Reducing Hypoxia in an Urban Estuary Despite Climate Warming

Seasonal hypoxia is a serious threat to coastal ecosystems. This study on hypoxia in Long Island Sound (LIS), a large urbanized estuary, focuses on responses to managed nitrogen load reductions and climate change. At the analyzed station in western LIS, warming in bottom waters (0.8 °C per decade) favors hypoxia. Total nitrogen concentrations have decreased (0.06 mg L^-1 per decade) with load reductions, but no linear temporal trend in chlorophyll is discernible. Bottom dissolved oxygen has increased (0.48 mg L^-1 per decade), despite warming-induced solubility decreases (0.13 mg L^-1 per decade). Decreasing trends in hypoxic area and volume (100 km² and 1 km³ per decade) reflect improved conditions and are coincident with reducing loads. Regressions link hypoxic extent to nitrogen loads, chlorophyll, salinity, and winds. Though mitigation has reduced hypoxia, these improvements will not be sustained in the warming climate without continued intervention. The warming-induced oxygen solubility decrease forecasted for 2099 (0.4 mg L^-1) would erode 35% of the observed oxygen gains. Implementing a nitrogen load reduction of 1.2 × 10⁶ kg year^-1 before the century's end would offset the oxygen solubility decline. This overall approach is applicable to areas experiencing warming and continued development that complicate efforts to reign in hypoxia.