Fate of legacy ammonium in the coastal Baltic Sea

Abstract. Eutrophication has enriched coastal sediments globally with organic matter (OM), fuelling internal nutrient loading once benthic hypoxia occurs. Internal ammonium (NH₄⁺) loading may be particularly prominent in the coastal Baltic Sea due to the existence of a substantial NH₄⁺ pool in its sediments, accumulated via OM burial and mineralization over long-term eutrophication. However, despite its potential to exacerbate eutrophication, internal NH₄⁺ loading has so far received little attention in the coastal Baltic Sea. It remains poorly understood for how long the benthic legacy OM may affect both the benthic NH₄⁺ pool and the internal NH₄⁺ loading via sediment-water NH₄⁺ effluxes, especially under varying oxygen availabilities. To reconstruct past and predict future NH₄⁺ effluxes in response to different OM loading and bottom water oxygen conditions, we developed a reactive transport model for muddy, organic-rich sediments of the coastal Baltic Sea. Our model results suggest that the legacy OM in the coastal sediments is the key driver sustaining benthic NH₄⁺ pools and effluxes both today and well into the future. As today’s OM loading is constantly adding new OM to the already existing legacy loading, the benthic NH₄⁺ pool will continuously grow and result in persistently elevated NH₄⁺ effluxes in the future, even under oxic conditions. If external measures strongly reduce OM loading to eventually pre-industrial levels, the NH₄⁺ pool still continues to grow for at least 80 years due to the continued mineralization of legacy OM, which keeps NH₄⁺ effluxes elevated for at least 180 years before eventually returning to pre-industrial levels in the year 2300. These model results highlight the persistence of eutrophication legacy effects and their importance for ecosystem management of the coastal Baltic Sea. The knowledge obtained is beneficial also for other anthropogenically impacted coastal seas with similar geomorphology as the Baltic Sea.