Thresholds Database > Hypoxia in the Gulf of Mexico (1980s)

Background

The inner and middle continental shelf of the northern Gulf of Mexico is the largest and most severely affected area in North America subject to seasonal hypoxia (dissolved oxygen levels < 2mg/L or < 1.4ml/L, Rabalais et al., 1991). From 1985 to 1988, hypoxic water was found from April to October from 5 to 60m water depth and from 5 to 60km offshore, extending up to 20m above the bottom and covering up to 9500km2. After major flooding in 1993, the hypoxic zone in the Gulf of Mexico doubled to 18,000 km2 and has not shrunk much since (Kaiser 1996). Although hypoxia was first discovered in 1973 (Diaz and Rosenberg 1995), it was not until 1978 that it caused extensive mortalities.

Alternate Regimes

1. Pre hypoxic period (pre-1973?); fauna includes large and long-lived sedentary species



2. Hypoxic period (post-1973);

a) October to April (winter); benthic community is dominated by opportunistic species characteristic of an early successional state (Boesch and Rabalais 1991)

b) April to October (summer); only highly resistant taxa persist (some polychetes and nematodes)

Mechanism

Hypoxia occurs when oxygen concentrations on the sea floor are reduced to levels low enough to cause severe stress and mass mortality of ocean fauna (Paine et al. 1998). The effects on fauna range from avoidance of the affected area to mortality of more sensitive taxa (e.g. crustaceans and echinoderms), to emergence of the redox potential discontinuity from the sediments, a condition where only chemoautotrophic bacteria can live. Many fish species suffocate when O2 drops below 2mg O2L-1 (Diaz 2001).



High nutrient levels, in particular nitrogen, flow down the Mississippi River and its tributaries into the Gulf of Mexico fueling phytoplankton production (Turner and Rabalais, 1994). The hypoxia is associated with strong salinity-controlled stratification during the warmer months of the year, which restricts re-oxygenation of the bottom waters. Large quantities of decomposing phytoplankton enhance the effects of stratification on oxygen depletion. The severity and extent of hypoxia varies depending on river-flow, shelf circulation and tropical storm mixing (Rabalais et al., 1991).



Abundances of finfish, shrimp and swimming crabs are severely reduced in hypoxic areas and the periods of oxygen stress includes critical life-history periods of several commercially important species (Paine et al. 1998). It is suspected that large declines in fish stocks with intensified commercial fishing in the 1970s and 1980s along with the hypoxic conditions have affected both nursery areas and food chains for commercial species.

Management Decisions in Each Regime