The production and potential loss mechanisms of bacterial biomass in the southern Gulf of Riga

Bacterial biomass and production were measured in the water column and sediment of the southern Gulf of Riga. The potential loss of bacteria in the water column by lysis, grazing and sedimentation was estimated. A generally higher biomass and production of bacteria (135–195 mg C m−3 and 53–80 mg C m−3 day−1) were measured during the midsummer when compared to the spring (56–123 mg C m−3 and 7–26 mg C m−3 day−1) and late summer (51–98 mg C m−3 and 4–16 mg C m−3 day−1) periods. Also heterotrophic nanoflagellate biomass (13–25 mg C m−3) and virus numbers (5–16×1010 l−1) had a maximum during midsummer. The average benthic bacterial production was highest in spring (1132 mg C m−2 day−1) when compared to the other seasons (706–806 mg C m−2 day−1). Benthic bacterial production exceeded the bacterial productivity in the water column above (404–1750 mg C m−2 day−1) in spring and late summer but not in midsummer. Throughout the productive season grazing was estimated to consume 6–50% and viral lysis 55–>100% of the total bacterial production. Loss of bacteria by sedimentation was less than 2% of the bacterial production in the water column.