Baltic Sea Hypoxia Takes Many Shapes and Sizes

The Baltic Sea is naturally prone to hypoxia, but the frequency and extent have increased multifold over the last century. Hypoxia manifests itself as perennial in the open central part, seasonal at the entrance area, and episodic at many coastal sites, and the expression of hypoxia is largely driven by differences in bottom water residence times and stratification patterns. Enhanced nutrient inputs from land and atmosphere are the main drivers of expanding hypoxia in the Baltic Sea although deoxygenation has also been exacerbated by increasing temperature over the past 3–4 decades. Hypoxia severely influences ecosystem functions such as fish production through reduced trophic efficiency and harmful cyanobacteria blooms sustained by phosphorus release from sediments. Nutrient inputs from land have created the largest man‐made hypoxic area in the world and the only viable long‐term solution to mitigation is to continue efforts to reduce nutrient loading.