Carbon and nitrogen isotope ratios in zooplankton of the Bering, Chukchi and Beaufort seas

Abstract

Zooplankton from the Bering, Chukchi, and Beaufort seas and a transect across the Arctic Ocean were collected from 369 stations on 18 cruises in the years 1985-1990 and 1993-1995. Carbon and nitrogen isotope ratio analyses were performed on the major taxonomic groups present - calanoid copepods, euphausiids and chaetognaths. The sampled waters around Alaska were divided into 11 subregions based on water mass characteristics and the zooplankton statistically tested for significant differences in the isotope ratios. Within all regions, copepods were significantly more depleted in 13C than euphausiids (average δ13C difference for copepods = -1.1%°than euphausiids), but showed no significant differences from euphausiids in δ15N except in the eastern Alaskan Beaufort Sea where copepods were relatively enriched in 15N. The greatest variability in isotope ratios was among geographic regions. All taxa tested were 13C-depleted in the eastern Beaufort Sea, the Arctic Ocean and in deep waters of the southwestern Bering Sea relative to the continental shelf waters of the Bering and Chukchi seas. The maximum enrichments were found in the found in the southwestern Chukchi Sea and central Bering Sea shelf waters. The advection of water northward through the Bering Strait was evident as a plume of enriched zooplankton extending to the shelf break in the Arctic Ocean. In contrast, the δ15N within taxa generally increased moving northward from the deep Bering Sea to the Chukchi Sea and eastward into the Beaufort Sea. The δ15N values for chaetognaths were 2.5 to 3‰°more enriched than copepods or euphausiids in all locations, consistent with carnivorous diet. Comparisons of zooplankton isotope ratios among years and cruises within the same region revealed no significant differences. Low δ15N and δ13C values in zooplankton of the pelagic Bering Sea are presumed to result from the isotopic discrimination arising in the presence of high nutrient abundances and slow phytoplankton growth rates whereas depleted values in coastal waters of the Canadian Beaufort Sea presumably derive from Mackenzie River inputs of terrestrially derived carbon and nitrogenous nutrients with low 15N and 13C 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.