Thin LayersThin Layers

Stefanie TanenhausStefanie Tanenhaus

BackgroundBackground

The existence of thin layers of The existence of thin layers of phytoplankton, zooplankton and phytoplankton, zooplankton and marine snow in coastal and open marine snow in coastal and open environments has been confirmedenvironments has been confirmed11

Formation due to physical and Formation due to physical and biological processesbiological processes

The mechanisms of formation and The mechanisms of formation and biological impacts are currently biological impacts are currently being investigatedbeing investigated

1 Alldredge 2002

The Thin Layer ExperimentsThe Thin Layer Experiments 1996, 1998 Field experiments in East Sound, WA 1996, 1998 Field experiments in East Sound, WA

to investigate characteristics and formation to investigate characteristics and formation mechanisms of thin layersmechanisms of thin layers

Information from: The 1998 Thin Layer Experiments; http://www.gso.uri.edu/criticalscales/program/progtxt3.html

Fig. 1: Acoustical Scattering (265 KHz TAPS)X axis = time, 24 - 26 June 1998Y axis = depth, in meters, bottom referenced

Volume Scattering Strength (dB)

40 90

12:00 12:00 12:00

15

0

Fig. 2: Optical Profiles of Thin Layerupper x-axis = Chlorophyll-a, in µg/Llower x-axis = sigma thetay-axis = depth, in meters

CharacteristicsCharacteristics Layers composed of phytoplankton, Layers composed of phytoplankton,

zooplankton or marine snow zooplankton or marine snow aggregatesaggregates11

Thickness: cm-m Thickness: cm-m (2)(2)

Horizontal length: up to km Horizontal length: up to km (2)(2)

Duration: up to several daysDuration: up to several days (2)(2)

Chlorophyll concentrations >3 times Chlorophyll concentrations >3 times ambientambient33

1Alldredge 20022McManus 20033Dekshenieks 2001

Data Acquisition Methods:Data Acquisition Methods: Glacier Bay, AK (Boas 2008)Glacier Bay, AK (Boas 2008)

Location: Glacier Bay, fjord with avg. 233 Location: Glacier Bay, fjord with avg. 233 m depth, 100 km length, 16 km widthm depth, 100 km length, 16 km width

Profiles (31) of:Profiles (31) of:• Temperature, Salinity, Depth measured using Temperature, Salinity, Depth measured using

Sea-Bird CTDSea-Bird CTD• Chlorophyll fluorometerChlorophyll fluorometer

Layer criterion: Layer criterion: • Fluorescence spike ≤ 2mFluorescence spike ≤ 2m• Fluorescence spike ≥ 30% above ambientFluorescence spike ≥ 30% above ambient

5Boas 2008

Statistical AnalysisStatistical Analysis55

““Chlorophyll zone” determinedChlorophyll zone” determined• Overall distribution of chlorophyllOverall distribution of chlorophyll

Thin layers vs. density Thin layers vs. density (Fig. 3)(Fig. 3)

Thin layers vs. distance from Thin layers vs. distance from pycnocline & chlorophyll-max pycnocline & chlorophyll-max (Fig. 4)(Fig. 4)

Fig. 3 (left)

Fig. 4 (right)

5Boas 2008

Data Acquisition Methods:Data Acquisition Methods: East Sound, WA (Alldredge 2002)East Sound, WA (Alldredge 2002)

Location: East Sound, Wa fjord with avg. 30m Location: East Sound, Wa fjord with avg. 30m depth, 12 km length, 1-2.5 km widedepth, 12 km length, 1-2.5 km wide

Profiles (~240) of:Profiles (~240) of:• Temperature, Salinity, Density, Fluorescence measured Temperature, Salinity, Density, Fluorescence measured

with CTD & fluorometerwith CTD & fluorometer• Particulate absorption (ac-9) and turbulent kinetic Particulate absorption (ac-9) and turbulent kinetic

energy dissipation (SCAMP)energy dissipation (SCAMP) Abundance and size of marine snow aggregates Abundance and size of marine snow aggregates

(>500µm d) in situ with camera and CTD(>500µm d) in situ with camera and CTD Zooplankton abundance (TAPS) and Zooplankton abundance (TAPS) and

Phytoplankton composition also measured (from Phytoplankton composition also measured (from samples)samples)

Statistical AnalysisStatistical Analysis11

Fig. 5 Vertical Distribution of Marine Snow over 24 h study

Fig. 6 Thin layer in relation to density and absorption

Phytoplankton layerMarine Snow layer

1. Moored instruments in triangular array measured 4-D profiles of:1. Moored instruments in triangular array measured 4-D profiles of:• Temperature, Salinity, Depth, OTemperature, Salinity, Depth, O22, absorption, chlorophyll, , absorption, chlorophyll,

current velocity (CTD, sensor, ac-9, fluorometer, 300 kHz ADCP) current velocity (CTD, sensor, ac-9, fluorometer, 300 kHz ADCP) • 3 Tracor Acoustical Profiling Sensors (TAPS-6)3 Tracor Acoustical Profiling Sensors (TAPS-6)

2. Stationary instrumentation2. Stationary instrumentation• 2 meteorological stations (air temp., wind speed/direction), 2 2 meteorological stations (air temp., wind speed/direction), 2

wave-tide gauges and 3 thermistor chainswave-tide gauges and 3 thermistor chains 3. Vessel anchored 150 m outside array 3. Vessel anchored 150 m outside array

• Water-sampling (CTD/transmissometer package)Water-sampling (CTD/transmissometer package)• Free-fall package (CTD, O2 sensor, 2 ac-9s, fluorometer, ADV, Free-fall package (CTD, O2 sensor, 2 ac-9s, fluorometer, ADV,

SCAMP profilerSCAMP profiler• Acoustics Package: TAPS-8, a SeaBird 911+ CTD, an irradiance Acoustics Package: TAPS-8, a SeaBird 911+ CTD, an irradiance

sensor, and bathyphotometersensor, and bathyphotometer 4. Two mobile vessels performed basin-wide surveys to define 4. Two mobile vessels performed basin-wide surveys to define

spatial extent of thin layers and the hydrography of the Soundspatial extent of thin layers and the hydrography of the SoundInstrumentation:Instrumentation:

• 1200 kHz ADCP, CTD, O1200 kHz ADCP, CTD, O22 and pH probes, fluorometer, 2 ac-9s) and pH probes, fluorometer, 2 ac-9s)

Data Acquisition Methods:Data Acquisition Methods:East Sound, WA (McManus 2003)East Sound, WA (McManus 2003)

Instrumentation Set-up (McManus 2003)McManus 2003)

Fig. 8 (right):

Temporal, Spacial and toxonomic coherence of thin layer

Fig. 7 (left):

σt and Chl concentrations

Layer appears

Dissipation (biological)

Fig. 9 Figure from McManus et al 2008

Findings and ConclusionsFindings and Conclusions

FormationFormation

Evidence of layers found in fjords, Evidence of layers found in fjords, river mouths, the continental shelf river mouths, the continental shelf and shelf basinsand shelf basins22

Most layer formation (in East Sound) Most layer formation (in East Sound) in regions where Ri>0.25 in regions where Ri>0.25 (3)(3)

Seasonal variationsSeasonal variations33

• May to September: thickness increases, May to September: thickness increases, intensity decreases intensity decreases

2McManus 20033Dekshenieks 2001

FormationFormation55

Form under varying circumstances and Form under varying circumstances and due to interactions between physical and due to interactions between physical and biological processesbiological processes• Physical: Shear, Turbulence due to wind and Physical: Shear, Turbulence due to wind and

tidal forcingtidal forcing• Biological: Predator-prey relationships, Biological: Predator-prey relationships,

sunrise/sunsetsunrise/sunset Density discontinuities trap organisms and Density discontinuities trap organisms and

fine sedimentsfine sediments Link between depth of pycnocline and Link between depth of pycnocline and

depth of layer formationdepth of layer formation5Boas 2008

Fig.10: Pycnocline association, Figure from 3Dekshenieks 01; Fig.6

Pycnocline AssociationPycnocline Association

Marine Snow Layer FormationMarine Snow Layer Formation11

1. Aggregate formation1. Aggregate formation

2. Layer formation 2. Layer formation • Aggregates reach neutral buoyancyAggregates reach neutral buoyancy

Proposed mechanisms:Proposed mechanisms:• Aggregate sinking from lower salinity Aggregate sinking from lower salinity

surface layer into haloclinesurface layer into halocline

1 Alldredge 2002

Figure from McManus et al 2003

Fig. 11: Vertical separation of layers demonstrates presence of biological and physical cues

FormationFormation Possible mechanisms responsible for the Possible mechanisms responsible for the

formation, maintenance and dissipation of formation, maintenance and dissipation of layerslayers44::• in situ in situ growth in thin layersgrowth in thin layers• physiological adaptation (photoadaptation) in physiological adaptation (photoadaptation) in

layerslayers• vertical differences in community structurevertical differences in community structure• sinking and accumulation at micropycnoclinessinking and accumulation at micropycnoclines• differential grazingdifferential grazing• turbulent mixingturbulent mixing• internal wavesinternal waves• horizontal (isopycnal) intrusionshorizontal (isopycnal) intrusions

4Franks 2005

Layer Formation Modeling Layer Formation Modeling 66

Directed Swimming Directed Swimming

Produced by balance Produced by balance and interactions and interactions between constant between constant turbulent diffusion and turbulent diffusion and thinning mechanisms thinning mechanisms of steady vertical of steady vertical shear, buoyancy, and shear, buoyancy, and directed swimming directed swimming toward target depthtoward target depth

Only directed Only directed swimming can result swimming can result in sharp profilesin sharp profiles

Image from: The 1998 Thin Layer Experiments; http://www.gso.uri.edu/criticalscales/program/progtxt3

pswim(z)= Pcosh[(z-z0)/δ]-wmaxδ/κ

δ B(1/2, wmaxδ/2κ)

Model of plankton distribution p where B is the beta function, P is the total amount of plankton in the water column

6Birch 2009

Fig. 12: Sharp Profile

Layer Formation ModelingLayer Formation Modeling77

Convergence-Diffusion Balance:Convergence-Diffusion Balance:

7Stacey2007

Fig. 13 Straining and buoyancy soln. comparison to swimming soln. (normalized)

• • Possible mechanisms Possible mechanisms (straining, motility, buoyancy) (straining, motility, buoyancy) applied to East Sound dataapplied to East Sound data• • Conclusion: Conclusion: Buoyancy and Straining Buoyancy and Straining dominatedominate

SignificanceSignificance

Influence biological structure, optical and Influence biological structure, optical and acoustical propertiesacoustical properties33

• Feed higher order speciesFeed higher order species55

3 to 1-D search for food3 to 1-D search for food

• Source of visual protectionSource of visual protection55

Promotes biological heterogeneity and Promotes biological heterogeneity and species diversityspecies diversity55

• Produce microenvironments that last at least Produce microenvironments that last at least as long as generation times of planktonas long as generation times of plankton

Species partitioning allows diversity to persistSpecies partitioning allows diversity to persist22

3Dekshenieks 20015Boas 20082McManus 2003