Phosphorus fertilisation (eutrophication) is expanding oxygen depletion in coastal systems worldwide. Under lowoxygen bottom-water conditions, phosphorus release from the sediment is elevated which further stimulates primary production. It is commonly assumed that re-oxygenation could break this 'vicious cycle' by increasing sedimentary phosphorus retention. Recently, a deep-water inflow into the Baltic Sea created a natural in-situ experiment that allowed us to investigate if temporary re-oxygenation stimulates sedimentary retention of dissolved inorganic phosphorus (DIP). Surprisingly, during this three-year-long study, we observed a transient but considerable increase, rather than a decrease, in the sediment efflux of DIP and other dissolved biogenic compounds. This suggested that the oxygenated inflow elevated the organic matter degradation in the sediment. As a result, the net sedimentary DIP release per m 2 was 35-70% higher over the years following the re-oxygenation than before. In contrast to previous assumptions, our results show that inflows of oxygenated water to anoxic bottom waters can increase the sedimentary phosphorus release. IntroductionEutrophication is one of the main causes of oxygen depletion in coastal systems worldwide (Breitburg et al., 2018;Diaz and Rosenberg, 2008). Excess input of nutrients from land stimulates primary production, resulting in a higher delivery of organic matter to deeper coastal waters and the seafloor (Breitburg et al., 2018;Middelburg and Levin, 2009). Oxygen is consumed as this organic matter is degraded and in severe cases, anoxia can develop. Depletion of oxygen can lead to decreasing biodiversity and thereby a loss of important ecosystem functions and services (