Anoxia-mediated release of dissolved organic matter from Baltic coastal sediments stimulate further hypoxia.

Coastal waters of the Baltic Sea are prone to widespread and recurring hypoxic and anoxic events. The development of anoxic conditions in bottom waters promotes the release of material into the water column that was otherwise particle bound in sediments. For example the diffusive release of both phosphate and reduced iron from sediment pore waters during anoxic events is well known and an important internal load of P and stimulating eutrophication and potential further oxygen depletion. Coastal sediments are also a source of dissolved organic matter (DOM) and the magnitude and reactivity of the DOM released during anoxia are poorly understood. Our experiments reveal that muddy sediments are a significant source of DOM at coastal sites in the Baltic Sea. Additionally during periods of anoxia DOM released is more bioavailable and therefore capable of contributing to further expansion and persistence of anoxic conditions. We relate the magnitude of the benthic flux of DOM to the water column to other regionally relevant fluxes of DOM, and show that this process is a potentially significant positive feedback on coastal hypoxia.