Thin Layers Stefanie Tanenhaus

Background The existence of thin layers of phytoplankton, zooplankton and marine snow in coastal and open environments has been confirmed 1 The existence of thin layers of phytoplankton, zooplankton and marine snow in coastal and open environments has been confirmed 1 Formation due to physical and biological processes Formation due to physical and biological processes The mechanisms of formation and biological impacts are currently being investigated The mechanisms of formation and biological impacts are currently being investigated 1 Alldredge 2002

The Thin Layer Experiments 1996, 1998 Field experiments in East Sound, WA to investigate characteristics and formation mechanisms of thin layers 1996, 1998 Field experiments in East Sound, WA to investigate characteristics and formation mechanisms of thin layers Information from: The 1998 Thin Layer Experiments; Fig. 1: Acoustical Scattering (265 KHz TAPS) X axis = time, June 1998 Y axis = depth, in meters, bottom referenced Volume Scattering Strength (dB) :0012:0012: Fig. 2: Optical Profiles of Thin Layer upper x-axis = Chlorophyll-a, in µg/L lower x-axis = sigma theta y-axis = depth, in meters

Characteristics Layers composed of phytoplankton, zooplankton or marine snow aggregates 1 Layers composed of phytoplankton, zooplankton or marine snow aggregates 1 Thickness: cm-m (2) Thickness: cm-m (2) Horizontal length: up to km (2) Horizontal length: up to km (2) Duration: up to several days (2) Duration: up to several days (2) Chlorophyll concentrations >3 times ambient 3 Chlorophyll concentrations >3 times ambient 3 1 Alldredge McManus Dekshenieks 2001

Data Acquisition Methods: Glacier Bay, AK (Boas 2008) Location: Glacier Bay, fjord with avg. 233 m depth, 100 km length, 16 km width Location: Glacier Bay, fjord with avg. 233 m depth, 100 km length, 16 km width Profiles (31) of: Profiles (31) of: Temperature, Salinity, Depth measured using Sea-Bird CTDTemperature, Salinity, Depth measured using Sea-Bird CTD Chlorophyll fluorometerChlorophyll fluorometer Layer criterion: Layer criterion: Fluorescence spike ≤ 2mFluorescence spike ≤ 2m Fluorescence spike ≥ 30% above ambientFluorescence spike ≥ 30% above ambient 5 Boas 2008

Statistical Analysis 5 “Chlorophyll zone” determined “Chlorophyll zone” determined Overall distribution of chlorophyllOverall distribution of chlorophyll Thin layers vs. density (Fig. 3) Thin layers vs. density (Fig. 3) Thin layers vs. distance from pycnocline & chlorophyll-max (Fig. 4) Thin layers vs. distance from pycnocline & chlorophyll-max (Fig. 4) Fig. 3 (left) Fig. 4 (right) 5 Boas 2008

Data Acquisition Methods: East Sound, WA (Alldredge 2002) Location: East Sound, Wa fjord with avg. 30m depth, 12 km length, km wide Location: East Sound, Wa fjord with avg. 30m depth, 12 km length, km wide Profiles (~240) of: Profiles (~240) of: Temperature, Salinity, Density, Fluorescence measured with CTD & fluorometer Temperature, Salinity, Density, Fluorescence measured with CTD & fluorometer Particulate absorption (ac-9) and turbulent kinetic energy dissipation (SCAMP)Particulate absorption (ac-9) and turbulent kinetic energy dissipation (SCAMP) Abundance and size of marine snow aggregates (>500µm d) in situ with camera and CTD Abundance and size of marine snow aggregates (>500µm d) in situ with camera and CTD Zooplankton abundance (TAPS) and Phytoplankton composition also measured (from samples) Zooplankton abundance (TAPS) and Phytoplankton composition also measured (from samples)

Statistical Analysis 1 Fig. 5 Vertical Distribution of Marine Snow over 24 h study Fig. 6 Thin layer in relation to density and absorption Phytoplankton layer Marine 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, O 2, absorption, chlorophyll, current velocity (CTD, sensor, ac-9, fluorometer, 300 kHz ADCP)Temperature, Salinity, Depth, O 2, absorption, chlorophyll, 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 instrumentation 2. Stationary instrumentation 2 meteorological stations (air temp., wind speed/direction), 2 wave-tide gauges and 3 thermistor chains2 meteorological stations (air temp., wind speed/direction), 2 wave-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, SCAMP profilerFree-fall package (CTD, O2 sensor, 2 ac-9s, fluorometer, ADV, SCAMP profiler Acoustics Package: TAPS-8, a SeaBird 911+ CTD, an irradiance sensor, and bathyphotometerAcoustics Package: TAPS-8, a SeaBird 911+ CTD, an irradiance sensor, and bathyphotometer 4. Two mobile vessels performed basin-wide surveys to define spatial extent of thin layers and the hydrography of the Sound 4. Two mobile vessels performed basin-wide surveys to define spatial extent of thin layers and the hydrography of the SoundInstrumentation: 1200 kHz ADCP, CTD, O 2 and pH probes, fluorometer, 2 ac-9s)1200 kHz ADCP, CTD, O 2 and pH probes, fluorometer, 2 ac-9s) Data Acquisition Methods: East Sound, WA (McManus 2003)

McManus 2003) Instrumentation Set-up (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 Conclusions Findings and Conclusions

Formation Evidence of layers found in fjords, river mouths, the continental shelf and shelf basins 2 Evidence of layers found in fjords, river mouths, the continental shelf and shelf basins 2 Most layer formation (in East Sound) in regions where Ri>0.25 (3) Most layer formation (in East Sound) in regions where Ri>0.25 (3) Seasonal variations 3 Seasonal variations 3 May to September: thickness increases, intensity decreasesMay to September: thickness increases, intensity decreases 2 McManus Dekshenieks 2001

Formation 5 Form under varying circumstances and due to interactions between physical and biological processes Form under varying circumstances and due to interactions between physical and biological processes Physical: Shear, Turbulence due to wind and tidal forcingPhysical: Shear, Turbulence due to wind and tidal forcing Biological: Predator-prey relationships, sunrise/sunsetBiological: Predator-prey relationships, sunrise/sunset Density discontinuities trap organisms and fine sediments Density discontinuities trap organisms and fine sediments Link between depth of pycnocline and depth of layer formation Link between depth of pycnocline and depth of layer formation 5 Boas 2008

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

Marine Snow Layer Formation 1 1. Aggregate formation 2. Layer formation Aggregates reach neutral buoyancyAggregates reach neutral buoyancy Proposed mechanisms: Proposed mechanisms: Aggregate sinking from lower salinity surface layer into haloclineAggregate sinking from lower salinity surface 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

Formation Possible mechanisms responsible for the formation, maintenance and dissipation of layers 4 : Possible mechanisms responsible for the formation, maintenance and dissipation of layers 4 : in situ growth in thin layersin situ growth in thin layers physiological adaptation (photoadaptation) in layersphysiological adaptation (photoadaptation) in layers 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 4 Franks 2005

Layer Formation Modeling 6 Directed Swimming Produced by balance and interactions between constant turbulent diffusion and thinning mechanisms of steady vertical shear, buoyancy, and directed swimming toward target depth Produced by balance and interactions between constant turbulent diffusion and thinning mechanisms of steady vertical shear, buoyancy, and directed swimming toward target depth Only directed swimming can result in sharp profiles Only directed swimming can result in sharp profiles Image from: The 1998 Thin Layer Experiments; p swim (z)= Pcosh[(z-z 0 )/δ] -w maxδ/κ δ B(1/2, w max δ/2κ) Model of plankton distribution p where B is the beta function, P is the total amount of plankton in the water column 6 Birch 2009 Fig. 12: Sharp Profile

Layer Formation Modeling 7 Convergence-Diffusion Balance: 7 Stacey2007 Fig. 13 Straining and buoyancy soln. comparison to swimming soln. (normalized) Possible mechanisms (straining, motility, buoyancy) applied to East Sound data Possible mechanisms (straining, motility, buoyancy) applied to East Sound data Conclusion: Conclusion: Buoyancy and Straining dominate

Significance Influence biological structure, optical and acoustical properties 3 Influence biological structure, optical and acoustical properties 3 Feed higher order species 5Feed higher order species 5 3 to 1-D search for food 3 to 1-D search for food Source of visual protection 5Source of visual protection 5 Promotes biological heterogeneity and species diversity 5 Promotes biological heterogeneity and species diversity 5 Produce microenvironments that last at least as long as generation times of planktonProduce microenvironments that last at least as long as generation times of plankton Species partitioning allows diversity to persist 2 Species partitioning allows diversity to persist 2 3 Dekshenieks Boas McManus 2003