Mesozooplankton community structure, abundance and biomass in the central Arctic Ocean

Abstract

During the "International Arctic Ocean Expedition 1991" (20 August–21 September 1991) mesozooplankton was sampled at six stations in the Nansen, Amundsen and Makarov Basins of the central Arctic Ocean from 1,500 m depth to the surface by multiple opening/closing net hauls. Total mesozooplankton abundance decreased from 268 ind. m–3 in the surface layer (0–50 m) to <25 ind. m–3 below 200 m depth. The small copepods Oithonasimilis and Microcalanuspygmaeus, as well as copepod nauplii, were most abundant close to the surface, while Oncaeaborealis and Spinocalanus spp. frequently occurred at greater depth. Mesozooplankton dry mass (DM) integrated over the upper 1,500 m of the water column was surprisingly stable throughout the investigation area and measured 2.0±0.3 g DM m–2. Dry mass in the upper 50 m measured 20.9 mg m–3 and was dominated by Calanushyperboreus (57.4%) and C. glacialis (21.1%). C. finmarchicus was very abundant only in the Nansen Basin. Below 200 m the calanoid copepods Metridialonga, Microcalanuspygmaeus and Pareuchaeta spp., the decapod Hymenodoraglacialis and chaetognaths of the genus Eukrohnia were the principal contributors to biomass values of <1 mg DM m–3. Hence, vertical changes in abundance, biomass and species composition were much more pronounced than regional differences between the basins. Three different mesozooplankton communities were differentiated according to their faunistic composition and are discussed in context with the major water masses: Polar Surface Water, Atlantic Layer and Arctic Deep Water.