Contrasting fine-scale distributional patterns of zooplankton driven by the formation of a diatom-dominated thin layer

Although plankton thin layers have been described and modeled in a variety of environments, the physical structure surrounding the layer, associated biological rates, and distributions of multiple trophic levels are rarely examined simultaneously. Similar combinations of measurements, such as growth, mortality, and spatial relationships among plankton, are key to understanding how physical processes generating thin layers can influence abundances, composition, and predator-prey interactions. An in situ imaging system was deployed along a southward-oriented transect to describe the full similar to 2.3-km extent of a thin layer arching from 8 to 4 m deep, with Chlorophyll a enhanced by an order of magnitude inside the layer (23.9 mg m(-3) peak concentration). Physical oceanographic measurements, distributions of different plankton groups, and output from a high-resolution model indicated that surface convergence and vertical shear drove the formation of the layer, which was dominated by Odontella sp. diatoms that were relatively scarce in the broader study region. Phytoplankton apparent growth (0.64 d(-1)) balanced microzooplankton grazing rates (0.52 d(-1)) within the layer, whereas grazing mortality (2.23 d(-1)) greatly exceeded phytoplankton growth (-0.39 d(-1)) outside the thin layer. Mesozooplankton had starkly differing distributions; copepods aggregated south of the layer near the surface, and doliolids followed the thin layer trajectory. A physical oceanographic model run over a 1-month time period, including the time of sampling, indicated that similar surface convergences occurred frequently. Thin layers driven by convergence and shear may be common, with behavioral and buoyancy differences among plankton likely contributing to community structure and modification of trophic transfer.