CARBON AND NITROGEN ISOTOPE RATIOS IN ZOOPLANKTON OF THE BERING, CHUKCHI AND BEAUFORT SEAS

Zooplankton from the Bering, Chukchi, and Beaufort seas and a transect across the Arctic Ocean were collected from 369 stations on 18 cruise s in the years 1985-1990 and 1993-1995. Carbon and nitrogen isotope ra tio analyses were performed on the major taxonomic groups present-cala noid copepods, euphausiids and chaetognaths. The sampled waters around Alaska were divided into 11 subregions based on water mass characteri stics and the zooplankton statistically tested for significant differe nces in the isotope ratios. Within all regions, copepods were signific antly more depleted in C-13 than euphausiids (average delta(13)C diffe rence for copepods = -1.1 parts per thousand than euphausiids), but sh owed no significant differences from euphausiids in delta(15)N except in the eastern Alaskan Beaufort Sea where copepods were relatively enr iched in N-15. Th, greatest variability in isotope ratios was among ge ographic regions. All taxa tested were C-13-depleted in the eastern Be aufort Sea, the Arctic Ocean and in deep waters of the southwestern Be ring Sea relative to the continental shelf waters of the Bering and Ch ukchi seas. The maximum enrichments were found in the southwestern Chu kchi Sea and central Bering Sea shelf waters. The advection of water n orthward through the Bering Strait was evident as a plume of enriched zooplankton extending to the shelf break in the Arctic Ocean. In contr ast, the delta(15)N within taxa generally increased moving northward f rom the deep Bering Sea to the Chukchi Sea and eastward into the Beauf ort Sea. The delta(15)N values for chaetognaths were 2.5 to 3 parts pe r thousand more enriched than copepods or euphausiids in all locations , consistent nrith their carnivorous diet. Comparisons of zooplankton isotope ratios among years and cruises within the same region revealed no significant differences. Low delta(15)N and delta(13)C values in z ooplankton of the pelagic Bering Sea are presumed to result from the i sotopic discrimination arising in the presence of high nutrient abunda nces and slow phytoplankton growth rates whereas depleted values in co astal waters of the Canadian Beaufort Sea presumably derive from Macke nzie River inputs of terrestrially derived carbon and nitrogenous nutr ients with low N-15 and C-13 abundances. The geographic heterogeneity in isotope ratios over short distances indicates a need for caution in the interpretation of isotope ratios in marine mammals and birds with regard to trophic status and habitat usage.