Origins of oceanic plankton in the middle Atlantic Bight

Abstract

Expatriated species of zooplankton found in the mid-Atlantic Bight include Arctic-Boreal species derived from shelf waters northeast of Cape Cod, transition zone species from the adjoining Slope Water and tropical-subtropical species that commonly reside in the Gulf Stream, and Sargasso Sea. Introduction of expatriates is largely associated with the pattern of advective movements of water onto the shelf: Arctic-Boreal species are brought in from the northeast largely by over-shelf transport; transition zone species by Slope Water penetration at the surface when horizontal density gradients are minimal and at mid-depth in response to physical processes such as estuarine-type circulation, wind-driven upwelling, cold shelf water ‘bubble’ formation and movement out into the Slope Water or to shelf-Slope Water interactions associated with warm core rings; warm water species by injection of warm core ring surface water in over the shelf. There is little evidence that Carolinian species are introduced into the mid-Atlantic Bight directly around Cape Hatteras. In general, the occurrence of expatriate warm water species is more important in terms of species numbers and total biomass when compared to the occurrence of expatriate cold water species. The Bight region can be divided into three regions with regard to oceanic influences: (1) the band of low salinity water along the coast south of the mouth of the Hudson River, extending to the mouth of the Chesapeake; (2) the Continental Shelf edge extending from about 37 ° 30′N to 40 °N and extending shoreward towards the eastern half of the Long Island and Block Island Sound, but not including the region south-east of Cape Cod and Nantucket: (3) the southern sector, including the shelf edge south of 37 °N and extending landward south of Chesapeake Bay. Each of these regions is characterized by types of expatriate species and by hydrographic features. A mechanism is postulated whereby warm water species which cannot withstand harsh winter conditions in the mid-Atlantic Bight can ‘overwinter’ by the movement offshore of adults or larvae in shelf water entrained at Cape Hatteras in late summer or early fall, by transit alongside or within the Gulf Stream, by incorporation into a warm core ring and by return to shelf waters in the spring when the ring impinges on the shelf margin.