Phytoplankton and bacterioplankton production and bacterial biomass in a fjord-like bay – open sea gradient

Bacterial production and biomass as well as primary production were compared along a gradient from a fjord-like bay influenced by river runoff (Pojo Bay) to the open sea area on the southern coast of Finland. In the archipelago and open sea areas spring phytoplankton bloom was observed in the end of April, but no spring bloom was observed inside the bay. Between 31 March and 4 September 1992, total primary production increased towards the open sea area (from 18 mg C m−2 in the bay to 73 mg C m−2 in the open sea). At the same time bacterial production was lower inside the bay (7 mg C m−2) than in the archipelago and open sea areas (12 mg C m−2). In Pojo Bay bacterial growth could explain 43–114% of the oxygen depletion in the water layer below 10 m during the productive season. Temperature had a significant effect on bacterial production, but other factors like substrate or nutrient availability may also have been important. The change in the share of bacterial production in the primary production along the gradient (40% in Pojo Bay, 20% in the archipelago, and 16% in the open sea area) indicates the decreasing input of the other sources than phytoplankton in the bacterial carbon need. Bacterial biomass did not vary significantly along the gradient from the bay to the open sea and seemed to be tightly controlled by grazing and/or viral infection especially during the summer.