Balance between denitrification and microalgal incorporation of nitrogen in microtidal sediments, NE Kattegat

The importance of denitrification and microphytobenthos for nitrogen remova l and retention was studied over an annual cycle in 2 microtidal shallow-wa ter sediments, 1 sandy and 1 silty, on the west coast of Sweden. Denitrific ation rates (isotope-pairing) and oxygen and inorganic nitrogen fluxes were measured in undisturbed sediment incubated in the laboratory at in situ te mperature, at mean monthly light intensities and in the dark. Algal N assim ilation rates were calculated from primary productivity and N retention in the microphytobenthic biomass from chlorophyll a concentrations. Denitrific ation ranged from ca 1 to 1360 mu mol N m(-2) d(-1), being generally 1 orde r of magnitude higher in the silty sediment. The sandy sediment was net oxy gen producing throughout the year and the silty sediment during most of the year. The rate of algal N assimilation calculated from daily net primary p roductivity was 30 to 7000 pmol N m(-2) d(-1), generally exceeding denitrif ication by 1 to 2 orders of magnitude. The low denitrification rates are ex plained by competition for external and internally circulated substrate by microalgae, and a general negative effect of microalgal photosynthesis on d enitrifiers. The results suggest that, in northern climate microtidal sedim ents, microphytobenthic N incorporation is more important than N removal by denitrification.