Coupling between climate variability and coastal eutrophication: Evidence and outlook for the northern Gulf of Mexico

It is generally believed that coastal eutrophication is primarily controlled by the magnitude of anthropogenic nutrient loading and this cause-effect relationship is often used as a common explanation for the widespread eutrophication observed during the second half of the 20th century. This paper examines the coupling between climate variability and coastal cutrophication, and discusses how future changes in climate may affect nutrient fluxes to the coastal zone, nutrient ratios, phytoplankton production and the severity of hypoxia. We focus on the northern Gulf of Mexico, a coastal ecosystem dominated by inflow of the Mississippi River, where recorded decadal and interannual variations in the size of a large hypoxic zone (> 2 x 10(4) km(2)) provide examples of anthropogenic and climatic controls on eutrophication. Using a mathematical model, four hypothetical future climate scenarios were examined. The scenarios were based on projected changes in the Mississippi River discharge, nitrate flux, and ambient water temperatures, and the simulation results were compared to the standard model. The forcing functions in the standard model included the observed time-series of temperature, riverine freshwater discharge and nitrate flux over the 45-y period 1955-2000. In all four model scenarios, simulated frequency of hypoxia differed significantly from the standard model, ranging from a 58% decrease to a 63% increase. The Gulf of Mexico responses to climate-driven variations in freshwater inflow may not be representative for other coastal ecosystems. A comparison of the northern Gulf of Mexico and the Hudson River estuary revealed that the increased riverine freshwater inflow, which causes cutrophication in the northern Gulf of Mexico, improves trophic conditions in the Hudson River estuary. Hence, the degree to which coastal eutrophication will be affected by future climate variability will vary from one system to another, depending on the characteristics of the physical environment and the current eutrophication status. (c) 2005 Elsevier B.V. All rights reserved.