Predicting the response of Gulf of Mexico hypoxia to variations in Mississippi River nitrogen load

The effects of nutrient loading from the Mississippi River basin on the areal extent of hypoxia in the northern Gulf of Mexico were examined using a novel application of a dissolved oxygen model for a river. The model, driven by river nitrogen load and a simple parameterization of ocean dynamics, reproduced 17 yr of observed hypoxia location and extent, subpycnocline oxygen consumption, and cross-pycnocline oxygen flux. With Monte Carlo analysis, we illustrate through hindcasts back to 1968 that extensive regions of low oxygen were not common before the mid-1970s. The Mississippi River Watershed/Gulf of Mexico Hypoxia Task Force set a goal to reduce the 5-yr running average size of the Gulf’s hypoxic zone to less than 5,000 km2 by 2015 and suggested that a 30% reduction from the 1980–1996 average nitrogen load is needed to reach that goal. Here we show that 30% might not be sufficient to reach that goal when year-to-year variability in ocean dynamics is considered. The evolution and management of summer hypoxia in bottom waters of the northern Gulf of Mexico have recently received considerable scientific and policy attention because of the enormous size of the hypoxic zone and because of its implications for watershed management within .40% of the continental United States—the Mississippi River Basin (Turner and Rabalais 1994; CENR 2000; Mississippi River/ Gulf of Mexico Watershed Nutrient Task Force 2001; Mitsch et al. 2001; Rabalais et al. 2002a). Extensive regions of bottom oxygen concentrations ,2 mg L21 form off the LouiAcknowledgments Data management, manipulation, contouring, and length determinations were done by Ben Cole. This paper is partially a result of research funded by the National Oceanic and Atmospheric Administration Coastal Ocean Program under award NA06OP0528 to Louisiana Universities Marine Consortium (LUMCON) and awards NA06OP0529 and NA06OP0526 to Louisiana State University. Funds for determining the size of the hypoxic zone since 1985 have come from NOAA Coastal Ocean Program and National Ocean Service, Louisiana Board of Regents; LUMCON, and Louisiana Sea Grant Program. Earlier versions of this manuscript benefited from reviews by Donald Pryor, Thomas O’Connor, David Scheurer, Michael Dowgiallo, Kenric Osgood, and two anonymous reviewers. siana coast each spring and summer (Rabalais and Turner 2001). These regions have recently extended 600 km westward from the mouth of the Mississippi River past the Texas border. These hypoxic regions averaged 8,300 km2 in 1985– 1992 and increased to an average of 16,000 km2 in 1993– 2001 (Rabalais et al. 2002a). An assessment of the causes and consequences of hypoxia concluded that the almost threefold increase in nitrogen load to the Gulf (Goolsby et al. 2001) has driven the long-term increase in hypoxia since the middle of the last century (CENR 2000; Rabalais et al. 2002a). Riverine nitrogen input stimulates coastal algal production and the subsequent settling of organic matter below the pycnocline. Because the pycnocline isolates deeper waters from the surface and inhibits vertical oxygen flux, decomposition of organic matter below the pycnocline consumes oxygen faster than it is replenished, and oxygen concentrations decrease dramatically. Variability in climate and ocean dynamics control much of the interannual variability in hypoxia extent (Rabalais et al. 1999, 2002a), which can confound the understanding of its response to the management actions being proposed for the basin (Mississippi River/Gulf of Mexico Watershed Nutrient Task Force 2001; Rabalais et al. 2002a). We use a