Beyond Science into Policy: Gulf of Mexico Hypoxia and the Mississippi River

TLDR

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Abstract

C oastal eutrophication is a major, global environmental problem that tracks increases in population and the concentration of those increases in coastal regions, increased agricultural production in adjacent river basins, and increasing food and energy consumption.Society has altered global nitrogen and phosphorus cycles and increased the availability of these two nutrients to marine ecosystems through generation of wastewater, application of fertilizers, nitrogen fixation by leguminous crops, and atmospheric deposition of oxidized nitrogen from fossil-fuel combustion (Peierls et al. 1991, Howarth et al. 1996).These increased nutrient loads often result in the degradation of water quality as more and more phytoplankton or macroalgae grow, including some noxious and toxic algal species.Additional effects, including increased turbidity, oxygen-depleted (i.e., hypoxic) waters, and loss of habitat, tend to decrease marine biodiversity and alter ecosystem structure and functioning.Changes in the relative proportions of nitrogen, phosphorus, and silicon can exacerbate eutrophication, favor harmful algal blooms, aggravate oxygen depletion, and alter marine food webs (Officer and Ryther 1980, Conley et al. 1993, Justic et al. 1995, Turner et al. 1998a).Over the last half of the 20th century, it became increasingly apparent that these symptoms of eutrophication were not minor and localized, but rather had large-scale ecosystem implications and were spreading rapidly (Diaz and Rosenberg 1995, Nixon 1995, Bricker et al. 1999, NRC 2000, Cloern 2001).The linked Mississippi and Atchafalaya Rivers and northern Gulf of Mexico system is a prime example of the worldwide trend of increasing riverborne nutrients and resulting diminution of coastal water quality.The Mississippi River system ranks among the world's top 10 in length, freshwater discharge, and sediment delivery, and it drains 41% of the contiguous United States (Milliman and Meade 1983).A third of the flow from the Mississippi River mainstem is diverted into the Atchafalaya River, where it joins the Red River for eventual delivery through a delta 210 kilometers west of the main Mississippi River birdfoot delta.Prevailing currents from east to west move most of the freshwater, suspended sediments, and dissolved and particulate nutrients onto the Louisiana and Texas continental shelf.At the terminus of this massive river system is the largest zone of oxygen-depleted coastal waters in the western Atlantic Ocean (Figure 1).