Arctic marine mesozooplankton at the beginning of the polar night: a case study for southern and south-western Svalbard waters

Our knowledge of the marine plankton in high Arctic regions under winter conditions is very limited, especially during the polar night. Recent winters in the Barents Sea and adjacent waters have been anomalously warm, strongly influencing zooplankton communities. The structure of the mesozooplankton in relation to environmental conditions was investigated in Svalbard waters during November-December 2015. A total of 39 taxa were identified in the samples. Diversity of the mesozooplankton, as assessed by the Shannon index and by evenness, was lower when compared with published summer estimates. Cluster analysis revealed three groups of stations that differed primarily in the abundance/biomass of major species rather than by the presence or absence of specific taxa. Copepods constituted more than 98% of the total mesozooplankton abundance with Calanus finmarchicus, Calanus glacialis, Microcalanus spp., Oithona similis, and Pseudocalanus spp. being the most frequent. Total mesozooplankton biomass varied over a wide range (0.4-59.6 g dry mass m(-2)) with a high average value (+/- standard error) of 12.2 +/- 4.2 g DM m(-2). The maximum biomass was found in Isfjorden; this is the greatest mesozooplankton stock estimation for the Arctic during winter conditions. High mesozooplankton biomasses were also recorded in waters south of Svalbard. Redundancy and linear regression analyses revealed that mesozooplankton variability was strongly connected with the depth of sampling layer, mean salinity, bottom temperature, and latitude, suggesting that the observed distribution pattern of mesozooplankton abundance and species composition were related to local topography, circulation, and frontal zones. Another possible factor that might influence the mesozooplankton assemblage is the recent warming in the Arctic.