Nitrogen dynamics in nontidal littoral sediments: Role of microphytobenthos and denitrification

Previous measurements from cool microtidal temperate areas suggest that microphytobenthic incorporation of nitrogen (N) exceeds N removal by denitrification in illuminated shallow-water sediments. The present study investigates if this is true also for fully nontidal sediments in the Baltic Sea. Sediment-water fluxes of inorganic (DIN) and organic nitrogen (DON) and oxygen, as well as denitrification, were measured in early autumn and spring, in light and dark, at four sites representing different sediment types. All sediments were autotrophic during the daytime both in the autumn and spring. On a 24-h time scale, they were autotrophic in the spring and heterotrophic in early autumn. Sediments functioned as sources of DIN and DON during the autumn and sinks during the spring, with DON fluxes dominating or being as important as DIN fluxes. Microphytobenthos (MPB) activity controlled fluxes of both DIN and DON. Significant differences between sites were found, although sediment type (sand or silt) had no consistent effect on the magnitude of MPB production or nutrient fluxes. The clearest effect related to sediment type was found for denitrification, although only in the autumn, with higher rates in silty sediments. Estimated N assimilation by MPB, based on both net primary production (0.7-6.5 mmol N m(-2) d(-1)) and on 80% of gross primary production (1.9-9.4 mmol N m(-2) d(-1)) far exceeded measured rates of denitrification (0.01-0.16 mmol N m(-2) d(-1)). A theoretical calculation showed that MPB may incorporate between 40% and 100% of the remineralized N, while denitrification removes <5%. MPB, assimilation of N appears to he a far more important N consuming process than denitrificafion in these nontidal, shallow-water sediments.