AME 30:91-101 (2002)  -  doi:10.3354/ame030091

ABSTRACT: The importance of the microphytobenthos (MPB) for nutrient turnover was studied in 2 shallow bays on the west coast of Sweden, during the period that floating green macroalgal mats generally become established in the area. Sediment-water nutrient fluxes (N, P and Si) and oxygen fluxes and denitrification were measured in light and dark laboratory incubations prior to and during the early growth of green-algal mats (April to June). The importance of microphytobenthic activity and hence, trophic status of the sediment system, magnitude of sediment fluxes and presence of macroalgal mats were shown to differ between the 2 bays. On a daily basis, the MPB decreased the efflux of inorganic N by 30 to 100%, P by 70 to 100% and Si by 10 to 95%. Thus, the MPB appears to efficiently control the availability of sediment-derived nutrients to ephemeral green algae during the period that is critical for the onset of the macroalgal growth. The MPB may even depress the development of green-algal mats, providing that these depend on the sediment nutrient efflux. Microphytobenthic N assimilation was compared with other N-consuming processes, such as denitrification and green-macroalgal N-assimilation. In autotrophic sediments, the MPB was the dominating N-consuming component (1 to 5.3 mmolN m^-2 d^-1), denitrification (0.1 to 0.7 mmolN m^-2 d^-1) being a minor N-sink. In the net heterotrophic sediments, however (with no or little net MPB productivity, higher infaunal biomass and hence larger nutrient effluxes), green macroalgal growth was the main N-incorporating process (mean 2.3 mmolN m^-2 d^-1). This was followed by denitrification (0.3 to 1.4 mmolN m^-2 d^-1), which removed roughly 20% of the remineralised N. Thus, denitrification may be a significant N-sink in bays harbouring green-algal mats on the west coast of Sweden.

KEY WORDS: Sediment · Microphytobenthos · Nitrogen · Denitrification · Macroalgal mats · Sediment-water fluxes