Coupling Between Climate Variability and Coastal Marine Eutrophication: Historical Evidence and Future Outlook

Dubravko Justic

Coastal Ecology Institute, and, Department of Oceanography and Coastal Sciences, Louisiana State University, Baton Rouge, LA 70803, USA

The incidence and severity of eutrophication have increased during the last five decades in many estuarine and coastal areas, particularly those affected by riverine freshwater and nutrient inflows. This worldwide trend has been paralleled by an increase in the riverine concentrations of dissolved nitrogen and phosphorus, which has occurred as a result of fertiliser and detergent use in the watersheds. Temporal and spatial associations between the use of nutrients in the watersheds and outbreaks of eutrophication provide convincing arguments for the hypothesis that eutrophication is primarily driven by the by the increasing anthropogenic nutrient loads.

At present, however, little is known about the linkages between climate variability, riverine nutrient fluxes, and coastal marine eutrophication. It was observed that short-term climate anomalies, such as droughts and floods, may substantially alter nutrient delivery to the coastal ocean, and influence surface net productivity, vertical flux of carbon, and oxygen depletion in bottom waters. The northern Gulf of Mexico, which is strongly affected by the Mississippi River, the sixth world largest river, provides ample opportunity to study such influences. Retrospective analyses have revealed pervasive control of variations in the Mississippi River freshwater and nutrient fluxes on oxygen and carbon cycling in the northern Gulf of Mexico. Model simulations for a 2xCO2 climate projected that a change in the global oxygen and carbon budgets would be of the same magnitude, or higher, than that resulting from five decades of anthropogenic eutrophication.