Nutrient fluxes from reduced Baltic Sea sediment: effects of oxygenation and macrobenthos

Effects of bottom water oxygenation and macrofaunal colonisation on benthic fluxes of nitrogen (N), phosphorus (P) and silicon (Si) from long-term anoxic Baltic Sea bottom sediment were investigated.Sediment boxcosms from an anoxic site at 150 m depth in the open Baltic proper were incubated in the laboratory to follow the development of benthic nutrient fluxes during 74 d exposure to flow-through of oxygen-rich water.In contrast to traditional end-point experimental designs, our repeated measurement approach allowed for separation of transient and long-term effects of oxygenation and bioturbation on benthic nutrient recycling.The composition, but not the rate, of the benthic total dissolved N efflux changed by oxygenation from being dominated by NH 4 in situ to being mostly composed of NO 2 + NO 3 and dissolved organic N (DON) under oxic conditions.Oxygenation in the boxcosms decreased the benthic efflux of dissolved silicate (DSi) and essentially shut off the in situ flux of dissolved inorganic phosphorus (DIP).After 20 d of oxygenation, 2 bottom macrofauna taxa, the polychaete Marenzelleria spp.and the amphipod Monoporiea affinis, were introduced to a subset of the boxcosms.Bioturbation by either taxa increased the efflux of dissolved inorganic N (DIN), DON and DSi to the overlying water.The Prich benthic flux under in situ anoxic conditions roughly approached Redfield N:P stoichiometry after oxygenation in the sediment boxcosms.Upon addition of macrofauna, bioturbation generated even higher N:P flux ratios.