Nutrients and the “dead zone”: the link between nutrient ratios and dissolved oxygen in the northern Gulf of Mexico

The "dead zone", an area with reduced concentrations of dissolved oxygen, forms every year off the mouth of the Mississippi River in the northern Gulf of Mexico. Some marine animals are stressed or killed by the hypoxic conditions, with negative consequences for this large and economically important marine fishery. In the past, the dead zone has been linked to nitrogen (N) input from the Mississippi River, but recent analyses suggest that phosphorus (P) also plays a role. It has therefore been proposed that controlling both the N and P entering the Gulf may be required to minimize hypoxia. However, the use of elemental ratios (stoichiometric analysis) of dissolved inorganic nutrients to reach this conclusion is scientifically tenuous. Stoichiometric analyses of total N and P and the results of several nutrient enrichment growth bioassays also suggest the importance of both N and P, but offer less evidence for a P effect, providing a stronger scientific basis for management.