Nutrient fluxes and microbial activity in sediment enriched with settled seston

The chemical and microbiological responses of profundal surface sediment were monitored in sediment enriched with natural settled material collected in May, July or August, and with diatom or cyanobacteria cultures. The activation of the sediment microbial community was clearest after the addition of settled seston collected in May which was richer in organic matter than those collected later in the summer. N was released when the sediment was enriched with settled seston which had been collected in July and August and had C:N ratios lower than 10:1. The diatom-rich May-material enhanced the release of P to the interstitial water, obviously because of chemical competition between P and diatom-derived Si at the sediment surface. The July- and August-materials did not release P, nor did the diatom and cyanobacteria cultures. The results thus indicate that the seasonal variations in the quality and quantity of the settling phytoplankton affect the P dynamics at the sediment surface, both chemically and biologically.