The Mississippi River Delta is one of the most biologically diverse areas in the United States, teeming with fish, waterfowl, and dense vegetation. The Mississippi River is essential for the maintenance of this estuarine environment, depositing nutrients and sediment into the delta and Gulf of Mexico. However, this mass nutrient deposition causes eutrophic conditions and a large hypoxic zone every year. This “dead zone” can exceed 20,000 square km and can leave potential habitat within the zone uninhabitable.
Hypoxic zone formation is a eutrophication process . Excess nutrients, primarily nitrogen and phosphorus, often enter the river from sources of agricultural runoff and waste. As nutrients flow down the Mississippi River, levels of primary production increase along with organic matter (Rabalais et.al. 2002). Degradation of the organic matter is mostly done by oxygen-consuming microbes which deplete dissolved oxygen.
Excess nutrients and affected primary production have the potential to change the dynamic of an entire ecosystem. One of the most noticeable effects is displacement of aquatic organisms. Fish breathe by buccal pumping where oxygen is filtered out of water by the gills, so depleted oxygen in water decreases survivability, and many swimming organisms cope by simply leaving the hypoxic area (Rabalais et.al. 2002, Bryant 2010). Shifts in partial oxygen can increase the oxygen-binding affinity of hemoglobin. Direct mortality also negatively affects the ecosystem. Fish populations that feed mainly on primary producers increase, while others decrease due to oxygen deficiency and depleted food sources (Rabalais et.al. 2002).
References
Rabalais, N. N., R. E. Turner, and W. J. Wiseman. 2002. Gulf of mexico hypoxia, a.k.a. “The dead zone.” Annual Review of Ecology and Systematics 33:235–263.
Bryant, M.D. 2010. Past and present aquatic habitats and fish populations of the Yazoo-Mississippi Delta. Gen. Tech. Rep. SRS–130.