Abundance of zooplankton during POLARSTERN cruise ARK-XIV/1a (PS51)

Zooplankton was collected by Birgit Stroscher (AWI) at three deep stations in the region of the Alpha Ridge in the Makarov Basin of the Arctic Ocean. Sampling was carried out between 15 and 18 July, 1998 during the cruise of RV "Polarstern" (ARK-XIV/1a). Nine depth strata from the bottom to the surface were sampled in a vertical manner with a multiple opening-closing net (Multinet Maxi, Hydrobios, Kiel, 0.49 m² mouth opening, 150 µm mesh). It was the first cruise when Multinet Maxi net model was used for zooplankton sampling in the ice-covered waters of the Arctic Ocean. Regular depth intervals were 0m-50-100-200-500-750-1000-1500-2000m-bottom. The zooplankton samples were preserved in 4% borax-buffered formalin. The samples were processed at Shirshov Institute of Oceanology, Russian Academy of Sciences (IORAS) by Dr. Ksenia Kosobokova. All zooplankton organisms from a sample were sorted to main taxonomical groups and for copepods further to species and stage level. Numerous small zooplankton organisms <1 mm were counted from an aliquot taken with a stempel-pipette of a sample (the smallest portion was 1:10), while all larger organisms, especially rare ones, were counted from the whole sample. Prosome length was used to distinguish between adult females (AF) and copepodite stage V (CV) of the two closely related copepods Calanus finmarchicus (AF < 3.1 mm, CV 3.1 mm, CV > 2.9 mm). Prosome length was measured from the tip of the cephalosome to the distal lateral end of the last thoracic segment. Earlier copepodite stages CI–CIV of Calanus belonged almost exclusively to C. glacialis and Calanus hyperboreus. They were separated by morphology and body size according to Hirche et al. (1994).