Benthic fluxes of oxygen and nutrients under the influence of macrobenthic fauna on the periphery of the intermittently hypoxic zone in the Baltic Sea

Abstract Understanding the role of benthic organisms in marine sediments is becoming increasingly important with the growing problem of eutrophication of marine ecosystems around the world, including the Baltic Sea. Therefore, we have conducted a series of incubation experiments on sediment cores collected from sites characterized by varying oxygen conditions and measured the influx (uptake by sediment) of oxygen as well as the sediment–water exchange of phosphate, ammonia and silicate. The research involved macrozoobenthic communities that clearly reflect oxygen deficiency episodes in the past. We also used Fick's first law to calculate diffusive fluxes of nutrients. The size and direction of benthic fluxes largely depend on physicochemical conditions. Silicate fluxes were affected by the biomass of macrozoobenthos in the surface layer of sediments, while those of phosphate – effluxes – by the biomass of the Baltic tellin Limecola balthica and Marenzelleria polychaetes. Despite oxic conditions in all incubated cores, we observed a phosphate release at most sites. The estimation of substance fluxes at the sediment–water interface based on the incubation experiments gave comparable values – to those of diffusive fluxes – only in areas without benthic communities. We have demonstrated how benthic macrofauna affects the exchange of substances at the sediment–water interface when bottom-water oxygen conditions fluctuate between normoxic and hypoxic.