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Thin Layers

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 Formation due to physical and biological processes

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Thin Layers

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  1. Thin Layers Stefanie Tanenhaus

  2. Background • The existence of thin layers of phytoplankton, zooplankton and marine snow in coastal and open environments has been confirmed1 • Formation due to physical and biological processes • The mechanisms of formation and biological impacts are currently being investigated 1 Alldredge 2002

  3. The Thin Layer Experiments • 1996, 1998 Field experiments in East Sound, WA to investigate characteristics and formation mechanisms of thin layers Fig. 1: Acoustical Scattering (265 KHz TAPS) X axis = time, 24 - 26 June 1998Y axis = depth, in meters, bottom referenced Fig. 2: Optical Profiles of Thin Layer upper x-axis = Chlorophyll-a, in µg/Llower x-axis = sigma thetay-axis = depth, in meters 15 0 12:00 12:00 12:00 40 90 Volume Scattering Strength (dB) Information from: The 1998 Thin Layer Experiments; http://www.gso.uri.edu/criticalscales/program/progtxt3.html

  4. Characteristics • Layers composed of phytoplankton, zooplankton or marine snow aggregates1 • Thickness: cm-m (2) • Horizontal length: up to km (2) • Duration: up to several days (2) • Chlorophyll concentrations >3 times ambient3 1Alldredge 2002 2McManus 2003 3Dekshenieks 2001

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

  6. Statistical Analysis5 • “Chlorophyll zone” determined • Overall distribution of chlorophyll • Thin layers vs. density (Fig. 3) • Thin layers vs. distance from pycnocline & chlorophyll-max (Fig. 4) Fig. 3 (left) Fig. 4 (right) 5Boas 2008

  7. Data Acquisition Methods:East Sound, WA (Alldredge 2002) • Location: East Sound, Wa fjord with avg. 30m depth, 12 km length, 1-2.5 km wide • Profiles (~240) of: • Temperature, Salinity, Density, Fluorescence measured with CTD & fluorometer • 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 • Zooplankton abundance (TAPS) and Phytoplankton composition also measured (from samples)

  8. Statistical Analysis1 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

  9. Data Acquisition Methods:East Sound, WA (McManus 2003) • 1. Moored instruments in triangular array measured 4-D profiles of: • Temperature, Salinity, Depth, O2, absorption, chlorophyll, current velocity (CTD, sensor, ac-9, fluorometer, 300 kHz ADCP) • 3 Tracor Acoustical Profiling Sensors (TAPS-6) • 2. Stationary instrumentation • 2 meteorological stations (air temp., wind speed/direction), 2 wave-tide gauges and 3 thermistor chains • 3. Vessel anchored 150 m outside array • Water-sampling (CTD/transmissometer package) • Free-fall package (CTD, O2 sensor, 2 ac-9s, fluorometer, ADV, SCAMP profiler • Acoustics 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 Instrumentation: • 1200 kHz ADCP, CTD, O2 and pH probes, fluorometer, 2 ac-9s)

  10. Instrumentation Set-up (McManus 2003)

  11. Fig. 7 (left): σt and Chl concentrations Fig. 8 (right): Temporal, Spacial and toxonomic coherence of thin layer

  12. Layer appears Dissipation (biological) Fig. 9 Figure from McManus et al 2008

  13. Findings and Conclusions

  14. Formation • Evidence of layers found in fjords, river mouths, the continental shelf and shelf basins2 • Most layer formation (in East Sound) in regions where Ri>0.25 (3) • Seasonal variations3 • May to September: thickness increases, intensity decreases 2McManus 2003 3Dekshenieks 2001

  15. Formation5 • Form under varying circumstances and due to interactions between physical and biological processes • Physical: Shear, Turbulence due to wind and tidal forcing • Biological: Predator-prey relationships, sunrise/sunset • Density discontinuities trap organisms and fine sediments • Link between depth of pycnocline and depth of layer formation 5Boas 2008

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

  17. Marine Snow Layer Formation1 1. Aggregate formation 2. Layer formation • Aggregates reach neutral buoyancy • Proposed mechanisms: • Aggregate sinking from lower salinity surface layer into halocline 1 Alldredge 2002

  18. Fig. 11: Vertical separation of layers demonstrates presence of biological and physical cues Figure from McManus et al 2003

  19. Formation • Possible mechanisms responsible for the formation, maintenance and dissipation of layers4: • in situ growth in thin layers • physiological adaptation (photoadaptation) in layers • vertical differences in community structure • sinking and accumulation at micropycnoclines • differential grazing • turbulent mixing • internal waves • horizontal (isopycnal) intrusions 4Franks 2005

  20. 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 Only directed swimming can result in sharp profiles Layer Formation Modeling 6 Fig. 12: Sharp Profile 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 Image from: The 1998 Thin Layer Experiments; http://www.gso.uri.edu/criticalscales/program/progtxt3 6Birch 2009

  21. Layer Formation Modeling7 Convergence-Diffusion Balance: • Possible mechanisms (straining, motility, buoyancy) applied to East Sound data • Conclusion: Buoyancy and Straining dominate Fig. 13 Straining and buoyancy soln. comparison to swimming soln. (normalized) 7Stacey2007

  22. Significance • Influence biological structure, optical and acoustical properties3 • Feed higher order species5 • 3 to 1-D search for food • Source of visual protection5 • Promotes biological heterogeneity and species diversity5 • Produce microenvironments that last at least as long as generation times of plankton • Species partitioning allows diversity to persist2 3Dekshenieks 2001 5Boas 2008 2McManus 2003

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