Vertical distribution of number, biomass and size-class spectrum of bacteria in relation to oxic/anoxic conditions in the Central Baltic Sea

Abstract

Total bacterial numbers and bacterial biomass were determined, and the size-class spectrum analysed, of the bacterial population in the Gotland Basin (Central Baltic Sea) during pronounced cyanobacterial blooms. Numbers of aerobic and anaerobic saprophytic bacteria and of sulphate-reducers are also reported. Below the oxic-anoxic interface a morphological change in the microfloral populations was observed. The size-class spectrum in anoxic bottom waters shifted towards larger cell forms. Average cell volume increased about 3-fold over that found in oxic waters. Anaerobic saprophytes and sulphate-reducers became more important in the H sub(2)S-rich bottom waters. Anaerobic micro-environments in O sub(2)-saturated water layers are discussed as important microbiotopes for anaerobic bacteria (e.g. sulphate-reducers).