Benthic microbial decomposition of organic matter and nutrient fluxes at the sediment-water interface in a shallow coastal inlet of the southern Baltic Sea (Nordrügensche Bodden)

Abstract In summer and autumn in 1996, the concentration, chemical composition and microbial decomposition of organic matter in sediments were investigated in the Nordrugenschen Bodden, shallow coastal lagoons in the southern Baltic Sea. Fluxes of DIN (dissolved inorganic nitrogen, NH 4 , NO 3 , NO 2 ) and DIP (dissolved inorganic phosphorus, o-PO 4 ) from the sediment into the overlying water were calculated based on laboratory incubations of undisturbed sediment cores. Microbial decomposition activity generally increased with increasing organic carbon concentrations, while there was a negative relationship between activity and C/N ratios. Decreasing microbial availability of organic matter with increasing sediment depth was reflected by a shift from the hydrolysis of α-D-glycosidic bounds at the sediment surface to that of β-D-glycosidic bounds in deeper sediment horizons. Microbial remineralization led to DIN and DIP liberation from the sediment into the overlying water. In October (autumn), nutrient release from the sediment supplied 6% of the nitrogen demand, and 9% of the phosphorus demand in the water column while in July (summer), the corresponding values were 8 and 20%. Diel variability of nutrient liberation could be explained by the activities of the benthic photoautotrophic communities.