Nutrient recycling by coastal sediments: effects of added algal material

The deposition of a Skeletonerna bloom onto sediment collected in a coastal bay was simulated and the effects on 02, N and P fluxes were recorded. A comparison of calculated net production and observed net efflux of inorganic nitrogen (IN = ammonium + nitrate) for the period prior to the addition of algae, indicated that two-thirds of the produced ammonium was nitrified and subsequently denitrified. Algal additions induced a dramatic and persistent rise in oxygen consumption (1.6 X ) and release of IN (4.5 X) . DON (dissolved organic nitrogen, 2 X), and Pod3(3 X). Of the added algal N (76 mm01 N m-2) , 33 % was recycled as NH,' and DON during the 19 d of incubation and, according to Pod3measurements, the same was true for the algal P, giving a half-life of the algal biornass of ca 4 wk. Comparison of calculated net production and observed net efflux of IN indicated a considerably decreased denitrification/mineralisation ratio due to algal additions. Extrapolating to the size of Laholm Bay, SE Kattegat (sampling site), suggests that the settlement of algae (realistic amount used here) would make the sediment the most important nutrient source of this bay for a few weeks period. DON was a significant part of the released, non-gaseous nitrogen, both before (46 %) and after (27 %) the addition of algae. In cores subjected to anaerobic conditions, the concentrations of DON decreased considerably, suggesting that the production of DON was inhibited during such conditions and that much of the DON in this sediment was easily utilized.