Seabird segregation at Gulf Stream frontal eddies

Eddies along the western frontal boundary of the Gulf Stream result in the episodic formation of physically and biologically distinct water masses at the continental shelf break (ca 200 m lsobath) off the southeastern United States. Standardized counts of seabirds were made simultaneously with ship-board and satellite-derived (VHRR) hydrographic measurements in 3 eddy-assoc~ated water masses during July 1983, and in 4 water masses during August 1984. Two statistical models were used to test for significant relations between seabird species composition and these water masses. Contingency analyses of abundance data from both cruises revealed that seabird specles were not distributed independently of water mass type (p < ,005). Tested against the null hypothesis of homogeneous species' abundances, 85 % of common (densities 2 0 5 birds km-2) seabird species exhibited significant affinities for a single water mass. The aerially foraging and feeding black-capped petrel Pterodroma hasitata and bridled tern Sterna anaethetus were most abundant in oligotrophic Gulf Stream and warm filament water, respectively. Species foraging on the ocean surface and feeding for more prolonged periods, shearwaters Puffinus gravis and Calonectrjs diornedea and storm-petrels Oceanites oceanicus and Oceanodroma castro, were most abundant in the upwelled cold core of the eddy. Interspecific variation in seabird affinities for meso-scale (50 to 150 km) water masses can be attributed to trophic segregation originating from species-specific foraging behaviors, and oceanographic processes that dlrectly or indirectly influence prey availability. These patterns indicate that apex marine consumers parallel other taxa and trophic groups in their responses to environmental heterogeneity induced by western boundary currents. Gulf Stream frontal eddies persist at 'event' time scales (2 to 14 d ) , thereby further suggesting that some s e a b ~r d s are functionally integrated with marine communities affiliated with relatively small, ephemeral, and kinetic physical features.