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

Biomass and productivity of microplankton were measured in theCraternaya Bay (Kurile Islands), which is influenced by hydrothermalactivity and volcanic heating. The hydrothermal fields are situatedaround its shores and underwater within the 0–20 m depth.A dense ‘bloom’ of photoautotrophic microplanktonwas observed there, dominated by diatoms, phytoflagellates andthe symbiont-containing ciliate Mesodinium rubrum. The biomassof these ciliates attained 3–11 g m^-3 in the upper waterlayer. The total biomass of the phototrophic microplankton reached30–46 g m^-3. The primary production in the water columnwas, correspondingly, enormously high: 6–10 g C m^-2 day^-1.The depth of the euphotic zone was 7 m. Pelagic photosynthesiswas inhibited in the upper 0–1 m by the spreading of alayer of low-salinity hydrothermal water. The numerical densityof bacterioplankton in the upper zone of the water column variedfrom 1 x 10⁶ to 2.9 x 10⁶ cells ml^-1, and its wet biomass from250 to 750 mg m^-3. Its production varied at stations from 70to 390 mg m^-3 day^-1. Chemosynthesis contributed up to 30% ofthis production in the sites neighbouring the hydrothermal vents.Outside their direct impact however, its share was negligible.The biomass of heterotrophic planktonic ciliates varied from30 to 270 mg m^-3. The mechanisms of possible influence of shallowvolcanic activity on development and function of microplanktonin the Craternaya Bay is discussed.