BENTHIC BACTERIAL PRODUCTION AND NUTRIENT SEDIMENT-WATER EXCHANGE IN SANDY NORTH-SEA SEDIMENTS

The relationship between benthic bacterial production and biomass and sediment-water exchange rates of inorganic nutrients was examined in the North Sea (Dogger Bank region), in summer. The sediments were sandy, poor in organic matter and with low buffering capacities for nutrients. Fluxes of dissolved inorganic N (DIN) and P (DIP) across the sediment-water interface were measured using undisturbed sediment cores. Bacterial production (H-3-thymidine incorporation method) in the sediment surface layer and bacterial biomass were analysed in the same sediment. By comparison of the benthic activities at 13 stations we assessed significant negative relations between bacterial production and sediment-water fluxes of nutrients. With increasing bacterial production the net efflux of DIN and DIP decreased. Bacterial production was significantly related to bacterial biomass and bacterial biomass was significantly related to chlorophyll a and pheopigment in the sediment, suggesting that enhanced bacterial production rates were associated with input of organic carbon. The observed relationship between bacterial production and nutrient sediment-water exchange could be explained by temporary immobilization of N and P in bacterial biomass. It was demonstrated that in response to a short-term sedimentation event, the increase in benthic bacterial biomass (over a 5 d period) may equal the decrease in sediment-water effluxes of DIN and DIP. Heterotrophic benthic bacteria can apparently act as sinks for N and P, which only release N and P upon death.