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Over 300 years of anthropogenic and naturally induced low-oxygen bottom-water events on the Louisiana continental shelf

Abstract:

Oxygen-depleted subsurface waters occur on the Louisiana continental shelf when the uptake of oxygen exceeds its resupply. When oxygen concentrations fall below 2 mg/L, it is operationally defined as hypoxic. Measurements of Louisiana continental shelf waters demonstrate that the size of the hypoxic zone has increased since 1985 (Rabalais et al. 1999; CENR 2000; Rabalais and Turner 2001) (Fig. 23.1). The likelihood of hypoxia occurring is increased with enhanced nutrient loading and the onset of greater water-column stratification (van der Zwaan 2000; Rabalais 2002). Increased nutrients can stimulate marine surface phytoplankton blooms whose organic matter, upon death, may sink to the bottom and decay. The modern recurrent hypoxia on the Louisiana shelf and expansion of the area of the hypoxic zone has been attributed to the increased use of fertilizer in the Mississippi River basin (Dinnel and Bratkovich 1993; Nelson et al. 1994; Rabalais et al. 1994; Rabalais et al. 1996; Rabalais et al. 1999; Goolsby et al. 2001). The increased frequency and expansion of hypoxic conditions has become an important economic and environmental issue to commercial and recreational fisheries in this region of the Gulf of Mexico (Malakoff1998; Diaz and Solow 1999). Copyright © 2011 by Texas A&M University Press. All rights reserved.