Zooplankton of the deep-water part of the White Sea at the end of the hydrological winter

The composition and distribution of zooplankton were studied in the open deep-water part of the White Sea at the end of the hydrological winter in April 2003. The species composition and the developmental stage structure of the zooplankton were typical of the winter season. The species of the cold-water zooplankton assemblage-the copepods Calanus glacialis, Metridia longa, Pseudocalanus minutus, and the chaetognath Sagitta elegans-dominated in terms of the biomass. The biomass distribution over the area studied showed the lower zooplankton abundance at relatively shallow sites of the Gorlo and the eastern White Sea Basin as compared to those in the deepest part of the Basin and in the Kandalaksha Bay depression. This kind of distribution pattern was caused by the fall-winter sinking of zooplankton and its concentration in the deeper region of the sea. The vertical distribution of the mass copepod species significantly differed from that at the beginning of the winter. The maximum abundances and biomasses of C. glacialis and P minutus ascended from the deepest layers above the 150-m level. Differences were also found in the degrees of development of the migration for the different age stages of these copepods. In addition to the onset of the spring upward migration of the wintered populations from the deep, we noted the presence of phytoplankton and ice algae in the guts of the ascending crustaceans and the commencement of the reproduction season of C . glacialis. The altitude of these biological characteristics indicate the onset of the spring biological season in the pelagic zone despite the retention of the winter hydrological conditions over the area studied.