Cross-frontal distributions and exchange of zooplankton on Georges Bank are a primary focus of Phase 3 of the U.S. GLOBEC Georges Bank program, which has the broader goal of understanding the response of marine populations to global climate change, especially interactions between zooplankton and fish. This proposal examines the cross-frontal exchange of zooplankton at two sites important to the biology of GLOBEC-targeted species: the Northeast Peak Front in March (coincident with expected inflow of the copepod Calanus finmarchicus and the presence of cod larvae and high predator abundance) and the Southern Flank Tidal Mixing Front in June (coincident with high concentrations of Calanus and predators). Questions addressed include the interaction between vertically-stratified zooplankton distributions and the cross-frontal exchange at different depths, the temporal variability of these interactions on diel, tidal, and fortnightly scales, and the effects over time of the repeated passage of tidal fronts at particular locations along a bathymetric slope. Zooplankton abundances and vertical distributions will be determined from a Lagrangian sampling scheme utilizing transects relative to frontal position conducted at different phases of the tidal cycle (semidiurnal and fortnightly), and will be related to hydrographic structure, tidal phase, and distance from the front. In a complementary Eulerian sampling scheme employing time series sampling at fixed locations along a bathymetric slope, zooplankton distributions and variability will be related to the time since frontal passage. Vertically-stratified MOCNESS tows will be used to collect zooplankton. Cross-frontal exchange of zooplankton (different species and lifestages with different vertical distributions) will be estimated from concentration gradients across the front at different depths and along isopycnal surfaces, using various estimates of flow and water exchange provided by physical oceanographers.
