The results of zooplankton studies in the Bering Sea under the NPAFC Program (BASIS expedition). Part 1. Eastern areas

Abstract

Data on zooplankton in the eastern Bering Sea are presented. Samples were collected and processed by the planktonologists of the Pacific Fisheries Research Center (TINRO Center) aboard American vessels in the summer and fall from 2003 through 2011. Based on the analysis of the averaged values of bottom temperature, surface temperature, ice cover index, and also on maps of temperature distribution on the surface and near the bottom, the years 2003–2006 are referred to as “warm” and 2007–2011 as “cold.” A comparison of these two periods showed that with the advent of the “cold” period, the overall stock of zooplankton in the eastern part of the sea grew, particularly in the shelf zone, where it increased more than three times exclusively at the expense of the large fraction. Chaetognaths and copepods constituted the major proportion of this increase; stocks of euphausiids, mysids, hyperiids, and pteropods also grew, while a loss was observed in decapods (larval crabs and shrimps) and coelenterates. In the southern part of the shelf zone, a vast area with a higher biomass of the large fraction occurred, which was mainly due to copepods and chaetognaths. At the same time, areas with denser concentrations were formed by euphausiids and amphipods. The horizontal distribution of small and medium-sized zooplankton almost did not change; meanwhile, the main species of the small fraction, viz., Pseudocalanus newmani and Oithona similis, exchanged their significance and the neritic species Centropages abdominalis, Eurytemora herdmani, and Acartia longiremis, as well as cladocerans and meroplankton (larval Cirripedia, Bivalvia, and Polychaeta) almost completely vanished from the plankton by 2011. In most of cases, the mean biomass of the profiling species of the large fraction (Calanus marshallae, Neocalanus flemingeri, N. cristatus, Eucalanus bungii, Thysanoessa raschii, Themisto libellula, Limacina helicina, and Sagitta elegans) grew during the cold period. In the large fraction, S. elegans constituted half of the biomass (52% in the warm period and 49% in the cold one); the following two species were the copepods E. bungii and C. marshallae (both amounted to 26% and 39%, respectively), while Th. raschii, which is usually predominant among euphausiids, was only the last in the top five species. According to the example of the eastern areas of the sea, it becomes evident that the hyperiid Themisto libellula is brought into the studied area with cold waters from the north and disappears when their inflow stops. This species responds later to cooling and earlier to warming as compared to other species or groups. It appears that the end of the “cold” period can be expected in 2012.