Nitrogen fluxes, denitrification and the role of microphytobenthos in microtidal shallow-water sediments: an annual study

The annual cycle of oxygen and nitrogen flux, denitrification and microphyt obenthic variables (primary production, biomass, composition and calculated N demand) were studied for 2 shallow-water microtidal sediment sites in NE Kattegat, 1 sandy and 1 silty, by incubating undisturbed sediment: in the laboratory in light and darkness. Both sites (2 stations within each) were characterised by low concentrations of inorganic nitrogen (IN) in the overl ying water during summer (NO3 generally <1 mu M), with winter concentration s of 5 and 30 mu M for the sandy and silty site, respectively. Through the activity of microphytobenthos, the sediment systems appeared to be net auto trophic during most of the year. Net oxygen production varied between -400 and 6600 pmol m(-2) h(-1), being highest during the warmest season. Althoug h the composition of the microphytobenthos depended on the sediment type, i t did not have a crucial effect on the magnitude of the microphytobenthic b iomass or function. The temporal pattern of the function of the microalgal community, on the other hand, was significantly influenced by the sediment type; sandy sediment exhibited a smooth seasonality, controlled mainly by t emperature and light, while the silty microtidal sediment was also controll ed by stochastic events, such as sediment resuspension. Microphytobenthos h ad a significant effect on the IN flux, the dearest effect being found for NH4. Total denitrification (isotope-pairing technique) generally varied bet ween <1 and 40 mu mol m(-2) h(-1), being dominated by nitrification-coupled denitrification (D-n), and being 1 order of magnitude higher at the silty site. Microphytobenthic activity generally inhibited denitrification in the low-N areas in this study. The results suggest that the microphytobenthos functions as a major control throughout the annual cycle, by forming commun ities that are net photoautotrophic throughout the year, and by significant ly influencing both the IN flux and denitrification rates. Sandy sediment a ppeared to function as an IN sink during winter and early spring, while no clear seasonal pattern was found for silty sediment. Calculated N demand of the microphytobenthos far exceeded the measured sediment net uptake of N, supporting the idea that sandy systems in low-nitrogen areas can be highly productive through a closed recycling of N. The ratio between calculated mi crophytobenthic N demand and measured denitrification rates suggests that d enitrification has a minor role as a N sink, particularly in sandy, cold-cl imate microtidal sediments.