Gulf Stream warm core rings: phytoplankton in two fall rings of different ages

Abstract

Warm core ring (WCR) 82-H was sampled in September–October (1982) as a Gulf Stream meander pinched off and became a ring. It is compared with the 3-month-old WCR 81-D, visited September–October (1981). Although the rings have different histories, their phytoplankton assemblages share some characteristics. Using cluster analyses based on quantitative group counts, a station from one ring occasionally clusters most closely with a station from the other ring, showing a similar balance of organisms. The younger ring at the time of sampling, WCR 82-H, had lower diversity, fewer shelf species, and greater consistency between stations, except for a high level of Oscillatoria in the meander before the ring pinched off. Interaction with slope water was seen principally at the ring margin. WCR 81-D, on the other hand, showed a great deal of structure, and immediate dilutions with slope water and the Gulf Stream were apparent, with higher diversity before and a week after such interactions. The upper water column of warm core rings, although showing evidence of physical mixing, can exhibit stratification of species, even after a storm.