Role of microphytobenthos in recovery of functions in a shallow-water sediment system after hypoxic events

Several studies in shallow-water sediments have followed the recovery from hypoxia on a macro- or meiobenthic level, but on the microbenthic level such studies are rare. Outdoor flow-through mesocosms were used to monitor the resilience of a microbenthic community and nutrient turnover after hypoxia in natural, previously undisturbed sediment. Variables included oxygen and nutrient fluxes, denitrification and biomass (ch1 a) of microphytobenthos (MPB). Results showed that the community recovered rapidly after hypoxia (< 15% of oxygen saturation). In a 7 d experiment preceded by 2 d of hypoxia, the initially changed oxygen and nutrient fluxes remained affected only during the first days after hypoxia. In a 50 d experiment, starting with 17 d of hypoxia, only nitrate under dark conditions remained significantly different at the termination of the experiment after a recovery period of 4.5 wk. The microbenthic community, as well as the basal functions, of shallow-water illuminated sediments possess a high resilience, at least after single events with short periods of hypoxia. This resilience appears to be related to the high resistance of MPB (especially diatoms) to hypoxia, which implies a rapid restoration of the oxygenation of the sediment surface and the base of the food web, securing food supply for new colonising grazers. The recovery was faster for functions under light conditions, indicating that the functions related to microalgal activity are less sensitive to hypoxic events than are heterotrophic processes.