Inorganic Nutrient and Carbon Controlled Bacterioplankton Growth in the Baltic Sea

Responses of natural bacterioplankton assemblages to additions of inorganic nutrients (NH4-N and PO4-P) and carbon (sucrose) were studied in seawater cultures. Cultures were obtained from surface water during different stages of phytoplankton succession and prepared by mixing 0·8 μm (Nuclepore filter) and 0·2 μm gravity filtered water (Nuclepore cartridge filter) in 1:10 proportion. Sucrose was taken up by bacteria at a rate up to 54% h-1. The mean percent respiration was 43% in the control units and 57% in the manipulated units. The single sucrose addition markedly stimulated bacterioplankton growth only in the pre-bloom experiment and in the late autumn experiment. During spring bloom, late summer and autumn periods, stimulation of thymidine incorporation rate by inorganic N + P addition was recorded. In all experiments the combined manipulation had the greatest effect on bacterioplankton thymidine incorporation. We thus concluded that the Baltic Sea bacterioplankton is predominantly limited by inorganic nutrients in a proper C:N:P ratio. The C:N ratios > 10 in the dissolved organic matter in the Baltic Sea imply that net uptake rather than release of mineral nitrogen by bacteria occurs during the summer period, when regenerated production period prevails. Due to the large dissolved organic carbon pool in the Baltic Sea and due to the rising trend of inorganic nutrient loading to the Baltic Sea, secondary production via the microbial loop may considerably accelerate the ongoing eutrophication process.