Contribution of fine-scale vertical structure and swimming behavior to formation of plankton layers on Georges Bank
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The roles of plankton behavior, stratification, and microstructure in the formation of fine-scale plankton layers were examined using a 3-dimensional video plankton recorder mounted on a remotely operated vehicle. Vertically compressed plankton patches were observed in association with a cold pool over the Southern Flank of Georges Bank, extending from the tidal mixing front to the shelf-slope break during the months of May and June, 1994, 1995, 1997. In June 1995, 3 major plankton layers were present: a 10 m thick layer above the thermocline, a 1 m thick layer within the thermocline, and a third, 2 to 5 m thick layer immediately below the thermocline. Energy dissipation rate was lowest in the central layer and increased in both top and bottom layers. Some passive organisms and particles, e.g. the colonial diatom Chaetoceros socialis and rod-shaped diatoms, were concentrated in all 3 layers, while marine snow particles were found only in transitional regions. All stages of Calanus spp. were present in high numbers on the fringes of all 3 layers, while Oithona sp. was found only in the thin, central layer. Plankton were significantly aggregated only when the motility number, Mn (i.e. ratio of plankton swimming speed/rms turbulent velocity) was greater than 3, suggesting dominance of plankton behavior over physical structure. Under both quiescent and turbulent conditions, the Lagrangian frequency spectra (f) for swimming plankton and passive particles decreased with a slope of f(-2). However, in quiescent conditions, the magnitude of the spectrum for swimming plankton was 10-fold greater than for passive particles, illustrating a decoupling of plankton swimming from turbulent eddies. The air/water interface, the pycnocline, and multiple shear interfaces at density discontinuities act as boundaries to vertical zones where plankton behavior may succumb to or dominate background microstructure, thus providing a mechanism for formation of plankton and particulate layers.