Heterotrophic nanoflagellates and bacteria in sediment of a brackish water sill basin in the Baltic Sea

This study was performed in 1992–1993 in Pojo Bay, a 30–35 m deep brackish water sill basin situated on the SW coast of Finland. Bacterial productivity as well as abundance of bacteria and heterotrophic nanoflagellates (HNF) in the sediment were studied in the top 10 mm of the sediment at three stations. Bacterial productivity was measured by incorporation of 3H-thymidine Bacteria and heterotrophic nanoflagellates were counted with epifluorescence microscopy. Bacterial abundance in Pojo Bay was higher (up to 1.63×109 cells ml−1) than estimates from more exposed marine intertidal or sandy sediments and seasonal differences were small. Although the bacterial productivity values were low in the cold sediment (range 3×103 to 2×107 cells day−1), they were, nevertheless, within the range measured in muddy or sandy sediments. HNF abundances were highest in spring/summer (5.1×105 cells ml−1) and lowest in autumn/winter summer (1.1×105 cells ml−1), being on the whole among the highest reported from marine sediments. The HNF abundances were significantly different between seasons, whereas spatial differences between stations were not.