Euphausiid distribution, abundance and succession in North Atlantic warm-core ring 82B

Zooplankton collections were made with a multiple opening/closingnet and environmental sensing system (MOCNESS) both day andnight in warm-core ring 82B in the North Atlantic Ocean andat night in the Slope Water between March and August 1982. Speciessuccession of euphausiids in 82B was presented during the lifespanof this warm-core ring, aiming at providing basic informationon possible response of North Atlantic marine ecosystem to globalwarming. Species succession of euphausiids (32 species) in thislong-lived warm-core ring was investigated. Major physical changesof 82B occurred in March to April by convective mixing and thermostadcooling, in April/May by stratification of the surface watersand in August by the interaction with Gulf Stream. Substantialchanges in species composition were observed that correspondedto these physical changes. Four different patterns were foundin temporal change in abundance of warm-water species. Therewere species that decreased in number from March to August,species that decreased from March to June, but increased inAugust, species that increased from Match to August and speciesthat showed no systematic trend. These differences may be attributableto a species tolerance to the thermostad temperature decreaseand their vertical distribution. There was also a large changefrom April to June with ascendance of the transition species,Thysanoessa gregaria. Cold-water species had variable patternsof abundance in 82B and occurred more abundantly in the SlopeWater than in the ring. The monthly percentage decrease in theabundance of warm-water species in 82B was high compared withthat of cold-water species in cold-core rings as a result ofthe more rapid changes in the physical structure and the shorterlifetimes of warm-core rings in the western North Atlantic.