TRENDS IN COPEPOD SPECIES ABUNDANCES ACROSS AND ALONG A GULF-STREAM MEANDER - EVIDENCE FOR ENTRAINMENT AND DETRAINMENT OF FLUID PARCELS FROM THE GULF-STREAM

Abundances of zooplankton populations are hypothesized to change across and along Gulf Stream meanders in response to meander-associated physical processes. Upwelling and northward detrainment of Gulf Stream water occur upstream of the meander crest (trailing flank) while downwelling and entrainment of Slope Water into the Gulf Stream occur downstream of the meander crest (leading flank). Two mesoscale zooplankton surveys of a Gulf Stream meander were conducted in autumn 1988 as part of the BIOSYNOP program to identify cross-stream trends and meander-associated changes in species abundances. The abundances of eight selected copepod species (five Sargasso Sea species, two Slope Water species and one widespread species) were enumerated from samples taken along three transects at different locations along the meander. The integrated abundances (0-1000 m) of seven of the selected species support the hypothesized entrainment and detrainment. Sargasso Sea species were more abundant across the upwelling flank than across the downwelling flank, whereas the opposite was true for Slope Water species. Species abundances on a trailing flank transect (upstream of the upwelling region) demonstrate effective transport of Sargasso Sea/Gulf Stream species out of the current into the Slope Water. The relative proportions of two copepod species (Pleuromamma borealis and P. gracilis) were sensitive to the processes of entrainment and detrainment in the meander, suggesting that these two species are useful indicator species of their respective water masses. Gulf Stream meanders may be sites of cross-stream exchange of plankton populations between the Sargasso Sea and the Slope Water.