ABSTRACT: The importance of denitrification and microphytobenthos for nitrogen removal and retention was studied over an annual cycle in 2 microtidal shallow-water sediments, 1 sandy and 1 silty, on the west coast of Sweden. Denitrification rates (isotope-pairing) and oxygen and inorganic nitrogen fluxes were measured in undisturbed sediment incubated in the laboratory at in situ temperature, at mean monthly light intensities and in the dark. Algal N assimilation rates were calculated from primary productivity and N retention in the microphytobenthic biomass from chlorophyll a concentrations. Denitrification ranged from ca 1 to 1360 µmol N m^-2 d^-1, being generally 1 order of magnitude higher in the silty sediment. The sandy sediment was net oxygen producing throughout the year and the silty sediment during most of the year. The rate of algal N assimilation calculated from daily net primary productivity was 30 to 7000 µmol N m^-2 d^-1, generally exceeding denitrification by 1 to 2 orders of magnitude. The low denitrification rates are explained by competition for external and internally circulated substrate by microalgae, and a general negative effect of microalgal photosynthesis on denitrifiers. The results suggest that, in northern climate microtidal sediments, microphytobenthic N incorporation is more important than N removal by denitrification.

KEY WORDS: Sediment · Nitrogen · Denitrification · Primary production · Microphytobenthos

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Published in AME Vol. 22, No. 3 (2000) on October 26
ISSN: 0948-3055. Copyright © Inter-Research, Oldendorf/Luhe, 2000