| The estuarine transition zone (ETZ) of the St. Lawrence Estuary is defined as the region where the freshwater from the river mixes with the saltwater from the estuary. The circulation of the ETZ was studied using field observations and the three-dimensional hydrodynamic model, Tidal, Residual, Intertidal Mudflat 3D (TRIM3D). TRIM3D was calibrated to the field observations and used to study cross-channel flow, stratification and baroclinic flow, and residual circulation.; Throughout most of the ETZ, the cross-channel component of the tidal current was driven by a cross-channel phase shift in water level elevation, which was produced by variable bathymetry. Over the tidal cycle, stratification was found to be periodic, and baroclinic flow was represented by a series of unsteady pulses. Over the spring-neap cycle, stratification and baroclinic flow followed a subtidal pattern of increasing on neap tide and decreasing on spring tide. Residual circulation in the ETZ was dominated by gravitational circulation in both the along-channel and cross-channel direction. In the more saline and deeper channels, gravitational circulation took the form of two-layer exchange flow.; Zooplankton retention in the ETZ was studied using a three-dimensional physical-biological model, which consisted of the circulation model (TRIM3D) and a particle tracking model. After comparison to a passive scalar and to field data for zebra mussel veligers, the physical-biological model was used to study the effect of baroclinic flow, constant sinking and swimming speeds, and tidal vertical migration (TVM) on zooplankton retention. Baroclinic flow was found to be a critical part of zooplankton retention, containing the passive zooplankton particles in low salinity zone of the southern ETZ. Small increases in constant sinking speed were observed to greatly increase the retention time of zooplankton particles, which was attributed to gravitation circulation. TVM, a migration pattern of upward movement on flood and downward movement on ebb, was found to be a potential retention mechanism. At low TVM speeds, the zooplankton particles were concentrated in the southern ETZ in the low salinity zone and became more concentrated in the fresher upstream waters as the TVM speed increased. |
