Microplankton and its function in a zone of shallow hydrothermal activity: the Craternaya Bay, Kurile Islands

Biomass and productivity of microplankton were measured in the Craternaya Bay (Kurile Islands), which is influenced by hydrothermal activity and volcanic heating. The hydrothermal fields are situated around its shores and underwater within the 0-20 in depth. A dense 'bloom' of photoautotrophic microplankton was observed there, dominated by diatoms, phytoflagellates and the symbiont-containing ciliate Mesodimum rubrum. The biomass of these ciliates attained 3-11 g m(-3) in the upper water layer. The total biomass of the phototrophic microplankton reached 30-46 g m(-3). The primary production in the water column was, correspondingly, enormously high: 6-10 g C m(-2) day(-1). The depth of the euphotic zone was 7 in. Pelagic photosynthesis was inhibited in the upper 0-1 in by the spreading of a layer of low-salinity hydrothermal water. The numerical density of bacterio-plankton in the upper zone of the water column varied from 1 X 10(6) to 2.9 X 10(6) cells ml(-1), and its wet biomass from 250 to 750 mg m(-3). Its production varied at stations from 70 to 390 mg m(-3) day(-1). Chemosynthesis contributed up to 30% of this production in the sites neighbouring the hydrothermal vents. Outside their direct impact however, its share was negligible. The biomass of heterotrophic planktonic ciliates varied from 30 to 270 mg m(-3). The mechanisms of possible influence of shallow volcanic activity on development and function of microplankton in the Craternaya Bay is discussed.