Gulf stream cold core rings: large-scale interaction sites for open ocean plankton communities

Abstract

Gulf Stream cold core rings are major oceanographic features in the northwestern Atlantic Ocean. During formation, cold water of Slope origin and the associated flora and fauna are entrapped within a ring of Gulf Stream water. These systems generally move southwesterly into the Sargasso Sea and are of biological interest because they offer a unique opportunity to assess the relative importance of physical and biotic factors in determining the distribution and abundance of plankton organisms. In effect, the formation and consequent decay of a cold core ring represents a large-scale ecological field experiment wherein major environmental changes are gradually imposed on a plankton community.

Four cruises have been made to sample Gulf Stream rings and the surrounding Sargasso Sea. Biomass has been estimated and selected species of Zooplankton, phytoplankton, and midwater fish populations have been assessed. Concurrent hydrocasts have yielded physical-chemical data.

Although a ring may remain physically identifiable for as long as two years, decay of the initially distinct biological community appears to be more rapid. A Zooplankton assemblage appears to persist longer than a phytoplankton assemblage. The season of formation appears to be critical in determining the biological changes of a Gulf Stream ring. The rapid demise of the Slope Water species assemblages in the rings may be the result of biological interactions with the Sargasso Sea assemblage initiated by the rapid modification of physical and chemical properties of ring surface waters.