Phosphorus fertilisation (eutrophication) is expanding oxygen depletion in coastal systems worldwide. Under low-oxygen 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 the 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 3-year 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, likely due to an increase in organic matter supply to the deeper basins, potentially combined with a transient stimulation of the mineralisation efficiency. As a result, the net sedimentary DIP release per m² was 56 %–112 % 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.

中文翻译：

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多年的深水流入事件增加了沉积物中磷的释放

磷肥（富营养化）正在扩大全世界沿海地区的氧气消耗。在低氧底部水条件下，沉积物中磷的释放增加，这进一步刺激了初级生产。通常认为重新充氧可以通过增加沉积磷的保留来打破这种“恶性循环”。最近，流入波罗的海的深水创造了一个自然的原位实验，使我们能够研究暂时的再氧化是否刺激了溶解的无机磷（DIP）的沉积保留。出乎意料的是，在为期3年的研究中，我们观察到DIP和其他溶解的生物成因化合物的沉积物流出量是短暂但相当大的增加，而不是减少。这表明含氧的入渗增加了沉积物中有机物的降解，这可能是由于向较深盆地的有机物供应增加，并可能与矿化效率的短暂刺激相结合。结果，每m的净沉积DIP释放量²高出56％-112％，在之后的复氧比前几年。与以前的假设相反，我们的结果表明，含氧水流入缺氧性底水可以增加沉积物中磷的释放。