Water column and sediment geochemistry in the Eastern Gotland Basin after a major Baltic inflow in June 2016

The Baltic Sea is characterized by the largest area of hypoxic (oxygen <2 mg/L) bottom waters in the world's ocean induced by human activities. Natural ventilation of these oxygen depleted waters largely depends on episodic Major Baltic Inflows from the adjacent North Sea. In 2014 and 2015, two such inflows led to a strong rise in oxygen and decline in phosphate in waters below 125 m depth in the Eastern Gotland Basin. This provided the opportunity to assess the impact of such re-oxygenation events on the cycles of manganese, iron and phosphorus in the sediment for the first time. We demonstrate that the re-oxygenation induced the activity of sulphur-oxidising bacteria, known as Beggiatoaceae in the surface sediment where a thin oxic and suboxic layer developed. At the two deepest sites, strong enrichments of total manganese and to a lesser extent iron oxides and phosphorus were observed in this surface layer. A combination of sequential sediment extractions and synchrotron-based X-ray spectroscopy revealed evidence for the abundant presence of phosphorus-bearing rhodochrosite and manganese(II) phosphates. In contrast to what is typically assumed, the formation of iron oxides in the surface sediment was limited. We attribute this lack of iron oxide formation to the high flux of reductants, such as sulphide, from deeper sediments which allows iron(II) in the form of iron monosulphide to be preserved and restricts the penetration of oxygen into the sediment. We estimate that enhanced phosphorus sequestration in surface sediments accounts for only ∼5% of water column phosphate removal in the Eastern Gotland Basin linked to the recent inflows. The remaining phosphate was transported to adjacent areas in the Baltic Sea. Our results highlight that the benthic oxygen demand arising from the accumulation of organic-rich sediments over several decades, the legacy of hypoxia, has major implications for the biogeochemical response of euxinic basins to re-oxygenation. In particular, phosphorus sequestration in the sediment in association with iron oxides is limited. This implies that artificial ventilation projects that aim at removing water column phosphate and thereby improving water quality in the Baltic Sea will likely not have the desired effect.