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Nutrient over enrichment from anthropogenic sources is a major stressor of aquatic, estuarine, and marine ecosystems. Nutrients enter ecosystems through off-target migration of fertilizer from agricultural fields, golf courses, and lawns; disposal of animal manure; atmospheric deposition of nitrogen; erosion of soil containing nutrients; sewage treatment plant discharges; and other industrial discharges. Excessive nutrients promote nuisance blooms (excessive growth) of opportunistic bacteria, cyanobacteria, and algae. When the available nutrients in the water column have been sequestered in plant biomass, the nuisance blooms die, decompose, and deplete dissolved oxygen in the water column and at the sediment water interface. This oxygen depletion, known as hypoxia, occurs when normal dissolved oxygen concentrations in shallow coastal and estuarine systems decrease below the level required to support many estuarine and marine organisms (≤ 2 mg/L).
Hypoxia can occur naturally in deep basins, fjords, and oxygen minimal coastal zones associated with upwelling. However, nutrient induced hypoxia in shallow coastal and estuarine systems is increasing worldwide. A large hypoxic area, averaging about 16,500 km2 (10, 250 mi2) and ranging from 8,500 to 22,000 km2 (3,100 to 7,700 mi2) forms annually between May and September in the northern Gulf of Mexico. Shown in Figure 1, the northern Gulf hypoxic zone is the largest in the United States and the second largest worldwide. Hypoxic conditions result from complex interactions between climate, weather, basin morphology, circulation patterns, water retention times, freshwater inflows, stratification, mixing, and nutrient loadings. Nutrient fluxes from the Mississippi-Atchafalaya River basin (MARB), coupled with temperature and density induced stratification have been implicated as the primary cause of hypoxia in the northern Gulf of Mexico (NGOM) (CENR, 2000).
Figure 1: Map of the frequency of hypoxia in the northern Gulf of Mexico, 1985-2005. Taken from N.N. Rabalais, LUMCON, 2006.
The MARB is one of the largest river systems in the world (Figure 2), draining approximately 40% of the contiguous United States, and is the largest contributor of freshwater and nutrients to the NGOM. About two thirds of the total Mississippi River flow enters the northern Gulf via the Mississippi River delta. The remaining third is diverted to the Atchafalaya River and eventually enters the northern Gulf about 200 km west of the main Mississippi River delta. Prevailing east-to-west currents in the Gulf move much of the freshwater, suspended sediments, and dissolved and particulate nutrients onto the Louisiana-Texas continental shelf.
Figure 2: Map showing the extent of the Mississippi-Atchafalaya River basin.
Land-use activities in the MARB influence water quality in the entire watershed as well as in the NGOM. Low oxygen events on the Louisiana-Texas continental shelf have been reconstructed over the past 180 years using the relative abundance of low-oxygen-tolerant benthic foraminifera in sediment cores (Osterman et al., 2005). These data show that the prevalence of low oxygen events has increased over the past 50 years. Several hypoxic events from 1870 and 1910 (prior to widespread fertilizer use) were attributed to natural variation in river flow that enhanced freshwater and nutrient transport. The increased prevalence over the past several decades is clearly related to increased nutrient loads. However, there is substantial variation in year-to-year inputs of both freshwater and nutrients from the MARB. Since these are correlated, it is not possible to tease apart the relative importance of increased eutrophication versus increased stratification in any given year over the recent past. Clearly, land-use practices in the MARB affect watershed dynamics and water quality within the Basin as well as the northern Gulf. Land-use practices in the Basin are also influenced by various, and conflicting, national environmental, conservation and agricultural policies.
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