Anoxia, hypoxia, and dead zones

Abstract

Hypoxia occurs in a wide range of systems and varies in temporal frequency, seasonality, and persistence. In temperate latitudes, bottom waters can remain hypoxic or anoxic for hours to months during summer and autumn. There is no doubt that the increase in the areas within coastal and open oceans with hypoxia is real. Coastal water quality with regard to oxygen is currently on the decline, and the future, based on the continued increase in the global occurrence of hypoxia and current and projected increased loads of nutrients, is trending to more hypoxia. The formation of hypoxic areas has been exacerbated by any combination of interactions that increase primary production and accumulation of organic carbon leading to increased respiratory demand for oxygen below a seasonal or permanent pycnocline. The overall forecast is for hypoxia to worsen, with increased occurrence, frequency, intensity, and duration. The consequences of global warming and climate change are effectively uncontrollable at least in the near term. On the other hand, the consequences of eutrophication-induced hypoxia can be reversed if long-term, broad-scale, and persistent efforts to reduce substantial nutrient loads are developed and implemented. The need for water and resource managers to reduce nutrient loads even if at a minimum, to maintain the current status, is critical in view of globally expanding hypoxia. The future pervasiveness of hypoxia in all ecosystems will depend upon a combination of climate change and land management. Climate change will affect water column stratification, organic matter production, nutrient discharges, and rates of oxygen consumption. Land management will also affect the concentrations of nutrients through agriculture.