Seasonal oxygen depletion in the North Sea, a review

•Thermal stratification controlled mainly seasonal North Sea oxygen depletion.•Deviation from stratification correlations revealed hot spots of oxygen depletion.•Oxygen consumption and nitrogen conversion was coupled in bottom waters.•Nitrogen equivalents of oxygen depletion revealed DIN deficits by denitrification.•Thermal stratification was modulated by water depth. Seasonal mean oxygen depletion in offshore and coastal North Sea bottom waters was shown to range between 0.9 and 1.8mg/L, corresponding to 95–83% saturation, between July and October over a 30-year assessment period (1980–2010). The magnitude of oxygen depletion was controlled by thermal stratification, modulated by water depth and nitrogen availability. Analyses were based on about 19,000 combined data sets. Eutrophication problem areas were identified mainly in coastal waters by oxygen minima, the lower 10th percentile of oxygen concentrations, and deviations of oxygen depletion from correlated stratification values. Connections between oxygen consumption and nitrogen sources and conversion, including denitrification, were indicated by correlations. Mean oxygen consumption reflected a minimum seasonal turnover of 3.1gN/m2 in the south-eastern North Sea, including denitrification of 1gN/m2. Oxygen depletion was underestimated in shallow coastal waters due to repeated erosion of stratification as indicated by local high variability.