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Marine Ecology Progress Series

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MEPS 301:199-215 (2005)  -  doi:10.3354/meps301199

Effects of physical processes on structure and transport of thin zooplankton layers in the coastal ocean

Margaret A. McManus1,*, Olivia M. Cheriton2, Patrick T. Drake3, D. V. Holliday4, Curt D. Storlazzi5, Percy L. Donaghay6, Charles F. Greenlaw4

1University of Hawaii, Department of Oceanography, Honolulu, Hawaii 96822, USA
2University of California Santa Cruz, Ocean Sciences Department, Santa Cruz, California 95064, USA
3University of California Santa Cruz, Long Marine Laboratory, Santa Cruz, California 95060, USA
4BAE SYSTEMS, 4669 Murphy Canyon Road, San Diego, California 92123, USA
5US Geological Survey, Pacific Science Center, Santa Cruz, California 95060, USA
6University of Rhode Island, Graduate School of Oceanography, Narragansett, Rhode Island 02882, USA

ABSTRACT: Thin layers of plankton are recurrent features in a variety of coastal systems. These layers range in thickness from a few centimeters to a few meters. They can extend horizontally for kilometers and have been observed to persist for days. Densities of organisms found within thin layers are far greater than those above or below the layer, and as a result, thin layers may play an important role in the marine ecosystem. The paramount objective of this study was to understand the physical processes that govern the dynamics of thin layers of zooplankton in the coastal ocean. We deployed instruments to measure physical processes and zooplankton distribution in northern Monterey Bay; during an 11 d period of persistent upwelling-favorable winds, 7 thin zooplankton layers were observed. These zooplankton layers persisted throughout daylight hours, but were observed to dissipate during evening hours. These layers had an average vertical thickness of 1.01 m. No layers were found in regions where the Richardson number was <0.25. In general, when the Richardson number is <0.25 the water column is unstable, and incapable of supporting thin layers. Thin zooplankton layers were also located in regions of reduced flow. In addition, our observations show that the vertical depth distribution of thin zooplankton layers is modulated by high-frequency internal waves, with periods of 18 to 20 min. Results from this study clearly show an association between physical structure, physical processes and the presence of thin zooplankton layers in Monterey Bay. With this new understanding we may identify other coastal regions that have a high probability of supporting thin layers.

KEY WORDS: Thin layer · Physical processes · Transport · Zooplankton · Coastal circulation

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