Seasonal dynamics of the trophic status of zooplankton in the Sea of Okhotsk (based on data from stable carbon- and nitrogen-isotope analysis)

Abstract

The composition of stable carbon and nitrogen isotopes was analyzed in the tissues of 42 species from the main plankton groups collected across the Sea of Okhotsk in the spring, summer, and autumn. This allowed a study of the seasonal variations in the trophic structure of the zooplankton community and the establishment of the trophic status of the dominant zooplankton species. The lowest δ15N values (6.9–8.0‰) were recorded for the filter-feeding copepod Eucalanus bungii throughout the seasons; the highest ones (12.0–13.5‰) were found for chaetognaths, Sagitta elegans. According to the δ15N value, the trophic status increases up the following sequence: omnivorous copepods < euphausiids < hyperiids < jellyfish < chaetognaths. The formed trophic hierarchy of main zooplankton groups does not undergo substantial seasonal changes; nevertheless, δ15N decreases noticeably in most species in the summer, which apparently reflects alterations in the base of the plankton food web that are caused by a phytoplankton bloom during the preceding spring season. Zooplankton species that inhabit the shelf zone have higher δ13C values as compared to the inhabitants of open deep-sea waters, whereas seasonal variations of δ13C values in zooplankton may be related to seasonal rearrangements in aggregations of some zooplankton species between deep-sea waters and the shelf zone.