Zooplankton collections were made with a Multiple Opening/Closing Net and Environmental Sensing System (MOCNESS) both day and night in warm-core ring 82B in the North Atlantic Ocean and at night in the Slope Water between March and August 1982. Species succession of euphausiids in 82B was presented during the lifespan of this warm-core ring, aiming at providing basic information on possible response of North Atlantic marine ecosystem to global warming.
Species succession of euphausiids (32 species) in this long-lived warm-core ring was investigated. Major physical changes of 82B occurred in March-April by convective mixing and thermostad cooling, in April/May by stratification of the surface waters, and in August by the interaction with Gulf Stream. Substantial changes in species composition were observed that corresponded to these physical changes. Four different patterns were found in temporal change in abundance of warm-water species. There were species that decreased in number from March to August, species that decreased from March to June, but increased in August, species that increased from Match to August, and species that showed no systematic trend. These differences may be attributable to a species tolerance to the thermostad temperature decrease and their vertical distribution. There was also a large change from April to June with ascendance of the transition species, Thysanoessa gregaria. Cold-water species had variable patterns of abundance in 82B and occurred more abundantly in the Slope Water than in the ring. The monthly percentage decrease in the abundance of warm-water species in 82B was high compared with that of cold-water species in cold-core rings as a result of the more rapid changes in the physical structure and the shorter lifetimes of warm-core rings in the Western North Atlantic.