Zooplankton distributions and transport across the northeastern tidal front of Georges Bank

Abstract

Tidal fronts are important sites of contact and mixing between zooplankton populations on and off Georges Bank in the northwest Atlantic. This study, a component of the US GLOBEC program, examined crossfrontal distributions of zooplankton, especially the copepod Calanus finmarchicus, across the northeastern tidal front of Georges Bank relative to the tidal cycle, frontal structure, flow field, and day/night cycle. A unique feature was a precisely executed sampling strategy designed to isolate spatial, tidal, and day/night effects. Vertically stratified zooplankton sampling was done along tidally coordinated crossfrontal transects and during time series studies at specific locations. Zooplankton taxa differed in their spatial and vertical distributions relative to the front and the hydrographic structure of the water column. There were significant associations between zooplankton assemblages and spatial location, depth, and water mass. Zooplankton distributions generally tracked water masses but usually showed crossbank abundance gradients. Consistent differences between day and night distributions were not apparent for most taxa. Meroplankton (larvae of benthic animals) were derived primarily from onbank populations. The zooplankton community was advected with the tidal flow. Crossfrontal distributions characteristic of onbank and offbank flow regimes clearly differentiated tidal phases and interacted with the flow field and topography. Salinity (a proxy for water mass) and depth interacted in controlling vertical distributions. As tidal forcing sloshed water masses up and down the bank slope, some taxa followed isohalines through depth changes while other taxa maintained themselves within a relatively limited depth or salinity regime. Surface convergence and downwelling, tidal upwelling of bottom water, and mid-depth tidal pumping influenced crossfrontal exchange.