Stable isotopic compositions of overwintering copepods in the arctic and subarctic waters and implications to the feeding history

Highly abundant zooplankton dominated by copepods was found in the upper 200 m water column off the west of Spitsbergen in winter, although copepods in high-latitude environments during the overwintering period are known to reside in deep waters. We examined the possible feeding activities of these surface dwelling, overwintering copepods through their stable isotope compositions (δ15N and δ13C), and their internal lipids. The differences in the δ15N values (δ15N enrichment factor) between the particulate organic matter (POM) at the surface layers and the dominant copepods in winter showed that the averaged δ15N enrichment factor in Calanus glacialis (+2.7‰) and Metridia longa (+2.8‰) were higher than that in Calanus finmarchicus (+1.7‰). This suggests that C. glacialis and M. longa might be occasional omnivorous feeders, because the δ15N enrichment factor of these two copepods were relatively close to the previously reported value (e.g. +3.4±1.1‰, [Geochim. Cosmochim. Acta 48 (1984) 1135]) for a stepwise increase of the value with the food chain. The estimated lipid isotopic ratios (δ13C) of C. finmarchicus and M. longa tended to decrease with the decreasing lipid contents (lipid carbon/total carbon), suggesting that some lipid components with heavy isotopes were removed from the total lipids. The isotopic changes of lipids with the lipid contents probably reflect the metabolic processes of lipid utilization of arctic and subarctic copepods during the overwintering period.