Fig. 16b).

During December 2007, diatom blooms occurred at stations 9, 13, 15 and 17; at

stations 11, 12 and 16, diatoms co-dominated with P. antarctica. D. speculum was

also present at the Amundsen Sea during December, 2007, co-dominating with

diatoms (Fig. 16d).

Figure 16. Distribution of P. antarctica (pink), D. speculum (orange) and diatoms (blue) blooms in the (a) Ross Sea during February, 2007 and (d) January, 2008 and the Amundsen Sea during (b) March and (c) December, 2007. Station with 2 dots of different colors indicates co-dominance.

a b

c

d

February 2007

March 2007

January 2008December 2007

48

4 - Diatoms and mixed layer depths

All diatom blooms were present in waters with shallow Zmix, varying from 4 to

14 m during early summer (December, 2007 and January, 2008). However, during

late summer and early fall, diatoms were present in a range of mixed layers, from 11

to 66 m (February and March, 2007) (Fig. 17). ANOVA indicates that the Zmix of

blooms dominated by diatoms was not significantly different from blooms that were

dominated or co-dominated by other taxa during the NBP07-02 and OSO-2007 cruise

(p>0.05, ANOVA).

0

10

20

30

40

50

60

D P P + D S + D S + P

Late spring/ Early summerLate summer/ Early fall

Figure 17. Distribution of blooms dominated by diatoms (D) or P. antarctica (P) or co-dominated by P. antarctica and diatoms (P + D), or silicoflagellate D. speculum and diatoms (S + D), or D. speculum and P. antarctica (S + P) during February and March of 2007 (late summer and early fall) and December of 2007 and January of 2008 (late spring and early summer).

Z mix (m

)

49

Although diatoms were present in shallow mixed layers during late spring and

early summer in the Amundsen and Ross Seas, stratified waters were not a physical

parameter unique to diatoms blooms. In fact, all blooms, irrespective of taxonomic

affinity, were in waters where Zmix < 25 m during December, 2007 and January, 2008

(Fig. 17). It is evident that the shallow depth of the mixed layer in late spring and

early summer promoted growth of diatoms. Relative diatom abundance (represented

by fuco:(fuco+19’-hex)) for blooms dominated by diatoms or co-dominated (with P.

antarctica) was greater in waters where the mixed layer was shallower (r2 = 0.69, p<

0.05) during late spring and early summer (Fig. 18 a). However, mixed layer depth

was not the only factor that promoted the formation of diatom blooms because there

were stations where mixed layers were low (from 6 to 21 m) but where surface chl a

concentration was also low ( < 1 µg L-1) (Fig. 18 b). This suggests that multiple

environmental factors govern the distribution of diatoms in the Amundsen and Ross

Seas. Nonetheless, shallow mixed layer depth appeared to be an important physical

variable that supported diatom blooms during December, 2007 and January, 2008.

50

Figure 18. Relationship between fuco/(fuco+19’-hex) and Zmix (m) in (a) blooms dominated by diatoms (D) or co-dominated by P. antarctica and diatoms (P + D) during December, 2007 and January, 2008 (b) all stations, including non-bloom and those dominated by diatoms (D) or P. antarctica (P) and co-dominated P. antarctica and diatoms (P + D) or D. speculum and P. antarctica (S + P) during December, 2007 and January, 2008 in the Amundsen and Ross Seas. Higher fuco/(fuco+19’-hex) indicates higher contribution of diatoms in the station. The solid line represents the linear logarithm regression (Zmix in the logarithmic scale) that best fit the model.

a

b

Zmix (m)

Zmix (m)

51

In the Amundsen Sea the average salinity of the upper 50 m varied from 33.75

to 34.05 psu during beginning to mid-December, 2007, except at station 4 located off

the continental shelf (average S = 34.6 psu). In the Ross Sea, which was sampled

during late December and early January, average surface and subsurface salinity

varied from 33.9 to 34.45 psu (Fig. 19). Therefore, although sampled earlier in the

season, the Amundsen Sea surface and subsurface waters (< 50 m) were relatively

fresher when compared to the Ross Sea (Fig. 19). During this period, the taxonomic

distribution of blooms in both areas was not correlated with salinity (r2 = 0.0002, p >

0.05). Nonetheless, blooms dominated or co-dominated by diatoms were more

frequently observed in the relatively fresher waters of the Amundsen Sea (Fig. 19). In

the Ross Sea diatom blooms were only found at stations 28 and 29, where waters

were relatively more saline when compared to the stations dominated by diatoms in

the Amundsen Sea (Fig. 19). The lack of relationship between salinity and the

contribution of diatoms in blooms from the Amundsen and Ross Seas indicates

distribution of diatom blooms is not a function of freshwater input during late spring

and early summer. However, during this period, sea ice is starting to melt and starting

to “seed” diatoms from sea ice into the water. Although seeding is a process that

plausibly explains the presence of diatoms in the water, it does not explain rapid

growth and bloom formation. Therefore, during late summer and early fall, it is

possible that shallower mixed layers in the Amundsen Sea (and maybe other factors

such as availability of Fe from melting sea ice) explains the presence of diatoms

blooms in the Amundsen Sea.

52

Figure 19. Relationship between fuco/(fuco+19’-hex) and salinity (psu) in all stations, including non-bloom stations and those dominated by diatoms (D), or P. antarctica (P), or co-dominated by D. speculum and P. antarctica (S+P) and P. antarctica and diatoms (P+D) in the Amundsen (AS) and Ross Seas (RS) during December, 2007 and January, 2008.

During late summer and early fall, Zmix varied broadly within blooms dominated

or co-dominated by different assemblages. About 40% of the diatoms were at sites

where the Zmix was > 25 m (data not shown). During this period, mixed layer depth

did not show any influence on the distribution of blooms dominated or co-dominated

by diatoms in the Amundsen and Ross Seas (r2 = 0.29, p > 0.05) (data not shown).

This evidence does not support the hypothesis that diatoms rely on shallow mixed

layers (< 25 m) to grow during late summer and early fall. However, all blooms

started to senesce during February and March of 2007, and therefore it is possible

that the shoaling of mixed layer depth promotes the growth of diatoms only during

spring and early summer. Conversely, the heterogeneous taxonomic distribution of

blooms varied significantly with salinity during late summer and early fall in the

Amundsen and Ross Seas (r2 = 0.76, p < 0.0001) (Fig. 20). This suggests that during

this period freshwater input from glaciers allowed the development of diatoms blooms

53

in the Amundsen Sea. Although the surface and subsurface waters of the Amundsen

Sea and the eastern part of Ross Ice Shelf (130 to 150˚W) are fresher (average

salinity from 33.2 to 33.9 psu) during late summer and early fall, mixed layers were

substantially deeper than during late spring and early summer. This may be due to

the increased wind stress and elevated brine injection cause by early ice formation.

Figure 20. Relationship between fucoxanthin/(fucoxanthin+19’-hexanoyloxyfucoxanthin) and salinity in all stations (including non-blooming) in the Amundsen and Ross Seas during February and March, 2007. The solid line represents the regression line.

5 - MCDW and Phaeocystis antarctica distribution

As expected, the hydrographic characteristics found in the Ross Sea were

different than those in the Amundsen Sea. Modified Circumpolar Deep Water, which

was found only in the Ross Sea, was closer to the surface and thinner in the western

Ross Sea as previously described by Orsi and Wiederwohl (2009). In this study the

MCDW was shallower than 100 m at stations 31 and 32 during December, 2007 and

54

slightly deeper at station 30. During February 2007, the MCDW was always from 100

to 200 m at stations 2, 6, 7, 8, 9 and 12 (Fig. 21).

Figure 21. θ-S plot in the upper 200 m. Different colors represent different depth. MCDW is located in the area where the neutral density is between 28 and 28.27. The numbers represents the stations where MCDW is found. Station 30, 31 and 32 are from OSO-07, whereas the other stations are from NBP07-02.

During this study P. antarctica was the dominant phytoplankton in all stations

where the MCDW was present in the upper 200 m, except in December at station 30,

where P. antarctica co-occurred with diatoms (Fig. 16) and at station 2 during

February, where no bloom was observed (data not shown). Nonetheless, during

February 2007, P. antarctica was also present in waters where the AASW was the

only water mass in the upper 200 m and the MCDW was deeper (ca. 400 m) (Fig. 22).

The significant linear relationship of 19’-hex and surface chl a shows that in waters

where the MCDW was shallower than 200m, P. antarctica (represented by

concentration of 19’-hex) dominated the biomass. However, in waters where only the

AASW is present within the upper 200 m (during February, 2007), the significant

Depth (m

)

55

linear relationship suggests that P. antarctica formed bloom in those waters as well

(Fig. 22). This means that while MCDW may contribute to the formation of P.

antarctica blooms, the shallow expression of this water mass (upper 200 m) is not

critical for the formation of these blooms during late summer (February, 2007) (Fig.

22).

Although it is recognized that 19’-hex content in P. antarctica changes

according to irradiance and Fe concentrations, the slopes of the regression between

19’-hex and chl a within different water masses were not significantly different

between each other (Fig. 22, ANCOVA, p > 0.05). However, 19’-hex and chl a

concentrations were significantly different in waters where the MCDW is < 100 m

during December 2007 and January 2008 (ANCOVA, p < 0.05), but not significantly

different when the MCDW is from 100 to 200 m or when AASW dominated the upper

200 m during February 2007 (ANCOVA, p > 0.05). This can indicate that the shallow

MCDW (< 100 m) may have contributed significantly to the growth and accumulation

of P. antarctica only during late spring and early summer, whereas during late

summer, P. antarctica was not found exclusively in waters where the MCDW is

shallow (upper 200 m). However, it is important to notice that there is the seasonal

effect, since the very shallow MCDW (< 100 m) was only present during the early

summer (January, 2008),; during late summer (February 2007), it was deeper than

100 m.

56

Figure 22. Relationship between surface chlorophyll a (μg L-1) and 19’-hexanoyloxyfucoxanthin (μg L-1) from Ross Sea samples. Data were grouped according to blooms dominated by P. antarctica (P) or co-dominated by P. antarctica and diatoms (P+D) or dominated by other assemblages (other) in different water masses. Red squares and purple inverted triangle are data from December 2007 and January 2008, whereas other data are from February and March, 2007.

To test the hypothesis that intrusions of MCDW are potentially bringing Fe into

surface waters and fueling blooms of P. antarctica, the Fe status of this group was

indirectly assessed. In the Ross Sea the Fe status of P. antarctica (based on 19’-

hex:chl c3 ratio) was variable within different water masses. Monte Carlo estimates

suggest that the frequency of 19’-hex:chl c3 ratio < 3 was not significantly associated

with different distributions of the MCDW at different depths (χ2 = 3.58, p > 0.05).

However, about 50% of the 19’-hex:chl c3 ratio was < 3 in waters where the MCDW

was < 100 m. Thus, these data do not provide evidence that P. antarctica was under

Fe-replete conditions in waters where MCDW is shallower than 200 m. However, the

19’-hex:chl c3 ratio was slightly lower in waters where the MCDW was < 100 m

during early summer compared to the late summer, where MCDW was > 100 m (Fig.

23). Therefore, one could speculate that either Fe availability was greater in waters

where the MCDW was shallower than 100 m, or that seasonality has a greater

19’-h

ex (μ

g L-1

)

Chl a (μg L-1)

57

influence in Fe stress (represented by lower 19’-hex:chl c3 ratio), since Fe

concentrations are higher during early summer than late summer and early fall.

Unfortunately, we are not able to separate the effect of shallow MCDW in the Fe

status of P. antarctica because we have the interaction of seasonality in these data.

Thus, it remains uncertain whether MCDW is source of Fe for P. antarctica during

late summer and early spring.

Figure 23. Relationship of 19’-hexanoyloxyfucoxanthin to chlorophyll c3 ratio and chlorophyll a (µg L-1) from P. antarctica blooms as a function of different water masses. Line represents a 19’-hex:chl c3 ratio of 3. Points located in the area above the line indicates samples where P. antarctica is under Fe stress (a).

The quantum yield of photochemistry (Fv/Fm), assessed by active fluorescence

methods, is a measure of the physiological status of phytoplankton. Given that the

Southern Ocean is rich in macronutrients, limitation of micronutrients, such as Fe,

may reduce the Fv/Fm (Olson et al., 2000). Because there is only Fv/Fm information

from early summer (December, 2007 - January, 2008), it was not possible to

compare the status of P. antarctica blooms over a wide range of MCDW depths (>

100 m and > 200m). In general, however, Fv/Fm of P. antarctica blooms were

Chl a (μg L-1)

19’-h

ex: c

hl c

3

58

relatively high during early summer (Fv/Fm average = 0.45) in waters where the

MCDW was < 100 m, but lower than in blooms dominated and co-dominated by

diatoms and P. speculum (Fig. 24). This value and range are similar to those found in

P. antarctica blooms during December, 2001/2002 (Peloquin and Smith, 2007). This

suggests that the shallow MCDW (< 100 m) provided Fe to P. antarctica blooms

during early summer. However, because P. antarctica blooms do not occur in waters

where the MCDW is > 100 m during early summer, there is no way to compare the

effect of the MCDW in increasing the Fv/Fm of P. antarctica.

P. antarctica blooms had lower Fv/Fm values compared to those in blooms

where the assemblage co-dominated with diatoms or those that were diatom-

dominated (Fig. 24). This suggests that either the Fv/Fm varies between different

assemblages or the health of the phytoplankton is inversely proportional to the size of

bloom (based on chl a biomass). However, Suggett et al. (2009) reported that

interspecific signatures of the Fv/Fm do not apply to HNLC areas, since changes in

Fv/Fm has been consistently demonstrated to be a function of Fe availability. In this

case, it is possible that diatoms were not under Fe stress because chl a

concentrations were noticeably lower in diatom blooms than in those that had mixed

composition (diatoms + P. antarctica) and/or P. antarctica blooms. However, the

variation in Fv/Fm likely resulted from a temporal progression (from December to

January) than from changes in bloom magnitude (Fig. 24). This is indicative that Fe

was more available to the phytoplankton in the Amundsen Sea during early

December, and as the summer season progressed, Fe availability decreased,

despite higher irradiance and potentially lower Fe demand. During early January,

2008, the Fv/Fm of P. antarctica was still high; however, these blooms could be

approaching an Fe-stressed condition, which is corroborated by pigment data (50%

of 19’-hex:chl c3 ratio < 50%).

59

Figure 24. Relationship between (a) Fv/Fm and chl a concentration (µg L-1) and (b) Fv/Fm with time, among blooms dominated by diatoms (D), or P. antarctica (P), or co-dominated by P. antarctica and diatoms (P + D), or D. speculum and P. antarctica (S + D) and water masses.

a

b

Fv/F

m

Chl a (µg L-1)

Fv/F

m

60

DISCUSSION

Phaeocystis antarctica and diatom bloom distribution

During most of the 20th century, the Southern Ocean was thought to be

primarily dominated by larger diatoms because of their frequent occurrence in net

tows and the high concentration of macronutrients in the water column, including

silicates (Hart 1934, 1942). However, this assumption was contested when Bunt

(1964) reported the presence of Phaeocystis blooms in Antarctic waters during

spring/summer of 1961, 1962 and 1963, and suggested that Phaeocystis was a

potentially important member of the phytoplankton. Since then, Phaeocystis and

diatoms have been shown to be major contributors to primary production in some, but

not all, regions of the Southern Ocean. However, the quasi-predictable distributions

of diatoms and Phaeocystis antarctica in some specific areas of the Antarctic, and

the factors that control their growth, remain poorly understood, and are sometimes

contentious.

The silicoflagellate Dictyocha speculum was present in relatively high

abundance in the eastern part of the Ross Sea during February, 2007, where it co-

dominated with P. antarctica and diatoms. Although it is surprising to find this species

in high abundance in the Ross Sea, silicoflagellates have been reported to occur in

low abundance in the sea ice in the West Antarctic Peninsula (Garrison, 1987).

Sillicoflagellates, along with diatoms, are used as proxies for past climatic events,

since different species from these assemblages respond differently to changes in sea

surface temperature (Baron et al, 2008). Distephanus speculum past occurrence has

been associated with cold, upwelling waters, whereas the genus Dictyocha is

interpreted to represent Pliocene warm events (Whitehead and Bohaty, 2003).

Although silicoflagellates have been observed in low abundance in regions south of

the Polar Front (Eynaud et al., 1999), virtually nothing is known about their presence

in shelf waters of the Ross Sea.

61

Mixed Layer Depth

It has been previously established that the depth of the mixed layer plays a

fundamental role in the growth of diatom blooms in the Southern Ocean. Kopczynska

(1992) was the first to point out that vertical mixing could potentially determine

phytoplankton spatial distributions when she concluded that the dominance of

microflagellates was due to deeper mixed layers in the Antarctic Peninsula. However,

these microflagellates were only referred to as species of Prasinophyceae and

Cryptophyceae. Leventer and Dunbar (1996) reported the spatially heterogeneous

distribution of diatoms and P. antarctica blooms associated with mixed layer depths

in the Ross Sea. According to their observations, diatoms were dominant in stable

waters close to the ice-edge, whereas P. antarctica was dominant in less stratified

open waters during spring and summer of 1991, 1992 and 1993. Their believed that

the release of diatoms from melting ice was the primary cause for the spatial

distribution, although they also suggested that positive buoyancy or adaptation to low

irradiance could have promoted the growth of P. antarctica in less stable waters as

well. The hypothesis that mixed layer depth spatially restricts the blooms of P.

antarctica and diatoms was bolstered when Moisan and Mitchell (1999), who

demonstrated that cultures of P. antarctica exhibit high quantum yields and high

chlorophyll-specific absorption under low-light conditions. According to Arrigo et al.

(1999, 2000) and Goffart et al. (2000), this would explain the dominance of P.

antarctica during spring blooms in the polynya, where irradiance levels are low due to

a deeper mixed layer when compared to summer. More recently, Kropuenske et al.

(2009) compared the photoprotective strategies of these two functional groups in a

laboratory study and provided a physiological explanation for the success of diatoms

in high-light environments and P. antarctica in low irradiance settings. P. antarctica

conducts rapid protein synthesis and photodamage repair, allowing them to thrive

under dynamic light conditions, characteristic of rapidly mixing waters. Conversely, F.

cylindrus, a common diatom from the Ross Sea, is able to maximize photoprotection

and minimize photoinhibition by maintaining high heat dissipation, which allows this

species to grow under high irradiance.

62

In this study I rejected the hypothesis that diatoms would be dominant in areas

where the mixed layer depth was shallow (< 25 m). In reality, diatom blooms

occurred in areas where the mixed layer depth varied from very shallow (4 m) to

relatively deep (66 m). During late spring and early summer, all blooms, including

those dominated by diatoms, were present in waters where the mixed layer was < 25

m. During late summer and early fall, about 40% of the diatoms were in sites where

the mixed layer was > 25 m. The remarkably shallow mixed layer found during early

summer was expected because most of the bloom sites were located in areas close

to melting ice. The deeper mixed layers found during late summer and early fall

occurred because of the increased strength of the winds and enhanced water column

convection as a result of brine rejection from newly formed ice. Because this study

included two different seasons (late spring/early summer and late summer/early fall),

it is likely that the environmental factors that regulate phytoplankton bloom

distribution differed with time.

During late spring and early summer, shallow mixed layer depths favored the

growth of diatoms because the contribution of these taxa increased as the mixed

layer depth decreased. However, a shallow mixed layer was not the only requirement

for bloom development, given that there were stations where the mixed layer was

shallow but surface chl a was low (< 1 µg L-1). That is, shallow mixed layers were not

exclusively required for diatom growth, since other phytoplankton assemblages also

formed blooms in shallow mixed layers. Therefore, other environmental factors

combined with shallow mixed layer governed the distribution of diatoms in the

Amundsen and Ross Seas during late spring and early summer. However, during

late summer and early fall, all blooms started to senesce (according to satellite

images), and therefore it is possible that shallow the mixed layers supported the

growth of diatoms only early in the season.

The hypothesis that mixed layer depth solely contributes to the spatial

variation of diatoms and P. antarctica blooms has been strongly scrutinized during

recent years. van Hilst and Smith (2002) found that both assemblages are adapted to

low-light levels, and therefore, changes in irradiance with mixed layer depth should

not be the sole reason for the distinct taxonomic distribution. According to these

63

authors, other factors, including Fe supply or algal seeding, may contribute to the

distinct P. antarctica/diatom distribution. Smith and Asper (2001) found that diatoms

were dominant in areas where the mixed layer was deeper during spring of

1995/1996, and no significant difference between the depth of mixed layer and

diatom bloom distribution was found during the summer. Their conclusion was that

instead of only one single factor, a complex temporal matrix of factors, including Fe

availability, nutrients, grazing, losses and growth rate controls the distribution.

Garrison et al. (2003b) also found no apparent association of P. antarctica and

diatoms with mixing. They also argued that other phenomena, such as availability of

Fe and algal seeding, are more likely to explain this distribution.

The lack of differentiation of temporal bloom patterns and spatial variability of

P. antarctica and diatom blooms may explain the controversy over the controls of the

distribution of these two assemblages. Arrigo et al. (2000) and Goffart et al. (2000)

argued that P. antarctica is usually found in the poorly stratified waters of the south

central Ross Sea during spring, whereas diatoms are found late in the summer

season in stratified waters along shelf of Victoria Land. Arrigo et al. (2003) also

suggested that different light regimes are the most important environmental factor

explaining this distribution during spring and summer, and that Fe has little influence

in determining assemblage composition. However, although ability to grow under low

light may explains the distribution of P. antarctica in less stable waters, it does not

explain the ability of this species to thrive in stratified waters. According to Arrigo et

al. (2003), additional input of Fe derived from melting ice during summer may

contribute to diatom growth, but ability to grow under high light explains their

competitive exclusion over P. antarctica. My results suggest that the distribution of P.

antarctica and diatoms in the Amundsen and Ross Seas is not uniquely related to

mixing; instead, a combination of factors varying with time explains this distribution.

64

Algal seeding

Algal seeding could be an important factor influencing the distribution of

diatoms and P. antarctica in the Amundsen and Ross Seas, since some diatom

species that are usually found in sea ice (e.g., F. cylindrus) were also found in high

abundance in the water. P. antarctica has also been found in the ice (Garrison et al.,

2003a) and shown to survive prolonged periods of darkness and freezing (Tang et

al., 2009), suggesting that this species might “seed” the water column as well.

Nonetheless, blooms dominated or co-dominated by diatoms were more frequent in

relatively fresher waters of the Amundsen Sea than the Ross Sea during the period

of study. During late summer and early fall, this pattern was even more evident, being

diatom-dominated in fresher waters of the Amundsen Sea and P. antarctica

dominating in more saline waters of the Ross Sea. Thus, one may suggest that

freshwater input from glaciers stimulated the development of diatom blooms.

However, Fonda Umani et al. (2005) observed that fresher waters, derived from

melting ice, are the main driving force that promoted the growth and the dominance

of P. antarctica and not diatoms in the Terra Nova Bay polynya. Although algal

seeding followed by freshwater input is a key process that reasonably explains the

presence of diatoms and P. antarctica, it does not account for their rapid growth and

distinct bloom distribution. Instead of algal accumulation derived from ice melt,

additional factors, including availability of Fe and proper light levels, may have

stimulated rapid growth of diatoms.

Zooplankton grazing

Zooplankton grazing is a key loss process that may influence phytoplankton

biomass and composition in the ocean. However, during late spring and early

summer, it is unlikely that this process strongly influenced the distribution of distinct

assemblages. That is because some overwintering populations, such as copepods,

are still migrating to the upper waters as consequence of their life cycle, and

therefore their abundance is still low when compared to summer (Quetin et al., 1996).

65

However, during late summer, zooplankton abundance might have increased and

potentially influenced the abundance and distribution of P. antarctica and diatoms.

There is strong evidence to show that diatoms are preferentially grazed over P.

antarctica (Haberman et al., 2003), although microzooplankton are able to graze P.

antarctica single cells (Smith et al., 2003; Shields et al., 2008). The ability to produce

chemical deterrents derived from DMSP (dimethylsulfoniopropionate), such as acrylic

acid and DMS (dimethylsulfate), may explain the reduced grazing pressure observed

on P. antarctica colonies. Moreover, the colonial forms are considered to be

unpalatable because of their tough outer skin, which is presumably of poor nutritional

quality and can act as a mechanical obstacle for grazing (Baustista, 1992). However,

colonies of P. antarctica are indeed grazed by mesozooplankton and

macrozooplankton (Hansen et al. 1994). Zooplankton biomass is thought to be low in

the Ross Sea compared to other areas of the Southern Ocean (Huntley and Zhou,

2000, Sertorio, 2000, Tagliabue and Arrigo, 2003). However, there are few data on

zooplankton abundance in the Ross Sea, and nearly nothing is known about the

zooplankton of the Amundsen Sea, and as such it is impossible to evaluate whether

grazing of P. antarctica and diatoms influenced their distribution.

Fe

In surface waters of many seas, Fe is derived from atmospheric deposition

(Johnson et al., 1997). However, in the Southern Ocean aeolian dust and Fe input is

minimal, and the biogeochemical equilibrium results in an average dissolved Fe

concentration < 0.2 nM in surface waters (Martin et al., 1990, de Baar et al., 1995;

Sedwick et al., 1997, de Baar et al., 1999). In the Ross Sea dissolved Fe

concentration is slightly greater (although often less than 0.3 nM at the surface),

because of the wide continental shelf and extensive input of melting sea-ice, which

contribute to Fe inputs (Johnson et al., 1999, Sedwick and DiTullio, 1997, Coale et al,

2005). However, the major source of dissolved Fe in the Ross Sea is derived from

depth and is mixed to the surface during winter (Edwards and Sedwick, 2001).

66

Modified Circumpolar Deep Water (MCDW) has been considered a potential

source of Fe to upper waters (Peloquin and Smith, 2007), although there is little

direct evidence that support this hypothesis. Fitzwater et al. (2000) observed slightly

higher concentrations of dissolved Fe in the upper 250 m along the Ross Ice Shelf

(up to 1.13 nM) in January of 1990 in regions where the MCDW usually occurs (Orsi

and Wiederwohl, 2009), although the MCDW was not sampled in that study. Because

the MCDW intrudes onto the continental shelf along several deep N–S troughs

(Dinniman et al., 2003) and promotes sediment resuspension, it is possible that this

water mass provides Fe to upper waters (Hales and Takahashi, 2004, Hiscock,

2004). However, there is no direct evidence that confirms this hypothesis.

A compelling argument that explains the distribution of P. antarctica and

diatoms is the combination of Fe availability with mixed layer depth. It is well

established that Fe demand is higher under low light (Sunda and Huntsman, 1997)

because Fe is required for the synthesis of additional light-harvesting pigments and is

used for synthesis of photosynthetic redox proteins (Falkowski and Raven, 2007).

Likewise, larger cells also require larger concentrations of Fe because of their smaller

surface-to-volume ratios (Sunda and Huntsman, 1997). However, phytoplankton

responses to Fe requirements vary within distinct taxonomic groups, since some

phytoplankton species evolve adaptations that to overcome Fe limitation. These

adaptations include competition between Fe-ligand complexes (Hutchins, 1999),

production of non-Fe containing substitutes for ferredoxin, extracellular Fe-binding

ligands (Trick and Wilhelm,1995), and increased Fe storage capacities (Marchetti et

al., 2008).

Diatoms and P. antarctica have been reported to differ not only in light-

harvesting capabilities but also in Fe requirements. Coale et al. (2003) observed that

Phaeocystis colonies have higher Fe requirements than diatoms (half-saturation

constants (Km) of 0.26 and 0.03 nM, respectively), and suggested that temporal and

spatial shifts from Phaeocystis to diatom in the Ross Sea were due to lower Fe

requirements of diatoms in well-illuminated waters. There is the possibility, however,

that the mucus-bound Fe in P. antarctica colonies may become available to cells for

67

later growth (Davidson and Marchant, 1987), which complicates the interpretation of

Fe availability for this species.

In this study MCDW was found only in the Ross Sea and along the Ross Ice

Shelf. Furthermore, it was shallower and thinner in the southwestern Ross Sea (Orsi

and Wiederwohl, 2009). In the Amundsen Sea, however, MCDW was not present in

shelf waters, and P. antarctica never dominated the blooms. This predictable

distribution of P. antarctica blooms in waters where the MCDW is shallow suggests

that this water mass was bringing Fe to the upper waters (at least to the subsurface;

~ 80 m) and potentially fueling blooms, given that this species requires higher Fe

concentrations than diatoms. This is consistent with what is known about P.

antarctica, as it is able to thrive under low light when Fe requirements become even

greater (Sunda and Huntsman, 1995, Sedwick et al., 2007).

P. antarctica blooms occurred in waters where the MCDW intruded from 80 to

150 m during late December 2007 and early January 2008. However, during

February 2007, P. antarctica blooms extended farther east along the Ross Ice Shelf

in waters where the MCDW was much deeper (~ 400 m). Although the MCDW was

not as shallow during late summer, it is intriguing that P. antarctica blooms

consistently occurred in waters where the MCDW was within 400 m. Thus, if it is true

that high Fe concentrations regulate the distribution of P. antarctica, additional Fe

inputs from the MCDW may favor this species over diatoms. Although Fe per se was

not measured in this study, maximum photosynthetic yield and 19’-hex:chl c3 ratios

were used to indirectly assess the photophysiological state and Fe status in P.

antarctica blooms. I hypothesized that in P. antarctica blooms Fe would not be

limiting where the MCDW is shallower than 200 m. Accordingly, 19’-hex:chl c3 ratio

was slightly less during December, 2007 and January, 2008, with 50% of those ratios

< 3. According to DiTullio et al. (2006), 19’-hex:chl c3 ratio > 3 is indicative of Fe -

stress. However, it is important to note that during early summer P. antarctica blooms

achieved the maximum biomass in the study (7 µg L-1); therefore, some Fe stress

would not be surprising. Nonetheless, during late summer and early spring, most of

the 19’-hex:chl c3 ratios were > 3, which indicates that the shallow MCDW may not

have fueled P. antarctica blooms during this period. Moreover, during this period all

68

blooms, including those dominated by P. antarctica, were declining according to

satellite images, possibly due to Fe limitation.

The Fe status of P. antarctica blooms (based on pigment ratios) shows that P.

antarctica blooms are approaching an Fe -stress condition during late spring and

early summer. During the same period Fv/Fm of P. antarctica blooms were relatively

high in waters where the MCDW was < 100 m (Fv/Fm average = 0.45) and where P.

antarctica was abundant. However, P. antarctica blooms had lower Fv/Fm values

compared to diatom and mixed blooms in the Amundsen Sea. There are three

potential reasons that could account for the higher Fv/Fm in the Amundsen Sea during

late spring and early summer: (1) higher biomass in the Ross than Amundsen Sea

during the sampling period resulted in light or micronutrient limitation; (2) temporal

progression of the blooms (Amundsen Sea was sampled in late spring while the Ross

Sea was sampled in early summer); and (3) higher Fe input in the Amundsen Sea

from glacial melt than from the MCDW. Because the magnitude of P. antarctica

blooms were greater in the Ross Sea during December, 2007 and January, 2008, it is

possible that P. antarctica was approaching Fe stress, although high levels of

chlorophyll per se does not imply strong competition per Fe and consequent Fe

stress for the phytoplankton. Furthermore, it is possible that the variations in Fv/Fm

resulted from a temporal progression, since Fe concentrations may be higher during

early and mid-December, 2007 than late December, 2007 and early January, 2008.

However, photoreduction, a mechanism mediated by irradiance that can convert

some ligand-bound Fe3+ species to Fe2+, is assumed to resupply available Fe (Fe2+)

to phytoplankton in surface waters during summer (Tagliabue and Arrigo, 2006,

Tagliabue et al., 2009). Because we do not have Fe measurements, it is impossible

to discuss the third possibility in detail. In the Amundsen Sea freshwater input is

significantly higher than in the Ross Sea due to glacial basal melting. A speculation

for this increased basal melting is the penetration of the CDW across the continental

shelf. In this study the CDW entered near Crosson Ice Shelf (transect 3). Therefore, if

Fe is indeed coming from basal melting and if this meltwater rises to the surface, it

should result in higher Fe concentrations.

69

Tagliabue and Arrigo (2005) argued that ice-derived Fe supports only 10% of

the regional net primary production in the Ross Sea. Unfortunately, nothing is known

about the contribution of MCDW-derived Fe for phytoplankton growth. Hiscock (2004)

observed a high photosynthetic efficiencies and chlorophyll a concentrations in

waters where the MCDW was shallow (from 150 to 400 m). Peloquin and Smith

(2007) speculated that the intrusion of the MCDW supplied Fe and fueled an unusual

secondary bloom of diatoms during February, 2004 after the decline of P. antarctica

in December, 2003. Shields (2007) assessed the relationship of temperature and

nitrate uptake by Phaeocystis and diatoms and found a significant negative

correlation between surface concentrations of nitrate and temperature in waters

where the MCDW intrusions were noted. She pointed out the possibility that

temperature may not influence nitrate uptake directly; rather, it may be a proxy for

intrusions of Fe-enriched water that consequently affect nitrate uptake. However,

intrusions of MCDW might strengthen water column stratification and restrict Fe input

to the surface (Smith et al., submitted). Although it remains unclear whether the

MCDW provided Fe to the P. antarctica during late December 2007 and early

January 2008, it is clear that during February, 2007, the shallow expression of the

MCDW was not a source of Fe for P. antarctica, since the assemblage was under Fe

stress. Therefore, it is possible that during summer either the MCDW did not provide

Fe at all to P. antarctica blooms, or the surface waters were too stratified to allow

significant input of Fe to the surface. Since my data provide only a snapshot rather

than a temporal progression of the bloom, it is not possible to know whether the

MCDW provided Fe to upper waters before February, 2007. Nonetheless, the results

of this study may alter the previously held view that MCDW is a potential source of Fe

to upper waters during summer.

70

SUMMARY

• P. antarctica and diatoms were not the only phytoplankton that were abundant

in the Amundsen and Ross Seas; silicoflagellates also co-dominated at some

locations.

• During late spring and early summer, shallow mixed layer depths supported

the growth of diatoms, but that was not the only factor required for diatom

bloom development.

• During late spring and early summer, shallow mixed layer depths were not

exclusively required for diatom growth, since other phytoplankton

assemblages also formed blooms.

• During late summer and early fall, mixed layer depths did not influence the

distribution of blooms dominated or co-dominated by diatoms in the Amundsen

and Ross Seas.

• Algal seeding per se does not fully explain phytoplankton growth and

distributions. Nonetheless, blooms dominated or co-dominated by diatoms

were more frequent in relatively fresher waters of the Amundsen Sea than in

the Ross Sea.

• Pigment data show that during late spring and early summer, P. antarctica

blooms were approaching Fe stress in waters where the MCDW was present.

However, this may have been influenced by the high biomass and/or temporal

progression of the bloom.

• MCDW intrusions may have supported P. antarctica blooms during the late

summer and early spring only, whereas during late summer and early fall,

despite the shallow expression of the MCDW, P. antarctica blooms appeared

to be under Fe limitation.

71

APPENDICES

Appendix 1. Surface chlorophyll a, mixed layer depth (Zmix), description of the most abundant assemblages based on microscopic observations

and average salinity of the upper 50 m for each station of the NBP07-02 cruise.

Station Date Surface chl a Zmix Dominance Average Salinity (µg L-1) (m) (microscope) (psu)

6 02/08/07 1.38 6 mostly P. antarctica 34.2 8 02/08/07 1.16 29 mostly P. antarctica 34.3 9 02/08/07 2.37 10 mostly P. antarctica 34.1 12 02/09/07 2.16 19 mostly P. antarctica 34.2 13 02/09/07 1.76 48 mostly P. antarctica 34.0 14 02/09/07 1.37 33 mostly P. antarctica 34.0 15 02/09/07 1.35 48 mostly P. antarctica 34.0 16 02/09/07 1.20 14 mostly P. antarctica 34.0 20 02/10/07 2.06 23 diversed flora (P. antarctica + D. speculum + diatoms 34.0 22 02/10/07 2.04 17 mostly D.speculum 33.8 25 02/11/07 1.27 35 mostly D. speculum + P. antarctica 33.7 26 02/11/07 1.09 8 diversed flora (P. antarctica + D. speculum + diatoms) 33.7 27 02/11/07 2.00 16 diversed flora (P. antarctica + D. speculum + diatoms) 33.6 29 02/11/07 1.49 38 diversed flora (P. antarctica + D. speculum + diatoms) 33.6 30 02/11/07 1.37 7 mostly D. speculum + Diatoms 33.6 32 02/12/07 1.01 43 mostly D. speculum + Diatoms 33.3 33 02/12/07 3.16 13 mostly D. speculum 33.6 39 02/15/07 2.04 10 mostly dead P. antarctica; some diatoms 33.8 66 02/23/07 1.28 66 mostly diatoms; some P. antarctica 33.8 88 02/28/07 1.35 32 P. antarctica + Diatoms 33.2 89 02/28/07 1.54 12 P. antarctica + Diatoms 33.3 90 02/28/07 1.42 16 P. antarctica + Diatoms 33.4 92 02/28/07 1.08 15 mostly Diatoms 33.7 93 02/28/07 1.13 11 mostly Diatoms 33.7 94 02/28/07 1.93 14 P. antarctica + Diatoms 33.7 95 03/01/07 1.60 30 mostly Diatoms 33.6

72

Appendix 2. Surface chlorophyll a, mixed layer depth (Zmix), description of the most abundant assemblages based on microscopic observations

and average salinity of the upper 50 m for each station of the OSO-2007 cruise.

Station Date Surface chl a Zmix Dominance Average Salinity (µg L-1) (m) (microscope) (psu)

9 12/12/07 2.10 14 Diatoms 34.0 10 12/12/07 4.80 10 mostly diatoms; some D. speculum 33.8 11 12/13/07 10.4 15 mostly Diatoms, some P. antarctica 33.8 12 12/14/07 4.16 13 mostly Diatoms, some P. antarctica 33.9 13 12/15/07 1.44 4 Diatoms 33.8 15 12/16/07 9.07 9 Diatoms 33.8 16 12/17/07 8.16 14 mostly P. antarctica + Diatoms 33.9 17 12/19/07 4.59 12 Diatoms 33.9 28 12/30/07 1.63 7 Diatoms 34.2 29 12/30/07 3.29 8 Diatoms 34.2 30 12/30/07 3.61 25 P. antarctica + Diatoms 34.0 31 1/1/08 5.08 10 P. antarctica 34.4 32 1/2/08 8.37 7 P. antarctica 34.2

73

Appendix 3. Integrated (from the upper 200 m) chlorophyll c3 (chl c3), 19’-butanoyloxyfucoxanthin (19’-but), 19’-hexanoyloxyfucoxanthin (19’-hex),

fucoxanthin (fuco), fuco:19’-hex ratio, fuco:(fuco+19’-but) ratio, 19’-but:19’-hex ratio, 19’-hex:chl c3 ratio and assemblage dominance based on

pigment approach for each station of the NBP07-02 cruise.

Station Date chl a 19’-but fuco 19’-hex fuco/ fuco/ 19’-but/ 19’-but/ Dominance (mg m-2) (mg m-2) (mg m-2) (mg m-2) 19’-hex (fuco+19’-hex) 19’-hex chl c3 (pigments)

6 02/08/07 13.0 0.2 6.2 41.4 0.15 0.1 0.00 3.2 P. antarctica 8 02/08/07 4.1 0.1 2.8 26.0 0.11 0.1 0.00 6.3 P. antarctica 9 02/08/07 36.1 1.5 11.1 150.5 0.07 0.1 0.01 4.2 P. antarctica 12 02/09/07 66.8 2.0 14.0 211.1 0.07 0.1 0.01 3.2 P. antarctica 13 02/09/07 13.8 0.7 5.6 70.8 0.08 0.1 0.01 5.1 P. antarctica 14 02/09/07 14.3 0.7 8.0 71.9 0.11 0.1 0.01 5.0 P. antarctica 15 02/09/07 11.2 0.8 10.7 61.6 0.17 0.1 0.01 5.5 P. antarctica 16 02/09/07 16.5 0.6 7.7 51.7 0.15 0.1 0.01 3.1 P. antarctica 20 02/10/07 18.6 0.9 12.8 59.1 0.22 0.2 0.02 3.2 P. antarctica 22 02/10/07 5.8 2.0 18.1 41.0 0.44 0.3 0.05 7.0 D. speculum + P. antarctica 25 02/11/07 9.5 2.0 19.9 37.5 0.53 0.3 0.05 4.0 D. speculum + P. antarctica 26 02/11/07 6.8 2.1 20.4 30.9 0.66 0.4 0.07 4.6 D. speculum + P. antarctica 27 02/11/07 5.0 2.1 22.6 22.2 1.02 0.5 0.09 4.5 Diatoms 29 02/11/07 7.8 3.1 44.2 32.4 1.37 0.6 0.10 4.1 Diatoms 30 02/11/07 6.1 3.5 32.1 33.4 0.96 0.5 0.11 5.5 D. speculum + P. antarctica 32 02/12/07 0.7 1.2 15.6 3.5 4.50 0.8 0.35 4.9 D. speculum + Diatoms 33 02/12/07 4.8 13.8 53.0 36.2 1.47 0.6 0.38 7.5 D. speculum + Diatoms 39 02/15/07 0.5 6.0 33.0 16.4 2.02 0.7 0.37 30.4 D. speculum + Diatoms 66 02/23/07 0.0 0.0 0.4 0.1 4.62 0.8 0.04 3.2 Diatoms 88 02/28/07 0.7 0.3 54.7 1.8 31.19 1.0 0.15 2.7 Diatoms 89 02/28/07 4.9 0.2 43.1 1.3 32.43 1.0 0.14 0.3 Diatoms 90 02/28/07 5.9 0.3 38.7 2.3 16.99 0.9 0.12 0.4 Diatoms 92 02/28/07 0.5 0.2 25.4 2.8 9.15 0.9 0.08 5.2 Diatoms 93 02/28/07 2.8 0.7 12.3 7.0 1.76 0.6 0.10 2.5 Diatoms 94 02/28/07 7.9 0.4 16.3 27.9 0.59 0.4 0.02 3.5 P. antarctica + Diatoms 95 03/01/07 4.1 0.8 48.3 36.5 1.32 0.6 0.02 8.9 Diatoms

74

Appendix 4. Integrated chlorophyll c3 (chl c3) (from the upper 200 m), 19’-butanoyloxyfucoxanthin (19’-but), 19’-hexanoyloxyfucoxanthin (19’-hex),

fucoxanthin (fuco), fuco:19’-hex ratio, fuco:(fuco+19’-but) ratio, 19’-but:19’-hex ratio, 19’-hex:chl c3 ratio and assemblage dominance based on

pigment approach for each station of the OSO-2007 cruise.

Station Date chl C3 19’-but fuco 19’-hex fuco/ fuco/ 19’-but/ 19’-but/ Dominance (mg m-2) (mg m-2) (mg m-2) (mg m-2) 19’-hex (fuco+19’-hex) 19’-hex chl c3 (pigments)

9 12/12/07 0.5 0.9 10.0 5.2 1.93 0.7 0.18 9.8 Diatoms 10 12/12/07 8.7 0.9 9.9 20.5 0.48 0.3 0.05 2.4 D. speculum + P. antarctica 11 12/13/07 9.1 0.7 8.1 25.9 0.31 0.2 0.03 2.8 D. speculum + Diatoms 12 12/14/07 11.0 0.6 12.8 31.5 0.41 0.3 0.02 2.9 D. speculum + Diatoms 13 12/15/07 0.4 0.1 6.1 1.2 4.93 0.8 0.06 2.8 Diatoms 15 12/16/07 11.5 0.3 28.5 12.6 2.27 0.7 0.02 1.1 Diatoms 16 12/17/07 12.6 0.3 12.8 15.6 0.82 0.5 0.02 1.2 P. antarctica + Diatoms 17 12/19/07 7.2 0.9 7.8 5.9 1.31 0.6 0.16 0.8 Diatoms 28 12/30/07 4.3 0.2 14.8 4.1 3.59 0.8 0.04 1.0 Diatoms 29 12/30/07 0.6 0.5 41.1 10.4 3.96 0.8 0.05 17.5 Diatoms 30 12/30/07 15.8 0.6 14.6 45.4 0.32 0.2 0.01 2.9 P. antarctica + Diatoms 31 1/1/08 27.5 1.2 14.5 84.8 0.17 0.1 0.01 3.1 P. antarctica 32 1/2/08 47.3 1.3 15.6 108.0 0.14 0.1 0.01 2.3 P. antarctica

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Appendix 5. Integrated PO4, Si(OH4), and (NO3 + NO2) drawdown from the upper 200 m, ΔSi:ΔN ratio, and assemblage dominance based on

nutrient ratios of each station from NBP07-02 cruise.

Station Date ΔPO4 ΔSi(OH)4 ΔNO2 + ΔNO3 ΔSi:ΔN Dominance (mol m-2) (mol m-2) (mol m-2) (nutrient)

6 02/08/07 0.14 1.10 1.90 0.58 P. antarctica 8 02/08/07 *** *** *** *** *** 9 02/08/07 0.15 0.62 2.21 0.28 P. antarctica

12 02/09/07 *** *** *** *** *** 13 02/09/07 *** *** *** *** *** 14 02/09/07 0.12 0.99 2.17 0.46 P. antarctica 15 02/09/07 0.11 0.96 1.94 0.49 P. antarctica 16 02/09/07 0.07 0.58 1.26 0.46 P. antarctica 20 02/10/07 0.02 0.66 0.46 1.45 Mixed 22 02/10/07 0.10 0.16 1.46 0.11 P. antarctica 25 02/11/07 *** *** *** *** *** 26 02/11/07 0.09 0.59 1.55 0.38 P. antarctica 27 02/11/07 *** *** *** *** *** 29 02/11/07 0.06 0.79 0.82 0.97 Mixed 30 02/11/07 0.08 0.69 1.22 0.56 P. antarctica 32 02/12/07 0.11 1.08 1.34 0.81 P. antarctica 33 02/12/07 0.05 0.69 0.68 1.01 Mixed 39 02/15/07 0.01 1.44 0.24 5.91 Diatoms 66 02/23/07 0.02 0.14 0.22 0.61 P. antarctica 88 02/28/07 0.06 1.74 0.96 1.81 Mixed 89 02/28/07 0.09 1.82 1.26 1.45 Mixed 90 02/28/07 0.03 1.27 0.55 2.30 Diatoms 92 02/28/07 0.02 0.86 0.30 2.89 Diatoms 93 02/28/07 0.03 1.16 0.54 2.16 Diatoms 94 02/28/07 0.03 1.69 0.51 3.29 Diatoms 95 03/01/07 *** *** *** *** ***

*** No data

76

Appendix 6. Integrated phosphate (PO4), Si(OH4), and (NO3 + NO2) drawdown from the upper 200 m, ΔSi:ΔN ratio, and assemblage dominance

based on nutrient ratios of each station from OSO-2007 cruise.

Station Date ΔPO4 ΔSi(OH)4 ΔNO2 + ΔNO3 ΔSi:ΔN Dominance (mol m-2) (mol m-2) (mol m-2) (nutrient)

9 12/12/07 0.02 0.70 0.22 3.15 Diatoms 10 12/12/07 0.02 0.33 0.21 1.56 Mixed 11 12/13/07 0.02 0.47 0.27 1.78 Mixed 12 12/14/07 0.04 0.49 0.70 0.70 P. antarctica 13 12/15/07 *** *** *** *** *** 15 12/16/07 0.05 0.35 0.80 0.44 P. antarctica 16 12/17/07 0.02 0.24 0.24 1.00 Mixed 17 12/19/07 *** *** *** *** *** 28 12/30/07 *** *** *** *** *** 29 12/30/07 *** *** *** *** *** 30 12/30/07 *** *** *** *** *** 31 1/1/08 *** *** *** *** *** 32 1/2/08 0.04 0.46 0.83 0.56 P. antarctica

*** No data

77

Appendix 7. Silicoflagellates abundance (ind mL-1) and average 19’-butanoyloxyfucoxanthin (19’-but)

concentration of the upper 50 m for each station of the NBP07-02 cruise.

Station Depth Silicoflagellates Average 19’-but (m)

(ind mL-1) (μg L-1)

2 0 0.0169 0 2 50 0.0029 0 5 20 0.0023 4 6 0 0.0034 0 7 20 0.0000 0 8 20 0.0031 0 9 20 0.0174 0 10 20 0.0108 0 11 0 0.0084 0 12 0 0.0155 0 13 0 0.0146 1 14 20 0.0108 2 15 20 0.0128 3 16 20 0.0093 2 17 20 0.0074 3 18 0 0.0077 5 19 0 0.0204 17 20 20 0.0162 1 22 0 0.0353 25 23 0 0.0309 33 24 0 0.0351 33 25 60 0.0329 35 26 0 0.0378 33 27 0 0.0518 65 29 0 0.0364 64 30 0 0.0421 86 31 0 0.0194 19 32 0 0.0484 51 33 0 0.1746 398 34 0 0.0406 61 35 0 0.0174 5 37 0 0.0120 4 38 0 0.0075 2 39 0 0.0780 2 40 0 0.0022 4 42 0 0.0010 12 43 0 0.0062 8 44 0 0.0103 15 45 0 0.0134 8 46 0 0.0078 12 47 0 0.0059 6 48 0 0.0103 5 49 0 0.0039 5

78





50 0 0.0049 1 51 0 0.0056 4 52 10 0.0042 3 53 0 0.0059 5 54 0 0.0033 3 55 0 0.0036 1 56 0 0.0031 3 57 0 0.0035 4 58 0 0.0079 4 59 0 0.0076 3 61 0 0.0089 3 63 0 0.0074 2 64 0 0.0045 0 84 0 0.0088 3 85 0 0.0024 0 87 0 0.0051 0 91 0 0.0024 1 92 0 0.0073 1 93 0 0.0103 1 94 0 0.0074 1 95 0 0.0105 4 96 0 0.0018 0 97 0 0.0121 1 98 0 0.0134 1 100 0 0.0188 1 101 0 0.0045 3 103 0 0.0100 4 104 0 0.0058 2 105 0 0.0134 6 107 0 0.0108 2 108 0 0.0120 3 109 0 0.0122 4 110 0 0.0134 2 111 0 0.0160 3 112 0 0.0090 16 113 0 0.0186 16 114 0 0.0149 4 115 0 0.0131 4 117 0 0.0046 7 121 0 0.0089 1 124 0 0.0136 3 125 0 0.0116 3 126 0 0.0122 5 129 0 0.0081 1 135 0 0.0055 0 139 10 0.0051 0

79





143 0 0.0025 0 147 0 0.0023 0 151 5 0.0048 0 152 0 0.0038 0 153 0 0.0079 0 154 0 0.0075 0 155 0 0.0128 0 156 0 0.0093 0 157 0 0.0119 0 158 0 0.0128 1 159 0 0.0098 0 160 0 0.0122 0 162 0 0.0091 0 167 0 0.0036 0

Appendix 8. Fv/Fm and phytoplankton dominance based on pigments during OSO-2007 cruise.

Station Depth Fv/Fm Dominance (m) (pigments)

4 8 0.4175 Not a bloom 4 8.5 0.4812 Not a bloom 4 50 0.4411 Not a bloom 4 100 0.3639 Not a bloom 4 150 0.3652 Not a bloom 5 9 0.2376 Not a bloom 5 20 0.4824 Not a bloom 5 50 0.4956 Not a bloom 5 100 0.4028 Not a bloom 5 150 0.3584 Not a bloom 6 8.9 0.5042 Not a bloom 6 20 0.4858 Not a bloom 6 35 0.5740 Not a bloom 6 50 0.5242 Not a bloom 6 100 0.5260 Not a bloom 7 10 0.3428 Not a bloom 7 30 0.5688 Not a bloom 7 34 0.5604 Not a bloom 7 51 0.5542 Not a bloom 7 100 0.5112 Not a bloom

80



8 10 0.2350 Not a bloom 8 20 0.4590 Not a bloom 8 35 0.5552 Not a bloom 8 49 0.5710 Not a bloom 8 99 0.5776 Not a bloom 8 150 0.5682 Not a bloom 9 10 0.2230 Diatoms 9 20 0.4868 Diatoms 9 35 0.5813 Diatoms 9 50 0.5813 Diatoms 9 100 0.5680 Diatoms 9 150 0.5466 Diatoms 10 10 0.2576 D. speculum + P. antarctica 10 20 0.4280 D. speculum + P. antarctica 10 35 0.5218 D. speculum + P. antarctica 10 50 0.5001 D. speculum + P. antarctica 10 100 0.5022 D. speculum + P. antarctica 11 10 0.2998 P. antarctica + Diatoms 11 20 0.4704 P. antarctica + Diatoms 11 35 0.5293 P. antarctica + Diatoms 11 50 0.5396 P. antarctica + Diatoms 12 9 0.3888 P. antarctica + Diatoms 12 19 0.4836 P. antarctica + Diatoms 12 34 0.5052 P. antarctica + Diatoms 12 49 0.5020 P. antarctica + Diatoms 13 2 0.5133 Diatoms 13 10 0.5987 Diatoms 13 20 0.6208 Diatoms 13 35 0.5895 Diatoms 14 10 0.1308 Not a bloom 14 18 0.5472 Not a bloom 14 33 0.4112 Not a bloom 14 50 0.4554 Not a bloom 15 10.6 0.5372 Diatoms 15 20.8 0.5652 Diatoms 15 34 0.5928 Diatoms 15 50 0.5908 Diatoms 16 10 0.5714 P. antarctica + Diatoms 16 20 0.5878 P. antarctica + Diatoms 16 35 0.5476 P. antarctica + Diatoms 16 50 0.5838 P. antarctica + Diatoms

81



17 5 0.6030 Diatoms 17 10 0.6098 Diatoms 17 20 0.6098 Diatoms 17 35 0.6088 Diatoms 19 5.5 0.5372 Not a bloom 19 9.7 0.5406 Not a bloom 19 20 0.5340 Not a bloom 19 35 0.4900 Not a bloom 20 10 0.4406 Not a bloom 20 20 0.4902 Not a bloom 20 35 0.5222 Not a bloom 20 50 0.5154 Not a bloom 20 75 0.5066 Not a bloom 23 10 0.4794 Not a bloom 23 20 0.4794 Not a bloom 23 35 0.5022 Not a bloom 23 50 0.4894 Not a bloom 24 10 0.5614 Not a bloom 24 20 0.5820 Not a bloom 24 35 0.5958 Not a bloom 24 50 0.5688 Not a bloom 25 10 0.4864 Not a bloom 25 20 0.5168 Not a bloom 25 35 0.5454 Not a bloom 25 50 0.5502 Not a bloom 27 10 0.5824 Not a bloom 27 20 0.5178 Not a bloom 27 35 0.5008 Not a bloom 27 50 0.4384 Not a bloom 28 10 0.5378 Diatoms 28 20 0.5200 Diatoms 28 35 0.4826 Diatoms 28 50 0.3418 Diatoms 29 10 0.4974 Diatoms 29 20 0.5000 Diatoms 29 35 0.5028 Diatoms 29 50 0.4582 Diatoms 30 10 0.5080 P. antarctica + Diatoms 30 20 0.4966 P. antarctica + Diatoms 30 35 0.4266 P. antarctica + Diatoms 30 50 0.4144 P. antarctica + Diatoms 31 10 0.4666 P. antarctica 31 20 0.3526 P. antarctica 31 35 0.3872 P. antarctica 31 50 0.3938 P. antarctica 32 10 0.4576 P. antarctica 32 20 0.4820 P. antarctica 32 35 0.4934 P. antarctica 32 50 0.4834 P. antarctica

82

Appendix 9. Fluorometric-based chlorophyll a concentrations (µg L-1) in the Amundsen and Ross Sea

during OSO-2007 cruise.

Station Cast Date Depth Chl a concentrations (m) (μg L-1)

4 7 12/08/07 8 0.21 4 7 12/08/07 8.5 0.23 4 7 12/08/07 50 0.22 4 7 12/08/07 100 0.18 4 7 12/08/07 150 0.06 5 9 12/09/07 9 0.29 5 9 12/09/07 20 0.25 5 9 12/09/07 50 0.25 5 9 12/09/07 100 0.12 5 9 12/09/07 150 0.03 6 11 12/10/07 2 0.22 6 11 12/10/07 10 0.16 6 11 12/10/07 20 0.19 6 11 12/10/07 20 0.22 6 11 12/10/07 35 0.15 6 11 12/10/07 50 0.25 6 11 12/10/07 75 0.14 6 11 12/10/07 100 0.08 6 11 12/10/07 150 0.01 6 11 12/10/07 250 0 7 19 12/11/07 2 0.38 7 19 12/11/07 10 0.27 7 19 12/11/07 20 0.3 7 19 12/11/07 35 0.33 7 19 12/11/07 50 0.47 7 19 12/11/07 75 0.21 7 19 12/11/07 100 0.06 7 19 12/11/07 150 0.01 7 19 12/11/07 250 0.04 8 21 12/12/07 2 0.64 8 21 12/12/07 10 0.88 8 21 12/12/07 20 0.78 8 21 12/12/07 35 1.02 8 21 12/12/07 50 0.70

83




8 21 12/12/07 75 0.51 8 21 12/12/07 100 0.16 8 21 12/12/07 150 0.03 8 21 12/12/07 250 0.01 9 23 12/12/07 2 2.03 9 23 12/12/07 10 2.10 9 23 12/12/07 20 2.05 9 23 12/12/07 35 2.24 9 23 12/12/07 50 1.47 9 23 12/12/07 75 0.29 9 23 12/12/07 100 0.18 9 23 12/12/07 150 0.02 9 23 12/12/07 250 0.01 10 25 12/12/07 2 4.26 10 25 12/12/07 10 4.80 10 25 12/12/07 20 3.06 10 25 12/12/07 35 2.79 10 25 12/12/07 50 2.88 10 25 12/12/07 75 1.81 10 25 12/12/07 100 0.13 10 25 12/12/07 150 0.14 11 26 12/13/07 10 10.35 11 26 12/13/07 20 5.44 11 26 12/13/07 35 3.64 11 26 12/13/07 50 0.46 11 26 12/13/07 100 0.18 11 26 12/13/07 150 0.09 12 30 12/14/07 2 4.17 12 30 12/14/07 10 4.16 12 30 12/14/07 20 4.05 12 30 12/14/07 35 4.10 12 30 12/14/07 50 3.96 12 30 12/14/07 75 1.98 12 30 12/14/07 100 0.32 12 30 12/14/07 150 0.20 12 30 12/14/07 200 0.10 13 38 12/15/07 2 1.44 13 38 12/15/07 10 2.02 13 38 12/15/07 20 1.74 13 38 12/15/07 35 1.05 13 38 12/15/07 50 1.05 13 38 12/15/07 75 1.07 13 38 12/15/07 100 0.47 13 38 12/15/07 150 0.02 13 38 12/15/07 200 0.05

84




14 39 12/15/07 50 1.96 14 39 12/15/07 75 1.54 14 39 12/15/07 100 0 14 39 12/15/07 150 0.01 14 39 12/15/07 250 0.01 15 41 12/16/07 5.5 9.11 15 41 12/16/07 10 9.07 15 41 12/16/07 20 9.28 15 41 12/16/07 35 8.80 15 41 12/16/07 50 7.18 15 41 12/16/07 75 7.09 15 41 12/16/07 100 5.86 15 41 12/16/07 150 2.60 15 41 12/16/07 200 0.70 16 47 12/18/07 10 8.16 16 47 12/18/07 20 6.73 16 47 12/18/07 35 9.03 16 47 12/18/07 50 7.07 16 47 12/18/07 100 0.68 16 47 12/18/07 150 0.15 16 47 12/18/07 250 0 17 48 12/19/07 5 4.59 17 48 12/19/07 10 4.76 17 48 12/19/07 20 4.64 17 48 12/19/07 35 3.71 17 48 12/19/07 50 4.17 17 48 12/19/07 75 3.55 17 48 12/19/07 100 0.48 17 48 12/19/07 151 0.08 18 51 12/19/07 2 7.13 18 51 12/19/07 10 7.76 18 51 12/19/07 20 7.44 18 51 12/19/07 35 0.41 18 51 12/19/07 50 6.60 18 51 12/19/07 100 4.80 18 51 12/19/07 151 2.08 19 54 12/20/07 5.5 0.59 19 54 12/20/07 9.7 0.66 19 54 12/20/07 20 0.59 19 54 12/20/07 35 0.62 19 54 12/20/07 50 0.68 19 54 12/20/07 76 0.52 19 54 12/20/07 100 0.28 19 54 12/20/07 150 0.10

85




20 55 12/20/07 10 0.36 20 55 12/20/07 20 0.39 20 55 12/20/07 35 0.29 20 55 12/20/07 50 0.26 20 55 12/20/07 75 0.16 20 55 12/20/07 100 0.15 20 55 12/20/07 150 0.08 21 57 12/22/07 10 0.43 21 57 12/22/07 20 0.46 21 57 12/22/07 35 0.34 21 57 12/22/07 75 0.18 21 57 12/22/07 100 0.11 21 57 12/22/07 150 0.05 22 58 12/25/07 10 0.41 22 58 12/25/07 20 0.39 22 58 12/25/07 35 0.39 22 58 12/25/07 50 0.29 22 58 12/25/07 75 0 22 58 12/25/07 100 0.12 22 58 12/25/07 150 0.04 23 59 12/27/07 10 0.28 23 59 12/27/07 20 0.24 23 59 12/27/07 35 0.22 23 59 12/27/07 50 0.19 23 59 12/27/07 75 0.13 23 59 12/27/07 100 0.08 23 59 12/27/07 150 0.04 24 61 12/27/07 10 0.46 24 61 12/27/07 20 0.35 24 61 12/27/07 35 0.68 24 61 12/27/07 50 0.74 24 61 12/27/07 75 0.32 24 61 12/27/07 100 0.24 24 61 12/27/07 150 0.06 25 62 12/28/07 10 0.33 25 62 12/28/07 20 0.78 25 62 12/28/07 35 0.29 25 62 12/28/07 50 0.16 25 62 12/28/07 75 0.16 25 62 12/28/07 100 0.11 25 62 12/28/07 150 0.08

86




27 64 12/29/07 10 0.60 27 64 12/29/07 20 0.46 27 64 12/29/07 35 0.27 27 64 12/29/07 50 0.14 27 64 12/29/07 75 0.14 27 64 12/29/07 100 0.15 27 64 12/29/07 150 0.03 28 65 12/30/07 10 1.63 28 65 12/30/07 20 2.27 28 65 12/30/07 35 0.44 28 65 12/30/07 50 1.33 28 65 12/30/07 75 1.71 28 65 12/30/07 100 0.66 28 65 12/30/07 150 0.28 29 67 12/30/07 10 3.29 29 67 12/30/07 20 3.57 29 67 12/30/07 35 3.84 29 67 12/30/07 50 1.26 29 67 12/30/07 75 0.78 29 67 12/30/07 100 0.83 29 67 12/30/07 150 0.18 30 68 12/30/07 10 3.61 30 68 12/30/07 20 3.21 30 68 12/30/07 35 2.95 30 68 12/30/07 50 3.73 30 68 12/30/07 75 4.05 30 68 12/30/07 100 4.92 30 68 12/30/07 150 4.22 31 70 01/01/08 10 5.08 31 70 01/01/08 20 5.56 31 70 01/01/08 35 4.86 31 70 01/01/08 50 3.96 31 70 01/01/08 75 3.99 31 70 01/01/08 100 5.72 31 70 01/01/08 150 0.69 32 74 01/02/08 10 8.37 32 74 01/02/08 20 6.83 32 74 01/02/08 35 6.75 32 74 01/02/08 50 4.50 32 74 01/02/08 75 2.89 32 74 01/02/08 100 1.91 32 74 01/02/08 150 0.11

87

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VITA:

GLAUCIA MOREIRA FRAGOSO

Glaucia was born in Macaé, northeast of State of Rio de Janeiro on March 17, 1984,

and graduated from high school at Centro Federal de Educação in Campos dos

Goytacazes, Rio de Janeiro in 2001. She received her Bachelor’s of Biological Science

in 2002 with emphasis in Environmental Sciences from the Universidade Estadual do

Norte Fluminense, and participated in the US-Brazil Consortium at Washington and Lee

University in 2006, where she studied Environmental Sciences for one semester. She

entered the School of Marine Science, Virginia Institute of Marine Science at the

College of William and Mary in 2007 under the supervision of graduate advisor Dr.

Walker O. Smith, Jr. She successfully defended her Master Degree in November,

2009.

Documents

Hydrography and phytoplankton distribution in the Amundsen and