Hypoxia means 'low oxygen'. In aquatic and marine systems, low oxygen generally refers to a dissolved oxygen concentration less than 2 to 3 milligrams of oxygen per liter of water (mg/L), but sensitive organisms can be affected at higher thresholds (4.5 mg/L). A complete lack of oxygen is called anoxia. Hypoxic waters generally do not have enough oxygen to support fish and other aquatic animals, and are sometimes called dead zones because the only organisms that can live there are microbes.
Hypoxia is a mounting problem affecting the world’s coastal waters, with severe consequences for marine life, including death and catastrophic changes. Low oxygen conditions are forecast to increase due to increased nutrient input and global warming. Hypoxia has been shown to trigger mortality events that deplete animals in the ecosystem, resulting in so-called ‘‘dead zones’’ devoid of fisheries resources, such as fish, shrimp, and crabs.
The conceptual diagram illustrates the process that can create low oxygen levels in an estuary.
The largest zone of oxygen-depleted coastal waters in the United States is found in the northern Gulf of Mexico on the Louisiana/Texas continental shelf. It is influenced by the freshwater discharge and nutrient load of the Mississippi River system. The mid-summer bottom areal extent of hypoxic waters (≤ 2 mg) in 1985–1992 averaged 8,000 to 9,000 km2 but increased to up to 16,000 to 20,000 km2 in 1993–2000. There is considerable evidence that nutrient-enhanced primary production in the northern Gulf of Mexico is related to the oxygen depletion in the lower water column.
As south Florida has developed, the amount of nutrients coming into the Caloosahatchee River and Estuary has increased. This, in turn, has increased the amount of hypoxic events and decreased biodiversity.
The Fort Myers and Beautiful Island RECON stations have documented significant hypoxic events annually. These events usually occur during the beginning of wet season when water column stratification (due to moderate freshwater flows) and benthic (lowest level of a body of water) oxygen uptake are both increasing.