Tidal migration and retention of estuarine zooplankton investigated using a particle-tracking model

Wim J. Kimmerer, S. EdwardGross and Michael L. MacWilliams

Limnol. Oceanogr., 59(3), 2014, 901-916 | DOI: 10.4319/lo.2014.59.3.0901

ABSTRACT: We used hydrodynamic and particle-tracking models to investigate vertical movement of plankton and resulting retention in the San Francisco Estuary. The hydrodynamic model was UnTRIM, a three-dimensional, unstructured grid model, which had been calibrated to historical conditions in this estuary for 1994–1997, a period of widely varying hydrology. Intensive field studies of hydrodynamics and vertical movements of organisms during 1994–1996 provided input data for the models. The particle-tracking model was run with 14 alternative behaviors for three 45 d periods of contrasting hydrology. The behaviors included passive behavior, several simple tidal migration patterns, and several sinking speeds. Vertical positions of migrating and sinking particles resembled those seen in the field studies for copepods and epibenthos, respectively. Passive particles were advected from the estuary at rates that depended on freshwater flow. All of the non-passive behaviors caused retention of the particles in the low-salinity region of the estuary under some flow conditions. Tidal migration was very effective at maintaining position, but model results and a re-examination of the field data suggested that migration speed should increase with increasing freshwater flow. Continuous sinking was also effective at retention, particularly in deep areas of low tidal velocity. We conclude that the vertical distributions observed in the field studies were a result of active patterns of movement, that these patterns can result in retention, and that retention in a bathymetrically complex estuary can be understood only in the context of the full time-varying three-dimensional flow field.

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