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In addition to the technology issues, we had a series of experimental
objectives related to beginning the process of describing the Thin Layers
themselves. These objectives are best described by relating our principal
conclusions from the 1996 field experiment:
- Thin Layers can be intense enough to dominate optical and acoustical
signatures.
- Thin Layers have the potential to dominate biological dynamics in
the water column.
- Temporal and spatial scales of Thin Layers can be hours to days, and
hundreds of meters to kilometers, respectively.
- Observed variations in the composition of different Thin Layers suggest
very different impacts on trophic transfer processes and sensor responses.
- Interactions between stratification, currents, current shear and any
resulting turbulence can be important in layer formation, maintenance
and dissipation.
- Particle sinking rates relative to density gradients may be important
in formation and maintenance of layers of larger particles (marine snow).
- Zooplankton often aggregate into Thin Layers, with different species
and size classes having different degrees of association with phytoplankton
layers.
- Zooplankton may avoid some optically detected Thin Layers.
Persistent Thin Layers which have considerable horizontal extent force
us to alter our perception of property distributions and controlling processes.
A partial list of important topics include:
- The average mixing conditions of the upper ocean suggest that Thin
Layers of small particles should be smeared or mixed away within tens
of minutes, at most. However, observations show otherwise. This leads
us to wonder:
- How does the vertical gradient in horizontal velocity interact
with density stratification over scales of ten's of centimeters
to a few meters to influence vertical mixing and horizontal extent
of a Thin Layer?
- To what extent are different types of persistent Thin Layers (e.g.
phytoplankton, marine snow, zooplankton biomass) affected by the
intermittency of local mixing?
- The growth and development of specific planktonic assemblages may
require the conditions created within Thin Layers. Examples include:
- Toxic dinoflagellates (red tide organisms) and toxic diatoms which
often bloom within narrow vertical intervals of the coastal ocean.
- Extensive mats of diatoms (usually less than 1 m thick) which
extend over most of the North Adriatic Sea
- An accurate evaluation of oceanic zooplankton production may require
assessment of the effects of thin phytoplankton layers on the feeding
and growth of zooplankton.
- An accurate evaluation of fish larval growth and survival may require
assessment of the effects of thin phytoplankton and/or zooplankton layers
on larval feeding.
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