RELATIONSHIPS BETWEEN PHYTOPLANKTON · PDF filewere supported by the Bureau of Land Management under Contract Number AA 851-CT1-39. i. ABSTRACT

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RELATIONSHIPS BETWEEN PHYTOPLANKTONDISTRIBUTION AND PRODUCTION AND THE

PHYSICAL OCEANOGRAPHY IN THEGEORGES BANK REGION

by

Jerome J . Cura, Jr .and

John H . Ryther, Jr .

EG&G Environmental Consultants300 Bear Hill Road

Waltham, Massachusetts 02254

Acknowledgements

The authors would like to acknowledge Dr . Charles Flagg forallowing us generous use of the hydrographic data presented hereand also for his comments during preparation of the manuscript . Wealso thank Charles Yentsch, Newell Garfield, Drs . John Ryther, Sr .,Bruce Magnell, Charles Menzie, George Mariani, and Hugh Mulliganfor reviewing various stages of the manuscript . Dr . Mariani allowed

us use of the nutrient data . Mr . John Clay provided technical assis-tance during the first two cruises . We also acknowledge the assis-tance of the captain and crew of the Sub - Sig II . These studieswere supported by the Bureau of Land Management under Contract Number

AA 851-CT1-39 .

i

ABSTRACT

This work considers the relationship of the physical oceanography of

the Georges Bank region to phytoplankton production and species distribution

and succession . The seasonal changes in the flora are examined in light of

the circulation, frontal systems, stratification, and intrusions upon the

Bank . Seasonal hydrographic changes are reflected in the distribution and

production of the phytoplankton . Of particular interest is the role that

fronts appear to play in determining species distribution and production .

The winter-spring intrusion of Scotian Shelf Water to the Bank explains the

present and historically observed developmental pattern of the spring bloom in

the Northeast Channel and along the south flank . The spring and summer fronts

along the north flank and north of the Great South Channel result in the

development of distinct phytoplankton communities . In all, there are clear

correlations between changes in the physical oceanography of the area and the

phytoplankton .

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Introduction

Most studies concerning Georges Bank phytoplankton have either considered

the Bank as part of a larger whole (Bigelow, 1926 ; Bigelow et al ., 1940 ; Gran,

1933 ; Braarud, 1934 ; Lillick, 1940) or as peripheral to the main object ofstudy (Gran and Braarud, 1935) . Sears (1941) has conducted one of the fewstudies specifically aimed at answering questions about the phytoplankton

species of Georges Bank (i .e ., what are the timing, species composition, and

magnitude of the spring bloom?) . However, enough observations have been madeto reveal the general pattern of seasonal dominants that, with allowance fornatural variation and sampling error, appears each year (Table 1) . Severalbasic conclusions concerning the phytoplankton species of Georges Bank emerge

from these studies :

1) The dominant phytoplankton species on Georges Bank exhibit seasonal-

ity (Table 1) .

2) In contrast to surrounding waters, diatoms dominate the Bank through-out the summer .

3) During the spring bloom, diatoms are as abundant or more abundant inthe areas surrounding the Bank to the southeast as they are in the

central Bank .

4) The pattern that the spring bloom develops is apparently related tothe drift of Scotian Shelf Water .

Except for Sears' 1941 study, the sampling intensity on Georges Bank andin surrounding waters has occurred on very coarse spatial scales and generallyin the absence of synoptic physical oceanographic measurements . The data thus

generated have not been appropriate for demonstrating relationships betweenthe distribution of physical and biological variables . In light of the factthat the Bank is hydrographically and bathymetrically distinct from the

seasonally stratified Gulf of Maine to the north and the considerably deeperand warmer sl ope waters to the south, one mi ght expect that di fferences in

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Table 1 . Seasonal pattern of phytoplankton species dominance on GeorgesBank, described by published sources (Braarud, 1934 ; Bigelow,1926 ; Falkowski and Von Bock, 1979 ; Gran, 1933 ; Gran andBraarud, 1935 ; Lillick, 1940 ; Marshall and Cohn, 1981a, 1981b ;Sears, 1941) .

MonthIM ~ A I M ~ J ~ J L A ~ S ~ 0 ~ N ~ D ~ J ~ F ~

Navicula sp. -

Thalassiosira ___nordenskio dii

Chaetoceros(sociale,deci iens,debile

Thalassiosirarag vida -

Leptocylindrusminimus

Guinardiaflaccida

Rhizosoleniaalata,

sty iTormi s ,stolterfothi ,hebatata,imi ta )

Leptocylindrusdanicum

Prorocentrummicans

Thalassionemanitzsc ioides

Coscinodiscussp .

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physical characteristics would be reflected in the biology . Cleve (1897)

introduced the term "plankton-type" as a group of jointly distributed speciesthat could be identified with a particular water mass and thus serve as a

hydrographic indicator . The nonconservative nature of phytoplankton and its

considerable temporal and spatial variability does not allow it to be usedhydrographically in the same sense as temperature and salinity . However,

spatial correlations between geographic areas defined floristically and thosedefined hydrographically may provide a means of placing initial physical

boundaries on phytoplankton communities . Once this correlation is made over aseries of vertical and horizontal sections, the boundaries of the biologicalcommunity can be established on the basis of the usually more tightly deter-mined and more easily and rapidly obtained physical data base .

An analysis of this correlation provides a rationale for choosing commun-

ity boundaries without resorting to arbitrary decisions . The biological

boundary can then be as dynamic as the hydrography and the problems posed

by fixed boundaries, such as bathymetry, are avoided . A hydrographically

defined community boundary has obvious advantages when the biologist considers

organisms such as phytoplankton whose movement and physiology are dictated by

their existence in a three-dimensional, fluid environment . Such an approach

also has implications for modelers, biogeographers, and environmental planners

concerned with exploitation of offshore resources .

The present work first considers the seasonal succession and distributionof phytoplankton species assemblages on and around Georges Bank . The distri-

bution patterns are then compared to temperature-salinity correlation groupsderived from synoptically taken hydrographic data . Floristic bound aries

defined by discontinuities in species distribution or by changes in taxonomiccomposition are considered in terms of the position of hydrographic fronts andcirculation patterns on Georges Bank .

Materials and Methods

Samples for biological purposes and hydrographic data were obtained onfour cruises to Georges Bank on the R/V SUBSIG II during December 1978, March

1979, May/June 1979, and August 1979 . The euphotic zone was sampled atseveral depths for chlorophyll-a, production, and phytoplankton cell counts .

Chlorophyll-a concentration was measured spectrophotometrically (Lorenzen,

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1967) and production rate was measured as uptake of C14-bicarbonate (Strick-land and Parsons, 1972) during daylight hours . During one cruise (May/June) a

continuQus measurement of surface chlorophyll-a was made using a continuous-

flow fluorometer . Productivity measurements were carried out under artificial

light in an on-deck incubator equipped with neutral density screening tosimulate 100%, 80%, 50%, 10%, and 1% of full incubation light intensity .

Ambient water temperature was maintained with a temperature controlled recir-culating water bath . Phytoplankton samples were preserved in buffered forma-

lin, settled in 100 ml Uttermohl chambers, and counted and identified using an

inverted phase-contrast microscope (Lund et al ., 1958) . The distributions of

phytoplankton species considered here were derived from samples taken near-surf ace (usually between 1 and 5 meters) . A smaller number of phytoplankton

samples from the 1% light level (usually 30 meters) were identified and

enumerated as well .

Temperature and salinity were measured throughout the water column at

each station with a Neil-Brown conductivity-temperature-depth (CTD) meter .

Details of the hydrography during each cruise are presented elsewhere (EG&G,

1979a,b,c, and 1980) . Nutrients (nitrate, phosphate, and silicate) were

measured according to standard methods (Strickland and Parsons, 1972) .

Distribution maps of phytoplankton assemblages were made by groupingstations with similar major species . A species was considered major if its

numerical abundance was >5% of a sample . Major species were then used to

define an assemblage . Assemblages were recognized for groups of stations

which had the same major species regardless of total cell abundance or of thecomposition of the minor (<5%) components of the flora at a station . There

were occasional outliers within an assemblage in that a station might sharethe same major species as the others in a group but have an additional domin-

ant as well . Unless this additional dominant occurred at more than onestation, its presence was ascribed to natural variability and the station was

not considered to represent a separate assemblage . It should be noted that a

small but numerous species occurring at only one station could skew the

averages shown in the summary table (e .g ., Table 9) . The occurrence of such

species at only one station was not considered sufficient to make them major

species in our definition of an assemblage . Boundaries were drawn around

assemblages which separated them f rom adjacent but distinct assemblages .

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The areal extent of each assemblage was compared to the areal extent of

temperature-salinity ( T/S) correlation groups, the position of fronts, theoccurrence of intrusions, and vertical structure defined from synoptically

taken hydrographic data . All the temperature-salinity plots which were used

to distinguish T/S correlation groups are presented elsewhere (EG&G, 1979a, b,

c, and 1980) . Temperature-salinity groups describe waters of similar sourceand mixing history on the basis of the T/S diagrams of each station . The

areal extents of the groups are a function of the T/S properties of the entirewater column and do not necessarily define water masses, nor do group bound-

aries necessarily reflect surf ace hydrography . However, such groupings do

represent similar physical histories for the water in a group . We postulatedthat these physic al f actors would also be reflected in the biology of the

area . To determine whether independently derived biological parameters co-

varied with an area's physical properties ( T/S space), we compared graphically

the phytoplankton assemblages with the distribution of T/S groups . Unlike a

T/S profile, however, a phytoplankton assemblage is a surf ace water property ;

therefore, where inconsistencies between the biological and physical boundar-ies occurred, we also examined physical properties that may not be reflectedin T/S groups such as surf ace fronts and circulation patterns .

This report defines areas on and around Georges Bank in terms of phyto-plankton species composition, productivity, and biomass ; compares the biologi-

cal assemblages to T/S groups, f ronts, and intrusions ; and describes physical

mechanisms which may account for any observed patterns . Because the number of

production and chlorophyll-a estimates made during any given season was smallrelative to the number of phytoplankton samples taken, floristic characteris-

tics were used to delimit a given community . The production estimate made

within the community boundaries was then assumed to be representative of the

whole community . The attempt was made to correlate particular hydrographicconditions with particular communities to determine the physical factors

controlling production and species distributions .

Results

Weather conditions encountered during the cruises determined the numberof phytoplankton samples taken each season . Coverage was sparse during the

December 1978 cruise (14 stations) ; moderate in March 1979 (21 stations) ; and

extensive in June 1979 (65 stations) and August 1979 (70 stations) . The broad

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spatial coverage attained during spring and summer coincided with the appear-ance of clear hydrographic boundaries ; for these seasons, the fine structure

of floristic distributions could be related to the location of these bound-

aries . Results obtained during each cruise are presented separately .

December 1978

General Hydrography . Figure 1 presents the areal extent of the T/S

correlation groups for December 1978 . Group 1, Georges Bank Water (GBW),*represents those stations where cooling, and tidal and wind mixing had pro-duced vertically uniform conditions . These hydrographic stations are confined

to the shallow areas of the central Bank, Great South Channel (GSC), and

Nantucket Shoals . Distribution of these stations within this group indicates

that the freshest water lay within the 65-meter isobath . Warmer and somewhat

saltier water was found in the GSC and south of Nantucket Shoals ; saltier but

colder water occurred at the boundary with the Gulf of Maine water groups,along the southern flank, and on Northeast Peak . Groups 2 and 3 showed acharacteristic decrease in temperature and increase in salinity with depthassociated with the progression from Maine Surface Water (MSW) to Maine

Intermediate Water (MIW) . Station 78, the easternmost station in thesegroups, showed influence of the Group 4 stations at depth . The salinity range

of Group 4 was similar to that of Group 3 ; however, these stations did not

show the characteristic temperature minimum . The origin of the warmer water

of intermediate depth was the warmer slope water flowing into the Northeast

Channel . The stations of Group 5 are confined to the Northeast Channel . This

group contained stations with the f reshest water found on the cruise in thenear-surf ace layer, indicating that fresher water from the Scotian Shelf (SSW)was flowing into the region from south of Browns Bank and heading toward the

Gulf of Maine . The hydrographic variation of this group is typical of thecomplex structure generally found in the Northeast Channel . Temperature-

salinity Group 6 occurred along the south flank of Georges Bank and belongs to

the intermediate water class between the shallow GBW and the slope water

farther offshore .

*Water mass names are based on those given by Hopkins and Garfield (1979) .It should be noted that not all T/S groups are one water mass, but some suchas groups 2 and 3 may include two or more water masses in the vertical .

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The surface density structure (Figure 2) displayed a frontal systemacross the south flank and an intrusion of low density SSW into the Northeast

Channel ._ The vertical hydrographic profiles across the north flank (Figures 3and 4) revealed the similarity between the MSW above 60 meters and the GBW at

this time . In contrast to MSW generally, Station 73 (Figure 4), a T/S Group 4station, had higher near-surface (25 meters) salinity and did not have atemperature minimum at depth .

Biological Patterns . The 9 phytoplankton stations (Figures 5 and 6)north of the density front on the south flank and outside the Northeast

Channel had similar major species : Asterionella bleakeleyi , Thalassionema

nitzschioides , and Prorocentrum micans . We considered the phytoplankton in

GBW and MSW to be one assemblage . They were similar with respect to major

species, species composition, and species number (Tables 2 and 3) . However,

total cell abundance, production rate, and chlorophyll-a concentration (rangeand average) differed among stations of different water depths . The 4 shallow-

water stations (60, 67, 140, 143 (see Figure 1)) had a higher average totalcell abundance (Table 2) than the 5 stations located in the deeper MSW (Table

3) . The GBW stations also had higher production and chlorophyll-a levels

(compare Tables 2 and 3) .

One MSW station in the GSC, 25, resembled the shallower stations interms of species composition and in magnitud'e of chlorophyll-a concentration

(33 .7 mg/m2) and production rate (38 .3 mgC/m2/hr) . We examined the hydro-graphic (Figures 7 and 8) and nutrient (Figures 9 and 10) profiles around thisstation for a physical basis for this similarity (see discussion) .

At Station 147, which was south of a distinct density front separating

T/S Groups 6 and 1, and Station 151, which was within the front (Figure 2),seven species were present : 5 diatoms (4 of which occurred only at Station

147), 1 dinoflagellate, and 1 coccolithophore (Table 4) . Average total cell

abundance at the surface (1760 cells/liter) was low . The major species were

Asterionella bleakeleyi and Peridinium cerasus . Primary production was not

measured at these stations ; however, Station 151 had a greater chlorophyll-a

concentration (33 .4 versus 10 .7 mg/m2) and ten times the cell abundance

(6400 versus 640 cells/liter) of Station 147 .

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Table 2 . The ten most abundant phytoplankton species collected at the sta-tions in the shallow (<60 m) region of Georges Bank during the Decem-ber 1978 cruise . Average cell abundance was calculated from the datafor the four stations . The average production rate and chlorophyll-aconcentration are also given .

SpeciesAverage Cell Abundance

(Cells/Liter)

*Asterionella bleakeleyi W . Smith 8,255

*Prorocentrum micans Ehrenberg 1,860*Thalassionema nitzschioides Hustedt 1,065

**Coscinodiscus eccentricus Ehrenberg 860Phaeocystis pouchetii (Hariot) Lagerheim 610Paralia sulcata (Ehrenberg) Cleve 440Rhizosolenia stolterfothii Peragallo 400Thalassiosira rg avida Cleve 300T . decipiens (Grunow) Jorgensen 275Corethron criophilum Castracane 215

Average Total Cell Abundance (23 species) 15,130

Average Production Rate (mgC/m2/hr) = ' 59 (Range = 37 .7-79 .9)(n -= 2 stations)

Average Chlorophyll-a (mg/m2) = 32 (Range = 19 .5-54 .0)(n = 4 stations)

*Major species .**C . eccentricus occurred only on the north flank in GBW .

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Table 3 . The ten most abundant phytoplankton species collected at the sta-tions in Maine Surf ace Waters during the December 1978 cruise . Averagecell- abundance was calculated from the data for the five stations . Theaverage production rate and chlorophyll-a concentration are also given .


(Cells/Liter)


* Prorocentrum micans Ehrenberg 832

*Thalassionema nitzschioides Hustedt 380

**Phaeocystis pouchetii (Hariot) Lagerheim 376

Coscinodiscus eccentricus Ehrenberg 204Corethron criophilum Castracane 88

Coscinodiscus curvatulus Grunow 76Flagellate 64

Coscinodiscus centralis Ehrenberg 56

Cyclotella sp . Kutzing 40


Average Production Rate (mgC/m2/hr) T 23 .6 (Range = 10 .5-38 .3)(n = 3 stations)

Average Chlorophyll-a (mg/m2) = 23 .8 (Range = 14 .5-33 .7)(n = 5 stations)

*Major species .**P . pouchetii occurred at two stations (69 and 71) .

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Table 4 . The seven phytoplankton species collected at the stations on theGeorges Bank south flank during the December 1978 cruise . Average cellabundance was calculated from the data for the two stations . The averagechlorophyll-a concentratiaon is also given ; average production rate wasnot measured .


(Cells/Liter)

*Asterionella bleakeleyi W. Smith 1,620

*Peridinium cerasus Paulsent 40

Corethron criophilum Castracane 20

Co scinodiscus sp. Ehrenberg 20

Pleurosigma sp . W. Smith 20

Thalassiosira decipiens (Grunow) Jorgensen 20Coccolithophore 20

Average Total Cell Abundance ( 7 species) 1,760

Average Chlorophyll-a ( mg/m2) = 22 .1 (Range = 10 .7-33 .4)(n = 2 stations)

*Major species .tP . cerasus was a major species at one of the two stations on thesouth f ank . The sparse data do not justify referring to these as anassemblage .

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Table 5 . The ten most abundant phytoplankton species collected at the sta-tions in the Northeast Channel during the December 1978 cruise . Averagecell- abundance was calculated from the data for the two stations . Averagechlorophyll-a concentration is also given ; average production rate was notmeasured .

Average Cell AbundanceSpecies (Cells/Liter)


*Phaeocystis pouchetii (Hariot) Lagerheim 510

*Coccolithophore 450

*Prorocentrum micans Ehrenberg 390

D istephanus speculum 160

Corethron criophilum Castracane 100

Thalassiosira decipiens (Grunow) Jorgensen 100

T . ra~ vida Cleve 80

Flagellate 80

Peridinium cerasus Paulsen 70


Average Chlorophyll-a (mg/m2) = 20 .4 (Range = 18 .9-21 .8)(n = 2 stations)

*Major species .

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The Northeast Channel had a third phytoplankton community, characterizedby a relatively evenly distributed species abundance (Table 5) . Two phyto-

plankton stations (112 and 115) were sampled . Sixteen species were observed :

7 diatoms, 3 dinoflagellates, 1 silicoflagellate, 1 haptophycean, 1 flagel-

late, 1 desmid, 1 coccolithophore, and 1 chlorophyte . Average cell abundanceof surface samples was low (6420 cells/liter) . Exclusive of the numerically

dominant Asterionella bleakeleyi , abundance was evenly distributed among the

other major species, Phaeocystis pouchetii , Pr orocentrum micans , and a

coccolithophore .

Mn'~L, 1070

General Hydrography . During March 1979, the region was characterized

by a strong, convoluted density front across the south flank slope (Figure11) ; by an obvious intrusion of cold (Figure 12), low salinity (Figure 13)Scotian Shelf Water (SSW) across the Northeast Channel and onto the westernend of Georges Bank ; and by a weak, salt-controlled density front across thenorth flank . Figure 14 presents the distribution of the T/S correlation

groups at this time . Group 1 represents those stations where cooling, andwind and tidal mixing had produced nearly uniform conditions vertically . Thegroup is confined to the central Bank in waters generally less than 100 meters

deep, to the sill region of the GSC, and, most probably, to the NantucketShoals . The salinity range of the group was between 32 .8 and 33 .4 0/oo,

with temperatures between 3 .5 and 5 .5°C . Group 2 surrounded Group 1 along thenorth flank and along the eastern half of the south flank . Group 2 may have

been continuous around the Northeast Peak, but was separated by the inflow ofSSW (temperature-salinity Group 5 [Figures 11 and 12]) . Groups 3 and 4

represent the transition to slope water .

Biological Patterns . Figure 15 shows the phytoplankton assemblages

derived from surf ace water samples . T/S Group 1 contained two phytoplankton

assemblages . The first assemblage was defined•by 7 stations, generally in the

GSC-western Georges Bank area . Sixty-three species were present : 51 diatoms,

10 dinoflagellates, 1 silicoflagellate, and 1 haptophycean (Table 6) . Phaeo-

cystis pouchetii was the overwhelming dominant . Its occurrence was restrictedto Group 1 stations and only on the western side . Stations 23, 26, 29, and 58

formed a well defined assemblage with very similar flora . Stations 20, 204,

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Table 6. The ten most abundant phytoplankton species collected at the sta-tions in the GSC-western Georges Bank during the March 1979 cruise .Average cell abundance was calculated from the data for the seven sta-tions . Note the dominance of Phaeocystis . 'The average production rateand chlorophyll-a concentration are also given .


*Phaeocystis pouchetii (Hariot) Lagerheim

Thalassiosira nordenskioldii CleveNitzschia delicatissima CleveRhizosolenia fragilissima Bergon

Chaetoceros sociale Lauder

Thalassionema nitzschioides Hustedt

Chaetoceros debile CleveNitzschia seriata CleveChaetoceros compressum Lauder

_ C . didymum Ehrenberg

Average Total Cell Abundance (63 species)

433,7206,8203,9403,5113,1832,6492,3032,0311,7801,297

471,162

Average Production Rate (mgC/m2/hr) = 388 (Range = 129 .4-803.5)(n = 7 stations)

Average Chlorophyll-a ( mg/m2) = 167 (Range = 65 .1-276 .2)(n = 7 stations)

*Ma j o r s pe ci es .

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and 207 were also similar except for the lower abundance or absence of

P . pouchetii . Production and chlorophyll-a levels within this Phaeocystis

assemblage were the highest found during the study and represent the springbloom .

Station 97, located on the central-eastern end of Georges Bank, repre-sented a second assemblage (Figure 15) with lower species richness, particu-larly among diatoms (12 species) relative to the eastern-end stations (X =23 species/station), lower cell abundance (1060 cells/liter), and a dominanceby Coscinodi scu s central is .

Outside the well-mixed area (Group 1), surface water phytoplanktondistribution appeared to be related to the front's position along the south

fl ank and to the intrusion of SSW south across the Northeast Channel . Sta-

tions 147 and 184 (Figure 14) were located across the north flank in MSW .

This area had a Thalassiosira flora with T . decipiens , T . gravida, and T .

nordenskioldii as major species (Table 7) . Station 144, in the Northeast

Channel, occurred just north of Station 147 . However, Station 144 was locatedin a frontal system (Figure 16) caused by the intrusion of low salinity SSW ;

Station 147 was in the more deeply mixed surface water characteristic of the

Gulf of Maine . The abrupt hydrographic change between Stations 147 and 144

was accompanied by floristic changes . Station 144 had greater cell abundance,higher species richness, higher production levels and chlorophyll-a concentra-tions, and a change in species dominance within the genus Thalassiosira

(Table 7) .

The distribution of phytoplankton along the south flank and slope wasclearly correlated with the position of the shelf/slope front and the intru-

sion of SSW . Those stations seaward of the front (Stations 69, 106, 109, 112,214) were characterized by two major species, A . bleakeleyi and Nitzschia

seriata , and by an impoverished flora within the genus Thalassiosira (Table

8) . This flora contrasted distinctly with that found in the Gulf of Maine .

South flank Stations 63 and 66 were not influenced by SSW and resembledthe above assemblage, except A . bleakeleyi was not collected . South flank

Station 100, shoreward of the front and under the influence of the SSW (Figure17), strongly resembled Station 144 in its floristic composition .

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Table 7 . The phytoplankton species collected at the stations in the Gulf ofMaine (Stations 147 and 184) and in the Northeast Channel (Station 144)dur-ing the March 1979 cruise . Cell abundances were calculated separatelyfor each station . The production rate and chlorophyll-a concentrationmeasured at each station are also given .

pecies

Cell Abundance(Cells/Liter)

Station Station184 147

Station144

Amphora sp . Ehrenberg 120Asterionella bleakeleyi W . Smith 20 400Bacteriosera fragilis Gran 20 120Chaetoceros atlanticum Cleve 800C . compressum Lauder 2,920C . concavicorne Mangin 60 260 280C . laciniosum Schutt 60 880C . simplex Ostenfeld 20Coscinodiscus centralis Ehrenberg 80 60 40C . lineatus Ehrenberg 40 60C . radiatus Ehrenberg 40C . wailessii Gran and Angst 20 20Cylindrotheca closterium Rabenhorst 100Eucampia zodiacus Ehrenberg 200Fragilaria sp . Lyngbye 340 940Guinardia flaccida (Castracane)Peragallo 40 80Melosira islandica 3,580Navicula sp . Bory 20 20Nitzschia sp . Hassall 280N . longissima (Brebisson) Ralfs 20N . seriata Cleve 20 820Paralia sulcata (Ehrenberg) Cleve 20Pleurosigma sp . W . Smith 40Porosira glacialis (Grunow) Jorgensen 40 580 2,980Rhizosolenia fragelissima Bergon 40 140R . hebetata (Hensen) Gran 240 100 80S ny edra sp . Ehrenberg 20Thalassionema nitzschioides Hustedt 580

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Table_7 . The phytoplankton species collected at the stations in the Gulfof Maine (Stations 147 and 184) and in the Northeast Channel (Station 144)during the March 1979 cruise . Cell abundances were calculated separatelyfor each station . The production rate and chlorophyll-a concentrationmeasured at each station are also given (Cont) .

Species

Cell Abundance(Cells/Liter)

Station Station Station184 147 144

Thalassiosira aestivalis Granand Angst

T, decipiens (Grunow) JorgensenT . gravida CleveT . nordenskioldii CleveT . rotula MeunierTropodoneis sp . CleveCeratium ranipesC . tripos (O .F . Muller) (Nitzsch)Goryaulax sp . DiesingPeridinium sp . EhrenbergP . cerasus PaulsenProrocentrum sp . EhrenbergP . micans EhrenbergDistephanus speculumUlothrix sp .

Total Cell Abundance

Production Rate (mgC/m2/hr)

Chlorophyll-a ( mg/m2)

2,8201,700 11,720 2,9803,420 3,380

20 3,040 11,400200 3,040

660 40060 20

60 40 4020

8040

120100

404,100

7,424 19,727 40,464(24 (15 (32species) species) species)

74 .7 198 .6 395 .0

48 .2 71 .6 75 .6

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Table 8 . The ten most abundant phytoplankton species collected at the sta-tions in the south flank-slope region during the March 1979 cruise .Average cell abundance was calculated from the data for the five stations .The average production rate and chlorophyll-a concentration are alsogiven .


(Cells/Liter)

* Nitzschia seriata Cleve 6,500*Asterionella bleakeleyi W. Smith 4,740

**Nitzschia sp. Hassall 820

N . delicatissima Cleve 416Chaetoceros debile Cleve 400Cylindrotheca closterium (Ehrenberg) Reimannand Lewin 340

Chaetoceros laciniosum Schutt 228

Amphora sp. Ehrenberg 224

Thalassiosira nordenskioldii Cleve 188

Chaetoceros affine Lauder 140


Average Production Rate ( mgC/m2/hr) = 81 (Range = 32 .7-148 .3)(n = 4 stations)


*Major species .**Occurred in unusual abundance only at station 103 .

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Although its cell abundance was lower by a factor of 3 than Station 144,

Station 100 had a similar high species richness and major species composition,and was equally rich within the genera Thalassiosira and Chaetoceros . Theresemblance extended even to the sharing of rare species (A . bleakeleyi ,Chaetoceros atlanticum , and Ulothrix sp .) . These similarities occurred even

though Stations 100 and 144 were geographically separated and in waters ofdifferent temperature and salinity . The floristic similarity paralleled thesimilar hydrographic structure imposed by the SSW influence . This floristicsimilarity did not appear to be a function of total abundance . Stations 100

and 144 had different abundances .

The limited samples from the 1% light level (Figure 18) resembled thesurface distributions over the slope where N . seriata was dominant and in the

SSW intrusion in the Northeast Channel where Th alassiosira sp . dominated .

However, in the GSC, P . pouchetii was absent f rom the single sample taken .there and, unlike the surface water, the central Bank was dominated byRhizosolenia spp . at depth .

June 1979

General Hydrography . During early June (29 May-7 June 1979), the hydrog-raphy in the surface water of the region was characterized by a well mixed

area surrounded by strong surface density fronts (Figure 19) . The north flankwas separated from Maine Surface Water (MSW) by a density front extending fromnorth of the GSC along the north flank to the Northeast Peak . Along the southflank, a front extended south of the GSC and east along the 60-meter contour .The eastern end of the south flank had a density front between the 100- and200-meter contours . Also, the surface water north of the GSC exhibited anintrusion of relatively fresh surface water extending to 20 meters .

Figure 20 presents the distribution of T/S groups . Group 1 occurredin that part of the Bank where wind and tidal• mixing had maintained a verti-cally well-mixed water mass overcoming the stratifying effects of the in-creased solar insolation . This group was located over the central portion ofthe Bank, over the sill of the Great South Channel, and probably extended overthe Nantucket Shoals . The salinity range of the group was f rom 32 .5 0/0o to

-19-

33 .0 0/oo, with a temperature range of 6 to 9°C . Group 2 occupied the region

along the north flank and extended over part of the Northeast Peak as well .The surface waters of this group were fresher than found on the Bank, whilethe mid-depth indicated the presence of Maine Intermediate Water (MIW) with anintermediate salinity (about 33 .0 °/oo) and cold temperatures (about 5 .0°C) .Group 8 almost fit into this group but was distinguished by minimum tempera-tures slightly higher than in Group 2, as well as somewhat higher surface

salinities . Group 3 occupied the mid-shelf region from the Northeast Peakwestward past the Nantucket Shoals . The surface waters of Group 3 had salini-ties similar to those of Group 1, although the temperatures were up to 5°Chigher . This suggests that water from the center of the Bank migrated south-ward, overflowing a colder and slightly saltier water type and warming as itwent . Groups 4 and 6 represent intermediate states in the mixing of the upperslope water with Bank waters . Group 7 is slope water that has been affectedby the passage of three weak warm-core eddies just prior to the cruise .Groups 5 and 9 typified clearly separate water groups from those associatedwith the Bank . Group 5 occurred in Scotian Shelf Water (SSW) . Group 9 was byfar the largest observed intrusion of fresh run-off into the area . Surfacesalinities were as low as 30 .4 0/oo . The intrusion was confined to the

upper 20 meters, while below 20 meters the water progressed to the cold MIW .

Biological Patterns . Twelve surface phytoplankton stations (16,19,24,78,84,105,111,114,117,188,191,261) within the well-mixed GBW formed a distinctassemblage (Figure 21) . The flora within this well-mixed water was distinctfrom the flora separated from it by density fronts . Cell abundance andspecies richness were higher than in surrounding water groups . The floraconsisted of 70 species : 47 diatoms, 21 dinoflagellates, 1 chlorophyte, and 1

silicoflagellate (Table 9) . Guinardia flaccida and Cerataulina pelagica

were the major species . Guinardia occurred as a dominant at all but onestation . All the stations within this assemblage generally shared a commonflora . However, several species, Rhizosolenia fragilissima , Dinophysis , afilamentous chlorophyte, and Skeletonema costatum , occurred only in the GSC .The only other area where these species occurred was in the intrusion of freshsurface water north of the GSC . Within this surface intrusion, Skeletonemacostatum was the major species and occurred in abundance ; Guinardia flaccida

and C . pelagica occurred only at one station each (Table 10) .

-20-

Table 9, The ten most abundant phytoplankton species collected at the sta-tions in the well-mixed region of Georges Bank during the May-June 1979cruise . Average cell abundance was calculated from the data for theeleven stations . This area is characterized by a Guinardia flaccida -Cerataulina pelagica assemblage . The average production rate and cho o-phyll-a concentration are also given .


(Cells/Liter) ,

*Guinardia flaccida (Castracane) Peragallo 4,407

**Chaetoceros compressum Lauder 3,582**Thalassiosira eccentrica (Ehrenberg) Cleve 3,396**Chaetoceros sociale Lauder 1,827*Cerataulina pelagica (Cleve) Hendey 1,373Thalassiosira rag vida Cleve 1,222Phaeocystis pouchetii (Hariot) Lagerheim 1,202Skeletonema costatum (Greville) Cleve 844Cylind rotheca closterium (Ehrenberg)Reimann Lewin 765

Rhizosolenia hebetata (Hensen) Gran 465


Average Production Rate (mgC/m2/hr) = 57 (Range = 30 .4-73 .3)(n = 6 stations)

Average Chlorophyll-a ( mg/m2) = 48 (Range 32 .7-69 .8)(n = 6 stations)

*Major species .

**C . compressum and T . eccentrica occurred in unusual abundance only inthe GSC . C . sociale occurred in unusual abundance only at onestation on the north flank .

-21-

Table 10 . The ten most abundant phytopl ankton speci es col 1 ected at thestations in the surface intrusion north of the GSC during the May-June1979 cruise . Average cell abundance was calculated from the data for thefour stations . Production rate and euphotic zone chlorophyll-a concentra-tion were not measured .


*Skeletonema costatum (Greville) Cleve 14,475

Cerataulina pelagica (Cleve) Hendey 1,160

Chaetoceros compressum Lauder 1,115C . laciniosum Schutt 600C . debile Cleve 410

Leptocylindrus minimis Gran 320

Ceratium tripos (O .F . Muller) Nitzsch 285

Chaetoceros breve Schutt 175

Ceratium longipes (Bailey) Gran 165Nitzschia seriata Cleve 155


*Major speci es .

-22-

Within the well-mixed GBW were centers of abundance and changes in thepattern of abundance as the different bounding density fronts were approached .Areas of similarly high cell abundance occurred in the GSC (Stations 16 and

19), along the north flank (Stations 78 and 105), and in the central Bank(Stations 84 and 111) . Cell abundance remained high on the north flank evenas the front was approached (Station 78, Figu re 22, x = 68,550 cells/liter ;Station 105, Figure 23, x = 84,000 cells/liter) . In contrast, as the front in

the direction of the Northeast Channel was approached (Figure 23), cellabundance progressively decreased (Station 114, x = 38,100 cells/liter ;

Station 117, x = 5350 cells/liter) . Similar progressive decreases were notedas the south flank front was approached (Figure 23, Station 191, x = 6300

cells/liter ; Station 194, x = 1350 ; Figure 20, Station 188, x = 1050) . Station261, also along the south flank, had relatively low cell abundance (x =

16,400) . These changes in abundance were not accompanied by changes in thegeneral floristic,nature of the samples (i .e ., all were dominated by diatomsand had Guinardia flaccida and Cerataulina pelagica as major species) . Gener-ally, cell abundance was highest in the GSC, north flank, and central Bank .

In contrast, abundance decreased along the perimeter of the Northeast Peak andthe south flank relative to the above areas . Production rates and chloro-

phyll-a concentrations (Table 9 and Figure 24) were relatively high within theGBW where G . flaccida and C . pelagica characterized the assemblage . Within

the community, production was highest in the GSC (20 .3-66 .9 mgC/m2/hr) and

along the north flank (30-73 mgC/m2/hr) . Chlorophyll-a was similarly highin these areas (Figure 24) and, like production, decreased abruptly across thenorth flank front .

Among the stations in the GSC (16,19,22,24), the phytoplankton of Station22 differed from the others . Cell abundance, chlorophyll-a, and primaryproduction were all lower at Station 22 . Skeletonema costatum was the major

species, and neither Guinardia flaccida nor Cerataulina pelagica occurred in

the sample . The station was located in a small area of relatively cold (6°C)surface water (Figure 25) . The low biomass and production observed within

this cold spot are suggestive of recently upwelled water . Vertical tempera-ture and density profiles (Figure 26) indicate that the source of this cold

-2 3-

water was MIW . Relative to Georges Bank and MSW, Station 22 had high nitrateand silicate concentrations (Figures 27 and 28) . The observed concentrationgradients of"these nutrients at depth in the Gulf of Maine (Figure 29) are

consistent with the idea that the source water was MIW .

The G . flaccida - C . pelagica assemblage was restricted to GBW and

coincided closely with the boundaries of the T/S Group 1 and the densityfronts (Figure 21) . Across the north flank front, the assemblage did not

occur either in the surface water or at depth (Figure 30) . Across thesouth flank, no representative species of the assemblage were present in

surface water . However, G . flaccida and, to a lesser extent, C . pelagica werepresent at depth (30 meters) over the south flank, although not over the

slope (Figure 31) .

The MSW phytoplankton stations (62,68,72,92,95) had a flora distinct from

that of the well-mixed area . It was less abundant and, perhaps as a conse-quence, displayed a low species richness . Dinoflagellates contributed agreater proportion of the flora than on-Bank (Table 11) . Peridinium speciesgenerally, and Peridinium trochoideum in particular, were the major species atmost surface stations . Ceratium longipes dominated the one station sampled atdepth (Figure 30) . A change in floristic composition occurred abruptly as thefront was crossed, reflected by the fact that stations within the front(56,88) resembled on-Bank stations in terms of species composition (althoughnot total abundance) while stations just outside the front (72,92) clearly

held MSW phytoplankton .

On the south side of Georges Bank, as the south flank was approached,there was a change from a diatom to a dinoflagellate flora . Ceratium long -

ipes was the major species in this mid-shelf area (Figure 21) . In contrast to

on-Bank communities, Guinardia flaccida was generally absent and cell abun-dance was 2 orde rs of magnitude lower.

Over the slope, where upper slope water mixes with Bank water, the florawas transitional (Figure 21) . This transition was reflected in the mixednature of the flora . Coccolithophores, Peridin ium sp ., and Prorocentrummicans were common, but no particular species dominated in the area .

-24-

Table 11 . The eleven phytoplankton species collected at the stations in MSWduring the June 1979 cruise . Average cell abundance was calculated fromthe data for the five surface stations . The average production rate andchlorophyll-a concentration are also given .


**Nitzschia seriata Cleve 76

*Peridinium sp. Ehrenberg 72

**Ceratium longipes (Bailey) Gran 40

*Peridinium trochoideum (Stein) Lemmermann 24

Paralia sulcata (Ehrenberg) Cleve 20

Chaetoceros concavicorne Mangin 16

Ceratium tripos (O .F. Muller) Nitzsch 16

Chaetoceros decipiens Cleve 8

Prorocentrum sp. Ehrenberg 8

Ceratium lineatum (Ehrenberg) Cleve 4

Peridinium pellucidum (Bergh) Schutt 4

Average Total Cell Abundance (11 species) 288



*Major species .

**N . seriata and C . longipes occurred in unusual abundance at only oneMSW station .

-25-

August 1979

General Hydrography . The surface density structure (Figure 32) wascharacterized by a front along the north flank, an intrusion from north ofthe GSC, and a large horizontal density gradient across the south flank .

The distribution of T/S correlation groups in August was similar to thatin June (Figure 33) . Group 1 (Figure 33) included the homogeneous waters on

top of the Bank and extended into the Great South Channel . The temperaturerange on the Bank within approximately the 60-meter isobath was 11 to 15°C

with a salinity of approximately 32 .5 0/00 . Group 2 represents the classicsummer waters found in the Gulf of Maine progressing from the warm Maine

Surf ace Water (MSW), to the temperature minimum associated with the MaineIntermediate Waters, and finally to the warmer and much more saline MaineBottom Water (MBW) . The surrounding water groups 3, 7, and 9 representmodifications of Group 2 because of topography and mixing with other watertypes . Group 3 over the Northeast Peak area, where the depth is less than 150meters, was primarily MSW, similar to the upper part of Group 2 althoughsomewhat more saline as a result of horizontal mixing with slope water . Group9 to the north of Great South Ch annel di ffered from Group 2 i n that freshwater had been added to the upper part of the water column . Group 4 came fromthe southern flank of the Bank between approximately the 60- and 100-meterisobaths . Surface temperatures of this group were 1 to 3°C higher than Group1, although the salinity was nearly the same . Group 6 represents stationsthat were generally characteristic of the slope waters with those stationsthat were farthest offshore having the highest salinities in the upper portion

of the water column . Group 5 was distinguished by the interleaving of warm/saline and cold/fresh waters that took place through the shelf/slope front .

Group 8 from the Browns Bank region of the Scotian Shelf represents a completedeparture from the water types previously observed in the area except for theSeptember 1977 cruise . Every other cruise showed cold, relatively freshwater on the Scotian Shelf with salinities in the surface waters less than32 .0 0/00 . Thus it would seem that the effect of St . Lawrence River runoffwas minimal in the later summer .

-26-

Biological Patterns . The phytoplankton of the well-mixed region in

August formed a diatom-dominated assemblage (Figures 34 and 35) . Sixty-three

species were present : 47 diatoms, 15 dinoflagellates, and 1 chlorophyte(Table 12) . The major species in this assemblage were Rhizosolenia alata , R .

styliformis , and G . flaccida. A flora characterized by the occurrence ofthese three as major species extended from the GSC, along the north flankfront to the Northeast Peak, and along the south fl ank wel l over the slope in

the west . As the front north of the GSC was crossed, the flora changed

considerably. Within the front, the assemblage was characterized by a

mixed-diatom flora, with occasional dominance by Chaetoceros sociale and

generally a complete absence or extremely low abundance (<3%) of R . alata .

North of this front a quantitatively and qualitatively impoverished flora of

mixed dinoflagellates (at all depths) and G . flaccida (at the surface only)

occurred . Rhizosolenia alata was also absent here . Within the front on thenorth flank (Figure 34, Stations 75, 77, 79, 87, 109), the flora was similarin abundance and species composition to that found within the well-mixedregion except that R . styliformis and G . flaccida were not major species .

They accounted generally for less than 2% of the flora . Rhizosolenia alata ,

and N . seriata were major species . Immediately north of the front in MSW

(e .g ., Station 72, Figure 36), the flora changed abruptly . Among stations

located here (63, 66, 72, 92, 95, 103, 106), abundance declined 2 orders of

magnitude relative to on-Bank (compare Tables 12 and 13) . Rhizosolenia alatawas still the dominant species, but it was accompanied by an impoverisheddinoflagellate flora rather than the rich diatom flora found on-Bank .

Although the area on Georges Bank defined by a diatom community charac-

terized by R . alata , R . styliformis , and G . flaccida was the area of highest

cell abundance, there were patterns of abundance and species richness similarto that found during June within the area . Centers of highest abundance were

the GSC (Station 39; x = 36,060 cells/liter), the north flank (Stations 57,

60, 81 ; = 45,960) and within the north flank'front (Stations 75, 77, 79, 87,109 ; x= 50,217) . Abundance decreased progressively along transects towardthe Northeast Peak (Stations 111, 116, 147 ; x= 7926), and along the southflank on the east (Stations 227, 193, 196, 225, 201 ; x= 8428) and west

(Stations 300, 302, 304 ; x= 10,166) .

-27-

Table 12 . The ten most abundant phytoplankton species collected at thestations in GBW during the August 1979 cruise . Average cell abundancewas calculated from the data for the 17 surface stations . Productionrate and chlorophyll-a concentration also were given .

Speci esAverage Cell Abundance

(Cells/Liter)

* Rhizosolenia alata Brightwell 5,629*R . styliformis Brightwell 2,459

*Guinardia flaccida (Castracane) Peragallo 985Nitzschia seriata Cleve 815Chaetoceros compressum Lauder 251C . affine Lauder 240Cerataulina pelagica (Cleve) Hendey 235Chaetoceros sociale Lauder 224C . seiracanthum Gran 168Flagellate 144



Average Chlorophyll-a ( mg/m2) = 42 (Range = 25 .6-66 .4)(n = 7 stations)

*Major species .

-28-

Table 13 . The ten most abundant phytoplankton species collected at thestations in MSW during the August 1979 cruise . Average cell abundancewas calculated from the data for the seven stations . Production rate and,chlorophyll-a concentration were not measured .


(Cells/Liter)

*Rhizosolenia alata Brightwell 109Ceratium tripos (O .F . Muller) Nitzsch 22Chaetoceros sp. Ehrenberg 8Coccolithophores 7Ceratium fusus (Ehrenberg) Dujardin 7Coscinodiscus radiatus Ehrenberg 4Rhizosolenia styliformis Brightwell 4Prorocentrum micans Ehrenberg 4Amphora sp. Ehrenberg 2Navicula sp. Bory 2

Average Total Cell Abundance (21 species) 180

*Major species .

-29-

August production followed a pattern similar to the one in June ; it was

highest within the well-mixed area relative to off-Bank areas . Production washighest on the north flank, and decreased progressively across it (Figure 36) .The northern GSC and the front separating it f rom the fresher intrusion northof it were also productive (56 .6-86 mgC/m2/hr), although the southern GSCand southwestern GBW were relatively low (18-38 mgC/m2/hr) . Within theintrusion north of the GSC, productivity (20 .2 mgC/m2/hr) was relativelyl owe r .

Across the south flank, R . alata continued to be dominant, but in a florawhich became less abundant and characterized more by dinoflagellates (mostlyCeratium species) than diatoms in a north/south transect . Rhizosolenia alatacontinued to be the dominant organism until slope water was encountered(Stations 212 and 286), where it was no longer observed in the flora . Fur-thermore, samples from below the pycnocline south of the Bank (Figure 35) weremore diverse in diatom species and more closely resembled the flora of thewell-mixed zone . As in June, the flora developed in the well-mixed zoneapparently sank below the pycnocline as it was t ransported south . Productionwas low (7 .2-16 mgC/m2/hr), probably as a result of the stratified water .

Iliernceinn

Temperature-Salinity Groups and Phytoplankton Assemblages

We examined the correlation between phytoplankton assemblage distributionand the areal extent of temperature-salinity groups . Temperature-salinitygroups represent waters of similar source and mixing history ; if the physicalcharacter of such a water parcel affects its biological properties, thereshould be some consistency between the spatial distribution of the water

parcel and the distribution of the non-motile organisms, such as phytoplank-ton . Correlations between the spatial patterns of T/S groups and phytoplank-ton assemblages did occur to varying degrees during each season examined .

The strongest correlation between the distributions of T/S groups andphytoplankton assemblages occurred in late spring (June) and summer (August),

(compare Figure 20 with Figure 21 and Figure 33 with Figure 34) . Although the

assemblage in the well-mixed area changed from Guinardia -Cerataulina -dominatedflora in June to a Rhizosolenia flora in August, the boundaries of each

-30-

assemblage coincided in general with T/S Group 1 (GBW) in both seasons .Apparently similar physical mechanisms (discussed below) maintained the arealextent of the assemblage although biological events (species succession) werecausing qualitative changes .

North of the GBW during June and August, two distinct phytoplanktonassemblages that correlated with the approximate positions of T/S Groups 2 and9 were observed . Temperature-salinity Group 9 correlated well with thedistribution of diatom assemblages (S . costatum dominated in June and G .

flaccida dominated in August) that were distinct from the GBW and MSW assem-blages . As above, changes in the species composition did not alter the fact

that the assemblage correlated with the T/S group . The low salinity signal ofthis intrusion indicates that its origin is coastal .

Changes in the distribution of surface phytoplankton assemblages across

the south flank also correlated with T/S group distributions . In June, T/S

Group 3, which occurred along the south flank, shared similar northern and

southern boundaries with a Ceratium -dominated assemblage . Farther south over

the slope at this time, T/S Groups 4 and 6, which represent mixing of Bank and

Upper Slope Water, correlated with an assemblage characterized by dinoflagell-

ates and coccolithophores . Similarly, in August, T/S Groups 4 and 5 over the

south flank correlated with a change in the phytoplankton assemblage . As

discussed -below, the phytoplankton differences between surface and the 1%

light level are probably due to the circulation .

During autumn (December), a single assemblage extending from the shelf/

slope front across Georges Bank and into MSW correlated with the extent of T/SGroups 1, 2, and 3 (Figure 5) . This probably reflects the fact that the

surface waters of Groups 1, 2, and 3 had similar physical properties . Al-

though their surface waters were similar, these T/S groups were differentiatedin that the depth to the bottom increased across the north flank (Figure 3) .

Therefore, the profiles determining Groups 2 and 3 included water masses (MIWand Maine Bottom Water) not present in Group 1 . Over the south flank, a

second December assemblage correlated approximately with Group 6, where shelfand slope waters mix .

-31-

During March (Figures 14 and 15), there was little correlation betweenthe T/S groups and phytoplankton assemblages . However, as discussed below,this was a transition period and the phytoplankton distribution can beexplained by particular hydrographic events which were occurring .

In general, the distribution of the phytoplankton assemblages correlatedwith the distribution of T/S groups . When particular T/S groups persisted,e .g ., from June to August, the areal extent of the assemblages continued tocorrelate with them even though within the assemblage species compositionchanged . This suggests that the geographic extent of an assemblage is due to

the structure imposed by the physical n ature and history (particularly in thesurf ace water) of the T/S group rather than to the response or tolerance of aspecies or assemblage of species to ambient (T/S) conditions . Certainly thepresence or absence of the species constituting an assemblage at any time is afunction of a multiplicity of biological and physical conditions (e .g ., light,temperature, salinity, grazing, nutrients) . However, phytoplankton speciescan grow over a wide range of environmental conditions . Given that the appro-priate ranges for growth and survival exist, the distribution of an assemblageis likely limited by physic al mechanisms which establish the surf ace waterboundary conditions and the physical-chemical nature of the surface waterwithin these boundaries .

Since phytoplankton are surface water organisms, those mechanisms thataffect the physical nature of the first 100 meters are probably most importantto an understanding of the phytoplankton distribution . These include fronts,intrusions, surf ace circulation, and vertical circulation . When any of theseare changing, the hydrography of an area will be transitional in nature, and

the correlation between groups and assemblages will break down until thephytoplankton can respond to the new physical conditions . We discuss belowsome of the physical characteristics of the surf ace waters in the Georges Bankregion which may explain the distribution of phytoplankton assemblages ob-served during the present study .

Fronts and Intrusions

Frontal systems and intrusions affect phytoplankton production, speciesdistribution, and chlorophyll-a concentration . Fournier et al . (1977) ob-served enhanced production along the Scotian Shelf Front . Relatively high

-3 2-

chlorophyll-a and a change in species composition from nannoplankton to large

diatoms have been associated with subsurface intrusions of North Atlantic

Central Water along the shelf/slope break in the South Atlantic Bight (Bishop

et al ., 1980) . Seliger et al . (1981) observed changes in species composition,

chlorophyll-a concentration, and total cell abundance associated with frontal

systems in Chesapeake Bay . Similar observations have been made in the Celtic

Sea (Pingree et al ., 1976) and the English Channel (Holligan and Harbour,

1977) . Suggested mechanisms for enhanced production along fronts include : a

decrease in the mixing depth on the inshore side of retrograde fronts during

periods of light limitation (Fournier, 1977) ; tidal stirring of nutrients to

the surface on the well-mixed (light-limited) side with consequent cross-front

transport to the more stratified (and, hence, light-sufficient) side (Simpson

and Pingree, 1977) ; and possible intrusions of nutrients into the front from

below as a consequence of internal tides (Herman and Denman, 1979) . We found

enhanced production along fronts in the Georges Bank region and also found

that fronts and intrusions resulted in changes in species composition in a

cross-front direction . Additionally, the establishment of fronts and intru-

sions in late winter appeared to be associated with the seasonal succession of

phytoplankton .

Late fall on Georges Bank and in MSW are characterized by uniform densityconditions presumably caused by a replenishment of GBW by Wilkinson Basin

Water (Hopkins and Garfield, 1981) . The uniform distribution of a singlephytoplankton assemblage in GBW and into MSW during December (Figure 5) isconsistent with the absence of any strong frontal features across the north

flank (Figure 3) . Where a frontal system existed (the shelf/slope front), the

species composition changed across it .

Late winter (March 1979) was a transitional period between the uniformly

distributed autumn-winter flora and the more restricted assemblages thatcharacterized late spring . The central area of the Bank had a characteristicwinter flora in that Coscinodiscus species dominated and, more importantly,Thalassiosira species, which characterize the spring flora, were entirely

absent (Figure 15) . To the east and west of the central area, two distinctfloristic communities were developing . In the Northeast Channel, the North-east Peak, and along the southeastern edge of the Bank, a classical spring

-33-

flora ( Thalassiosira -dominated) was developing (Figure 15), while the western

end of the Bank di spl ayed a Phaeocysti s -domi nated flora that overshadowed an

otherwise spring-like flora . Bigelow (1926) also observed the development ofthis spring flora on Browns Bank, the Northeast Channel, and along the south-east slope of Georges Bank, while there were comparatively few of thesespecies over neighboring parts of the Bank in early spring (April 1920) .Sears (1941) noted a similar distribution of spring flora, dominated in this

case by Chaetoceros decipiens . Neither Sears nor Bigelow noted a Phaeocystis

bloom at this time . Bigelow (1926) considered that this distribution of

diatoms indicated a center of propagation in the Browns Bank region whichdrifted south along the edge of the Bank and west into the Gulf of Maine with

what he referred to as the Nova Scoti a current (a southeastward movement ofcold, relatively fresh water originating in the Cabot Straits [Bigelow,

1927]) .

We found the spring phytoplankton in the Northeast Channel and on thesoutheast edge of the Bank to be proximal to the frontal system caused bymixing of the intruding SSW with GBW, MSW, and slope water . This intrusion of

Scotian Shelf Water is a discrete event occurring for a few weeks each year in

late winter or early spring (Hopkins and Garfield, 1979) . The establishment

of similar spring phytoplankton communites in the Northeast Channel and alongthe southeast edge of the Bank is probably due to similar biological response

to an imposed physical structure, the front . The hydrographic conditionscaused by the intrusion resulted in the physical conditions (shallower mixed

layers) necessary for the spring bloom in the eastern end of the Bank . Thephysical condition most likely responsible for the high production in the

Northeast Channel is the stratification caused by the front . The station with

high production (144) lies within the front, while the nearby Station 147 justoutside the front is deeply mixed . The stratification results in the cells'exposure to a greater average in situ light intensity within the front . As

indicated above, cell abundance and species richness were higher at the frontin both the Northeast Channel and along the southeast edge of the Bank,

relative to central Bank and slope areas . This confirms a similar observationmade by Bigelow (1926) during spring 1920 (see Bigelow, 1926, Figure 109) .

Production (measured as C14 productivity) was also higher (395 mgC/m2/hr) at

-34-

the front in the Northeast Channel . Production along the southeast edge wasnot as high (77 mgC/m2/hr) despite the remarkably similar species composition,

and probably represents an earlier stage of development as the intrusion movedsouthward . Seaward of the front along the southeast edge, production general-ly decreased and the species composition became less representative of springconditions . This pattern of production is similar to that found in transects

across the Scotian Shelf Front (Fournier et al ., 1977) .

The data indicate that the frontal system is both encouraging the propa-

gation of the spring flora and enhancing production . The intrusion of colder,

fresher SSW mixing with the GBW and MSW disrupted the relatively uniform

hydrographic conditions observed in December . This imposed physical struc-

tures (fronts) capable of delivering nutrients to the euphotic zone and of

developing the stratification necessary to allow growth and reproduction even

in deeper waters (e .g ., the Northeast Channel) . The source of the phytoplank-

ton in these blooms was a function of the propagules available in both the

intruding SSW as well as those present in the MSW-GBW and the slope water . An

Arctic species, Bacteriosira fragilis , which is rarely observed in the Gulf of

Maine (Gran and Braarud, 1935 ; Cura and McAlice, in preparation), and which

was probably introduced by the SSW was observed along the front . Chaetoceros

atlanticum , an oceanic species (Gran and Br aarud, 1935) also observed at the

frontal stations, was found to be common on the south flank of Georges Bank by

Bigelow (1926) during the spring, and probably originates in slope water .

Guinardia flaccida , generally confined to Georges Bank (Bigelow, 1926 and the

present data set), also appeared in this frontal system . The importance of

the introduction and propagation of diatom species due to the presence of

the SSW intrusion is illustrated by the f act that 75 % of the diatoms which

eventually appeared in the late spring (June) were present in the f rontal

stations sampled in March . Although SSW does not appear to be a major source

of water for Georges Bank (Hopkins and Garfield, 1981), the presence of the

intrusion has clear implications for the production on Georges Bank in that it

is correlated with those species responsible for the spring bloom and encour-

ages the propagation of diatoms, at least one of which dominated the phyto-

plankton and presumably the production later in the year .

-35-

The SSW intrusion was not the only source of the spring flora nor was its

associated frontal system the only area of high production . As indicatedabove, the GSC and southwestern Georges Bank were also areas of high cellabundance and were dominated by Phaeocystis op uchetii . Bigelow (1926) foundmassive blooms of this species during spring in Cape Cod Bay . This speciesoccurs abundantly in coastal waters of the North Atlantic, and, of particularnote, in Vineyard Sound (Guillard and Hellebust, 1971) . An apparent sourcefor P . pouchetii and its accompanying spring flora may be the coastal water of

the Gulf of Maine off Cape Cod . Its restriction to the GSC southwestern Bankarea is consistent with the observation that MIW which forms in WilkinsonBasin may exit the Gulf of Maine through the GSC during early spring (Hopkins

and Garfield, 1979) . Considering that the Wilkinson Basin lies off Cape Cod,this movement of MIW may have entrained a population of Phaeocystis whichthen bloomed in the favorable (shallow) conditions of the Great South Channel .Those stations dominated by Phaeocystis were enveloped by an isotherm (Figure12), suggesting an intrusion of cooler (than Bank) water from north of theGSC . This intrusion extended 80 meters deep, and may conceivably represent

MIW moving into the GSC . Production in the intrusion was higher (range =248-803 mgC/m2/hr) than anywhere else and was the highest observed through-out the study period . Therefore, movement of this water into the well mixedarea may provide a second source of spring species and high production toGeorges Bank .

Density fronts were evident all around the Bank in June and were oftencoincident with the T/S .group and phytoplankton assemblage boundaries (Figures

19, 20, and 21) . One of the more interesting aspects of the phytoplankton

distribution at this time was the apparent restriction of G . flaccida and itsaccompanying diatom flora to the well-mixed GBW . Examination of samples fromthe bottom of the euphotic zone revealed that the flora was evenly distributedin the vertical . It did not appear elsewhere near-surface nor did it appearat depth (about 30 meters) in the Gulf of Maine . Bigelow (1926) considered G .flaccida as occasionally dominant in the summer (July) on Georges Bank . Ithas been observed as only a minor component in the Gulf of Maine and adjoining

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water in summer (Bigelow, 1926 ; Lillick, 1941 ; Gran and Braarud, 1935) .Restriction of the GBW diatom community to south of the north flank occursover a small spatial scale (5 km) and represents a discontinuity rather than agradual gradient . The frontal system and particularly the associated jet(Magnell et al ., 1980) which is strongest in spring probably provides abarrier to the northward movement of this flora . Hopkins and Garfield (1981)concluded that the jet provides a barrier for any lateral intrusions of water .

This conclusion, coupled with the general southward movement of water acrossthe Bank (Bigelow, 1927 ; Butman et al ., 1982 ; Hopkins and Garfield, 1981) isconsistent with the abrupt change in flora observed across the north flank .

The distribution of the GBW assemblage (Rhizosolenia-Guinardia flora) inAugust corresponded closely to the surface distribution of the sigma-t = 24contour (compare Figure 32 with Figure 34) and to the well-mixed area gener-ally . This correspondence indicates that the development of the flora was afunction of the distribution of the well-mixed area . As in June, the well-developed front along the north flank (Figure 32) restricted the assemblage tosouth of the north flank . However, in August, the density gradients acrossthe north flank were greater than in June and an assemblage of diatoms differ-ing from both GBW and MSW assemblages was found within the front . Across thesmall (40 km) distance between GBW and MSW three different hydrographicregimes were found : the well-mixed GBW ; the front ; and the stratified MSW .Despite the small spatial scales, the differences in mixing depth on each sideof the front and the dynamics associated with fronts apparently resulted inthe development of distinct phytoplankton assemblages .

During June and August, a density front developed north of the GSC due toan intrusion of fresher water from the north . The phytoplankton communitywithin the intrusion differed from the GBW assemblage not only in speciescomposition, but also in the fact that microscopic examination of the diatomsin the intrusion revealed them to be weakly'siliceous and chlorotic . Alsoin these samples, the Ceratium species were observed in various stages of celldivision - a condition not observed in the GBW samples . Chlorophyll-aconcentration (Figure 24) and production rates were also lower in the intru-sion . It appears that the intrusion of relatively fresh water and consequent

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development of the frontal system north of the GSC had resulted in a phyto-plankton assemblage different from both Georges Bank and Gulf of Maine flora .In addition to this difference in community structure, the morphology(weakened silica frustules) and probable vitality (chlorotic appearance) ofindividuals of a major group, diatoms, suggest physiological differences as

well .

r; ..,.1m+ ; ., .,

Although the distribution of assemblages on the north flank and north of

the GSC was related to the presence or absence of frontal systems, the floris-tic changes occurring along the south flank between the central Bank and slopewere related to the surface circulation pattern on and around the Bank . Ofparticular importance is the tendency of Bank water to migrate south overthe more stratified south flank . In a cross-isobath transect from GeorgesBank seaward, we generally found that dinoflagellates assumed a greaterproportion of the flora over the slope than on-Bank . As indicated above,

remnants of the on-Bank diatom-dominated assemblage were found below thepycnocline under this slope assemblage at about 30 meters . In stratified,

nutrient-poor surface water, such as slope water, dinoflagellates generallypredominate over diatoms (Margalef, 1967 ; Holligan and Harbour, 1977) . Thedevelopment of dinoflagellate assemblages over the south flank and slope isprobably a- result of diatoms sinking in the stratified water as GBW migrates

over the south flank . This was particularly clear in June when Guinardia wasfound only below the pycnocline over the south flank . A similar verticaldistribution of Rhizosolenia alata was observed over the south flank-slope in

August . This species was more abundant at depth over the slope than in the

surface water . The gradual disappearance of the species even at depth in an

offshore direction is consistent with the idea that the GBW flora was sinkingout of the surface layers as it was transported south .

Upwelling

As described earlier, f rontal systems have been associated with enhancedproduction . We found enhanced production across the shelf/slope front inMarch and at a front on the south flank in June, which was in agreement withthe patterns observed on the Scotian Shelf and in the Celtic Sea . Across the

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north flank in both June and August, production rate was highest on thewell-mixed (GBW) side of the front and decreased within the front and acrossit to the more stratified side . For example, in a cross-f ront transect inAugust, production was 93 .6 mgC/m2/hr in GBW, 68 .9 mgC/m2/hr within thefront, and 22 .8 mgC/m2/hr in MSW . Similarly, in June production droppedcross-front from GBW to MSW . This generally observed pattern of enhancedproduction on the GBW side of the north flank front rather than on the strati-fied side is not consistent with the conditions advanced by Simpson andPingree (1977) for enhanced production near frontal systems . These condi-

tions, a nutrient source on the well-mixed side and consequent cross-f rontflow of nutrient-rich water to the stratified side, are not met on the northflank . Flow on the Bank is generally south and the nutrient source is underthe pycnocline on the stratified side . The high biomass and production alongthe north flank are probably a function of upwelling under the jet, which isassociated with the front . Hopkins and Garfield (1981) state that the jettends to entrain water from between MSW and MIW, to transport and mix it whilein the jet, and to cause a surface discharge onto Georges Bank . They indicatethat the northeast region appears to be the most common site of entry for thiswater . Water from depth on the stratified side is nutrient-rich . The conceptof nutrient-rich water entrained in the jet and discharged along the northflank, and particularly the northeast region ; is consistent with the observa-tion that this area (north flank-Northeast Peak) supported a relativelynumerous, diverse, and productive flora during both June and August . Furthersupporting this concept was the fact that high production on the NortheastPeak in June was accompanied by high nutrient levels (Figures 27 and 28) andcolder surface water (Figure 25) in this area relative to the rest of theBank . The biological significance of the north flank front and associated jet

is three-fold : 1) it provides a barrier to lateral movement and hence limitsthe distribution of the on-Bank assemblage when the front is well developed(June and August) ; 2) the different hydrographic regimes result in differentphytoplankton assemblages in GBW, MSW, and, at times, within the front ; and,

3) the front-jet system provides a mechanism for nutrient delivery to theBank from MIW and hence enhanced production .

A second site of generally high production rates, cell abundance, andchlorophyll-a levels occurred in the GSC, particularly on the northern end .

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Upwelling may possibly support this productivity, although the mechanism isprobably not related to the jet as is the case farther north, since Hopkinsand Garfield (1981) contend that most of the discharge of jet-entrained water

occurs farther northeast than the GSC . There existed in June biological andphysical evidence of upwelling of MIW into the GSC . As indicated earlier,there was an area within the GSC of cold, nutrient-rich water whose source was

apparently MIW . The biological characteristics - low cell abundance, produc-

tion rates, and chlorophyll-a levels relative to nearby GBW stations - aresuggestive of recently upwelled water .

Further evidence of upwelling into the GSC was obtained in December .

Although the phytoplankton assemblage in GBW and MSW was qualitatively uniformat this time (Figure 5), differences in cell abundance were observed . Theaverage cell abundance on-Bank (15,130 cells/liter) is three times that in theGulf (5592 cells/liter) and production was also higher on-Bank (Tables 2 and

3) . Also, if Gulf of Maine Station 25 (Figure 1) was excluded from analysis,the GBW is richer in diatom species (16) than the Gulf of Maine (10) . How-

ever, the abundance and production of Station 25 bear a remarkable similarityto the flora of the Georges Bank water . With the exception of Nitzschia

delicatissima , which does not occur at Station 25, there is a one-to-onecorrespondence between the species at this station and those found on Georges

Bank . The salinity and temperature transects intersecting Station 25 (Figure7) indicate that the surface water here is similar to the water over the GSC(part of the GBW) and is physically separated from it by a section of colder(8°C) and saltier (32 .8 0/00) water . The hydrographic contours (Figure 8) aswell as the phosphate and silicate contours (Figures 9 and 10) are consistent

with the idea that a parcel of upwelled water separated the water at Station25 from i ts ori gi n as part of the GBW . An epi sodi c occu rrence of thi s typeexplains the hydrographic and biological similarity between the surface waterat this station and those on Georges Bank, while simultaneously explaining itsphysical occurrence among surrounding waters of somewhat different hydrographyand production . The significance of these combined physical and biological

observations is that they support the concept of at least episodic upwellingover the GSC and that upwelling events can result in a patchy distribution of

phytoplankton abundance and production .

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Significance of Physical Oceanographic Phenomena for Phytoplankton_Saecies Distribution, Succession, and Production

Production in coastal and oceanic waters is dependent upon incidentradiation and vertical circulation . Incident radiation determines the maximumamount of energy available for photosynthesis and is modified by transparency(depth of penetration) . Riley (1957) suggested that in oceanic waters acritical mean in situ average light level of 40 langleys/day (ly/day) must beavailable to the phytoplankton before growth exceeds losses . This critical

light level has since been extended to temperate estuaries (Barlow, 1958 ;

Riley, 1967 ; Gieskes and Kraay, 1975 ; Hitchcock and Smayda, 1977 ; Cura, 1981) .

This in situ light level is calculated as

I = IO (1 - e-KZm)KZm

wh e reI = average in situ light10 = incident lightK = extinction coefficientZm = mixed layer depth

Using extinction coefficients calculated from the light-depth profiles,mixed layer depths from hydrographic data, and the theoretically maximumincident light at 42° north latitude for each month (Kimball, 1928), wecalculated I for each station where we obtained production data . Within the

well-mixed GBW, the average in situ light intensity never achieved Riley's

critical value of 40 ly/day at any station . The average in situ light

intensity varied little seasonally (14, 27, 26, and 26 ly/day for December,March, May-June, and August, respectively) . The similar seasonal values were

due to increased late spring and summer extinction coefficients . Extinction

coefficients increase in summer probably as a result of increased suspended

matter (Bothner et al ., 1981) . The areas of highest production and biomass inGBW apparently never attain Riley's critical light intensity . Assuming the

concept of critical light level to be correct, then losses on the Bank must besmall, relative to off-Bank areas, in order for the high production and

biomass to be maintained . The physical conditions on Georges Bank would tend

to minimize losses . The water in the well-mixed area vertically mixes rapidly

, -41-

(7 hours within the 60-meter contour, 14 hours in the GSC, and 15 hours on theNortheast Peak - see Magnell et al ., 1981) . This complete turnover from sur-face t o bottom on short (<1 day) time scales prevents any loss of phytoplank-ton cells to deep layers while they are in the GBW . These vertical mixingrates are considerably faster than those over the south flank (35 hours) andslope (112 hours) where phytoplankton populations similar to those from thewell-mixed area were found at depth in late spring-summer . The cells appar-ently had sunk from the near-surface waters as they flowed south and were thuslost to the near-surface water over the south flank and slope, which were

dominated by dinoflagellates . The long residence times, gyre-like circula-tion, and slow southerly flow of GBW and the presence of fronts on the northflank retard advective losses . This minimization of physical losses appar-ently compensates for the low average in situ light intensity experienced byphytoplankton populations on Georges Bank .

The frontal systems that intrude upon Georges Bank at various seasonshave significant influences on both production and species composition .Production and biomass within the f ront on the north flank are as high ornearly as high as on the well-mixed side, and in August the f ront contains adistinct community . Similarly, the f rontal system associated with the intru-sion of coastal water north of the GSC contains a distinct community . Highproduction in the Northeast Channel is a result of the front formed by theintrusion of SSW, which results in a relatively shallow mixed depth . Thisintrusion of SSW is also associated with the appearance of spring flora .

Perhaps the most striking correlation between the physical and biologicaloceanography of the area is that between species assemblages and T/S groups .Historically, a number of attempts have been made to correlate the distribu-

tion of phytoplankton and zooplankton species with physical oceanographicphenomena . In instances where the physical dynamics are clear and relativelywell understood, as in upwelling areas or in• rings and eddies, the correla-tions have been clearly made and underlying causes demonstrated . Blasco et

al . (1980) demonstrated that changes in the hydrographic regime of the north-west African upwelling a rea have major effects on phytoplankton composition

and diversity . In the Peruvian upwelling system, the hydrographic and advec-tive characteristics of the area have implications for the phytoplankton on

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both a gross (Huntsman et al ., 1981) and fine (Barber and Smith, 1981) taxo-nomic scale . Zooplankton species distribution is also affected by the advec-tive properties off Peru (Smith et al ., 1981) . Changes in species distribu-tion have also been observed as a result of rings and eddies off southernCalifornia (Sargent and Walker, 1948) and in the Atlantic (Wiebe et al .,1976) . These distribution patterns are clearly a result of the large-scaledisplacement of water masses in the case of rings or relatively dramatic

hydrographic changes in the case of upwelling zones .

Distributional patterns have also been observed in the absence of such

obvious events as eddies and upwelling . Cleve (1897) attempted to character-ize the waters of the Atlantic as having distinct "plankton-types ." Hisscheme was subjected to early criticism (Gran, 1912) and has fallen out of

use . However, species assemblages are readily recognized even where theirvalidity as communities is questioned (Williams et al ., 1981) . The termsoften loosely applied to assemblages, such as boreal plankton or springplankton, imply, albeit vaguely, some underlying physical mechanisms contri-buting to the development of the particular group or at least the dominantspecies in the group . Perhaps the most detailed description and analysis ofphytoplankton distribution in a continental shelf area has been provided by

Braarud et al . (1953) for the North Sea . These investigators found 16 "vege-tation areas" in the North Sea and attempted to explain their distribution onthe basis of flow patterns in the North Sea which resulted in a sequence ofplant societies . They argued that each assemblage represented a differentwater mass that was developed by the mixing of various source waters . Theydeveloped their argument by superimposing maps of surface flow over theirvegetation maps . No use was made of temperature or salinity as water mass

properties . These authors suggest, however, that "the prehistory of the water

mass and the turbulent activities are of outstanding importance" to thedifferences in vegetation .

The idea that water-mass history is a determinant of biological distribu-tion continues to appear in the literature . Hart and Currie (1960) contended

that the groups suggested by Cleve occur when environmental gradients areabrupt (implying the juxtaposition of water masses) . In studying zooplankton

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distributions of the North Pacific Ocean, Fager and McGowan (1963) demon-strated that species groups and water masses were similarly distributed

although no correlations could be made between species groups and temperature,salinity, oxygen, or themocline depth . They suggest that the history of thewater mass may be important in setting the environment for organisms . McGowan(1971), in reviewing the biogeography of the Pacific, contended that zooplank-ton species composition is strongly influenced by the history of the waterbody .

We suggest that it is the mixing history of the waters in the GeorgesBank region and their eventual relative isolation from each other which deter-

mines to a large extent the occurrence of phytoplankton species assemblages .The present work shows a close spatial correlation between independent mea-sures of mixing history (T/S groups) and species assemblages (major species) .In some instances, particularly during late spring and summer on Georges Bank,it is relatively easy to determine some of the differences in mixing history .For example, the low salinity signal in the intrusion north of the Great SouthChannel in June and August clearly indicates that this T/S group has beenunder the influence of coastal water to a greater extent than the adjacentMaine Surface Water . Georges Bank Water (GBW) is derived from Wilkinson BasinWater and, to a lesser extent, Upper Slope Water and Scotian Shelf Water(Hopkins and Garfield, 1981) . As indicated above, the water over the southflank is a mixture of GBW and Slope Water . These physical mixing histories

will determine the chemical nature of the water and initial biological inocu-lum . Among other parameters (light, bathymetry, stratification, etc .), the

chemistry of a water mass is of considerable importance in determining itsfloristic composition . It is becoming recognized that micronutrients (trace

metals and vitamins) are a determinant of phytoplankton succession . Frey andSmall (1980) demonstrated that trace metals and vitamins can control speciessuccession and spatial distribution in coastal waters . Murphy and Brown(1982) demonstrated that even the ratio of metals can be determinants ofspecies success . The trace metal composition of a water mass or T/S group

will depend upon the chemistry of the waters contributing to that water massduring its mixing history .

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The species succession on Georges Bank and the surrounding region is

f airly consistent from year to year (Cura, in preparation) . Various recentstudies-indicate that patterns of succession may be chemically controlled bymicronutrients . For example, Thalassiosira species, which have a dependencyon vi tami ns ( Swi ft, 1981) and on trace metal s( Frey and Smal l, 1980) becomeless prominent in the flora after spring until members of the genus are nearlyexcluded by August . At that time, Rhizosolenia species, which are not sotrace-metal-dependent (Frey and Small, 1980), dominate . Considering that thewater on Georges Bank is relatively isolated in late spring and summer (longerresidence time, fronts), trace metals may be depleted with time . Therefore,the successional changes which occur during this time are probably related torelative isolation of the water mass and to the ability of a species ( Rhizo-solenia sp .) to thrive in trace-metal-poor water . Substantiation of thissuggestion awaits investigation of the trace metal concentration within thewell-mixed zone and experimental confirmation of the metal requirements of theindigenous flora .

Summar

The shallower regions of Georges Bank have been considered the mostproductive and as having the highest phytoplankton biomass (Riley, 1941 ; Cohenand Wright, 1978) . This was generally found to be the case during the fourcruises described here . However, there existed seasonal changes in the

spatial distribution of biomass (cell number, chlorophyll-a) and production .

During December the areas of highest production occurred in the well-mixed central region (within the 60-meter isobath) . The relatively enhancedproduction in the shallow regions at that time was probably due to the rela-

tively shallow mixed layer . In the other areas where production and/or

biomass were measured (the Gulf of Maine, Northeast Channel, Northeast Peak,

and slope), the more deeply mixed surface layer imposed light limitation uponthe phytoplankton .

During the spring bloom (March), the areas of highest measured production

occurred in the Great South Channel (GSC)-western end of Georges Bank and inthe Northe ast Channel . The high production in the GSC-western Georges Bankarea was probably due to the bathymetry and vertical mixing rate . In the deepNortheast Channel, the high production was caused by a relatively shallowmixed depth as a consequence of the southward movement of SSW .

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During late May-early June, the highest production was measured in theGSC, on the well-mixed side of the north flank front, and on the Northeast

Peak . Production on the north side was one-half that in the GSC and NortheastPeak, but still 3-4 times that found in MSW or over the south flank and slope .

The enhanced production in the GSC at this time correlates with evidence forthe upwelling of nutrient-rich water . The enhanced production on the North-

east Peak is consistent with a high-nutrient, low-temperature parcel in thatarea and is also consistent with the hypothesis of Hopkins and Garfield (1981)that jet-entrained water is discharged onto the Northeast Peak .

The distribution of production in August was similar to that in May-June

(high production in the GSC and on the well-mixed side of the north flankfront) . Although neither production nor chlorophyll-a was measured at thistime on the Northeast Peak, cell numbers were low relative to the GSC andnorthern Georges Bank .

In general species assemblages correlated with the distribution of T/Sgroups except during periods of hydrographic changes - particularly spring .

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McGowan, J .A . 1971 . Oceanic biogeography of the Pacific . In : The micropale-ontology of the oceans . B.M . Funnel and W .R . ReidelTeds .), CambridgeUniversity Press, Cambridge : 3-74 .

Murphy, L .S ., and J .F . Brown . 1982 . The influence of iron and manganese oncopper sensitivity in diatoms : Differences in the response of closelyrelated coastal and oceanic isolates . Eos, 63 : 112 .

Pingree, R .D ., P .M . Holligan, G .T . Mardell, and R .N . Head . 1976 . The influ-ence of physical stability on spring, summe r and autumn phytoplanktonblooms in the Celtic Sea . J . mar . biol . Ass . U .K ., 56 : 845-873 .

Riley, G .A. 1957 . Phytoplankton of the north-central Sargasso Sea . Limnol .Oceanogr ., 2 : 252-270 .

Riley, G .A . 1967 . The plankton of estuaries, p . 316-326 . In : G .H . Lauff (ed .),Estuaries . Publ . Am . Assoc . Adv . Sci . 83 .

Sargent, M .C ., and T .J . Walker . 1948. Diatom populations associated witheddies off southern California in 1941 . J . mar . Res ., 7 : 490-505 .

Sears, M. 1941 . Notes on the phytoplankton on Georges Bank in 1940 . J . mar .Res ., 4 : 247-257.

Seliger, H .H ., K .R . McKinley, W .H . Biggley, R .B . Rivkin, and K .R .H . Aspden .1981 . Phytoplankton patchiness and frontal regions . Mar . Biol ., 61 :119-131 .

Simpson, J .H ., and R .D . Pingree . 1977 . Shallow sea fronts produced by tidalstirring . In : Ocean fronts in coastal processes, M .J . Bowman and W .E .Esais ( eds .f, Springer-Verlag, Berlin : 23-28 .

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Strickland, J .D .H ., and T .R . Parsons . 1972 . A practical handbook of seawateranalysis . Fish . Res . Bd Can ., Ottawa, Canada, 310 pp .

Swift, D .G . 1981 . Vitamin levels in the Gulf of Maine and ecological signi-ficance of vitamin B12 there . J . mar . Res ., 39 : 375-403 .

Wiebe, P .H ., E .M . Hulbert, E .J . Carpenter, A•.E . Jahn, G .P . Knapp, III, S .H .Boyd, P .B . Ortner, and J .L . Cox . 1976 . Gulf Stream cold core rings :Large-scale interaction sites for open ocean plankton communities . Deep-Sea Res ., 23 : 695-710 .

Williams, W .T ., J .S . Bunt, R .D . John, and D .J . Abel . 1981 . The communityconcept and the phytoplankton . Mar. Ecol .-Prog . Ser ., 6 : 115-121 .

-50-

Figure 1 . Temperature-salinity correlation groups (italic numbers) andphytoplankton stations (dots), December 1978 . Transects referred toin text are shown .

Figure 2 . Hydrographic map for the Fall 1978 Hydrographic Cruise, 11 -- 19December 1978 . Dots indicate the stations for which data were availableat this depth level .

Figure 3 . Temperature and salinity profiles across Section I, December 1978 .

Figure 4 . Temperature and salinity profiles across Section II, December 1978 .

Figure 5 . Phytoplankton assemblages, based on surface water (80% light level)samples during December 1978 . Dots indicate the stations for whichdata were available at this depth level .

Figure 6 . Phytoplankton assemblages, based on 1% light level samples, December1978. Dots indicate the stations for which data were available at thisdepth level .

Figure 7 . Vertical temperature and salinity profiles across Section III, December1978.

Figure 8 . Vertical density profile across Section III, December 1978 .

Figure 9. Vertical phosphate profile across Section III, December 1978 .

Figure 10. Vertical silicate profile across Section III, December 1978 .

Figure 11 . Hydrographic map for the Winter 1979 Hydrographic Cruise, 17-29 March1979 . Dots indicate the stations for which data were available at thisdepth level .

Figure 12 . Temperature at 3 meters during the Winter 1979 Hydrographic Cruise,17-29 March 1979 . Dots indicate the stations for which data wereavailable at this depth level .

Figure 13 . Salinity at 3 meters during the Winter 1979 Hydrographic Cruise, 17-29 March 1979 . Dots indicate the stations for which data were availableat this depth level .

Figure 14 . Temperature-salinity correlation groups (italicized numbers) andphytoplankton stations (dots), March 1979 . Sections (Roman numerals)referred to in the text are shown .

Figure 15 . Phytoplankton species assemblages, based on surface (80% light level)samples, March 1979 . Dots indicate the stations for which data wereavailable at this depth level .

Figure 16 . Vertical density profile across Section I, March 1979 .

Figure 17 . Vertical salinity and density profiles across Section II, March 1979 .

-51-

Figure 18 . Phytoplankton species assemblages, based upon samples from the 1% lightlevel, bottom of euphotic zone, March 1979 . Dots indicate the stationsfor which data were available at this depth level .

Figure 19. Hydrographic map for the Spring 1979 Hydrographic Cruise, 29 May to7 June 1979 . Dots indicate the stations for which data were availableat this depth level .

Figure 20 . Temperature-salinity correlation groups ( large numbers) andphytoplankton ( dots) stations, June 1979 . Sections (Roman numerals)referred to in the text are shown .

Figure 21 . Phytoplankton species assemblages, based upon surface 80% light levelsamples, June 1979 . Dots indicate the stations for which data wereavailable at this depth level .

Figure 22 . Vertical density profiles Sections I, II, and III, June 1979 .

Figure 23 . Vertical density profiles Sections VI, VII, and VIII, June 1979 .

Figure 24 . Contours of surface (1-2 meters) chlorophyll-a content (mg chlor-a/m3)during June 1979 made by continuous measurement fluorometry . Discrete,extracted chlorophyll-a surface values are given .

Figure 25 . Temperature at 3 meters during the Spring 1979 Hydrographic Cruise,29 May to 7 June 1979 . Dots indicate the stations for which data wereavailable at this depth level .

Figure 26 . Vertical temperature and density profiles across Section IV, June 1979 .

Figure 26 . Vertical temperature and density profiles across Section IV, June 1979(Cont) .

Figure 27 . Surface map, May-June 1979 .

Figure 28 . Surf ace map, May-June 1979 .-

Figure 29 . Profiles of vertical silicate and nitrate concentrations ( µg-at/1)across Section IV, June 1979 .

Figure 30 . Phytoplankton species assemblages, based on samples from the 1% lightdepth, June 1979, bottom of the euphotic zone . Dots indicate thestations for which data were available to this depth level .

Figure 31 . Shelf/slope density profile showi_ng distribution of dominant specieswith depth, June 1979 . G= G . flaccida , C = Ceratium, P = Peridinium ,S = Syracosphaera , T= transitional (mixed f-Tora) . Numbers inparentheses above station numbers are production ( mgC/m2/hr) .

Figure 32 . Density at 3 meters during the Summer 1979 Hydrographic Cruise, 14-23August 1979 . Dots indicate the stations for which data were availableat this depth level .

-52-

Figure 33 . Temperature-salinity correlation groups (large numbers) and phyto-plankton stations (dots), August 1979 . Sections (Roman numerals)referred to in the text are shown .

Figure 34 . Phytoplankton species assemblages, based upon surface (80% lightlevel) samples, August 1979 . Dots indicate the stations for whichdata were available at this depth level .

Figure 35 . Phytoplankton species assemblages, from the 1% light level, August1979, bottom of euphotic zone . Dots indicate the stations forwhich data were available at this depth level .

Figure 36 . Vertic al density profile, Section I, August 1979 . Associatedtable gives production (mgc/m2/hr) at stations in the crossfront transect .

-53-

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TEMPERATURE (°C ) AT 3 METERS 4 p~~•~ p •~ *~ p p~'.~.«K M

3.•w1M 7Q' ' {t' f!l• {7• {f• {0• H•

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DISTRNCE [kL lometersl


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100.0

200.0

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300.0

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FIGURE 26 (CONT)


0)srfiNCE (kLLometers)

I00F-+i

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C,

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NITRATE (ug-at/1) AT 2 METERS 4~~`~~~~ ~4 Ip 1) !0 ~0 70 e0 10 W e0 100

' [I •

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G = G. FLACCIDAC = CERATIUMP = PERIDINIUMS= SYRACOSPHAERAT = TRANSITIONAL

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...r-•.-.~_~l, ..µ~'~ ~r v

1 TO' M• M' p• N' $S• M•

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„ t G L .0 .0 . . . -=r-~r' r'-r 0.0

PRODUCTIONSTATION (mgC/m2/hr)

81 9479 6977 3275 23

100.0 -f NXZP I I F 100.0

x10 .0 i I / F 200.0

mLmJmE

2FG.IJC3

a~m..~mE

F(LLiO

w0.n i F 300.0

++10.u i F 400.0

500.0

I)ISTRNCE tk'LLometers)

FIGURE .36-90- J .J . Cura, Jr . and J .H. Ryther, Jr .

Documents

RELATIONSHIPS BETWEEN PHYTOPLANKTON · PDF filewere supported by the Bureau of Land Management under Contract Number AA 851-CT1-39. i. ABSTRACT