Zooplankton retention in the estuarine transition zone of the St. Lawrence Estuary

Simons, Rachel D., Stephen G. Monismith, Ladd E. Johnson, Gesche Winkler, François J. Saucier

Limnol. Oceanogr., 51(6), 2006, 2621-2631 | DOI: 10.4319/lo.2006.51.6.2621

ABSTRACT: We used a three-dimensional physical-biological model consisting of a Eulerian circulation model and a Lagrangian particle-tracking model, which included vertical sinking and swimming, to explore zooplankton retention in the estuarine transition zone of the St. Lawrence Estuary (SLETZ). To test the accuracy of the model, the results were temporally and spatially compared to a passive scalar released simultaneously in the circulation model and to field data for zebra mussel veligers. The model was then used to study the effects of baroclinic density-driven flow, vertical sinking and swimming, and tidal vertical migration on retention of simulated zooplankton. Baroclinic flow, created by longitudinal and lateral salinity gradients, was a critical part of retention in the SLETZ. In the presence of baroclinic flow, vertical sinking and swimming speeds of $0.2 mm s21 had a large effect on the residence time of the simulated zooplankton as a result of gravitational circulation. Tidal vertical migration—a pattern of upward movement on flood and downward movement on ebb—was a viable retention mechanism for the SLETZ, and its effectiveness was amplified by baroclinic flow. As the speed of tidal vertical migration increased, the simulated zooplankton were concentrated in smaller areas of the SLETZ and moved further upstream.

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