BENTHIC AMPHIPODS (CRUSTACEA, MALACOSTRACA) IN ICELANDIC WATERS - DIVERSITY IN RELATION TO FAUNAL PATTERNS FROM SHALLOW TO INTERMEDIATE DEEP ARCTIC AND NORTH-ATLANTIC OCEANS

Patterns in diversity, species replacement and species composition of gammaridean amphipods (Crustacea: Malacostraca) were studied in two ar eas on different sides of the Greenland-Iceland-Faeroe Ridge. One was sited south of the Ridge in the North Atlantic Ocean off southern Icel and and the other north of the Ridge in the Arctic Ocean (subarctic an d arctic waters of the Iceland Sea) off northern Iceland. Samples were taken with a Rothlisberg-Pearcy epibenthic sled within the depth rang e of about 50 to 1200 m at a total of 28 stations. In all 21 150 indiv iduals were found belonging to 156 species. Of these 102 were found in the northern area with 64 of the species restricted to this area. In all 94 species were found in the southern area with 52 of the species restricted to this area. This suggests a similar regional diversity, b ut a different faunal composition. The amphipods showed similar specie s replacement patterns with depth on both sides of the Ridge, while th e diversity patterns differed between the areas. In the North Atlantic the number of species increased with depth, while in the Arctic there seemed to be no pattern at all. It is suggested that diversity patter ns of amphipods on each side of the Ridge are shaped by different fact ors, and these may not be the same as those determining diversity patt erns of other groups in the area. Salinity (as indicative of water mas ses) was found to be the most important environmental variable in expl aining the species composition when both areas were considered, sugges ting a number of contributing factors (adaptation to water masses, dis persal, etc.). Water temperature was the most important variable in ex plaining the variance in the species composition in the northern area, while depth was the most important variable in explaining the varianc e in the species composition of the southern area.