The Effects of Hypoxia on Animal Distributions
Nancy N. Rabalais
The largest zone of oxygen-depleted coastal waters in the United States is in the northern Gulf of Mexico on the Louisiana-Texas continental shelf. From 1993 to 1997, the size of the hypoxic zone was greater than 16,000 km2 in mid-summer. Hypoxic waters (< 2 mg l-1) occur near the bottom and extend to as much as 20 m from the bottom over extensive areas. Oxygen depletion begins in the spring, reaches a maximum in mid-summer and disappears in the fall and winter. The operational definition for hypoxia is based upon the lack of fish and shrimp in trawls when oxygen levels fall below the 2 mg l-1 level.
Hypoxia affects the behavior and distribution of zooplankton similar to results from the Chesapeake Bay. The dominant zooplankton in the northern Gulf of Mexico, copepods, are normally in low abundance or absent from oxygen depleted waters < 1 mg l-1. Copepod nauplii are more affected than adult copepods. Anoxia disrupts the diel migratory behavior.
A fairly predictable pattern in responses of components of the benthic and demersal communities follows a decrease in oxygen concentrations from 2 mg l-1 to anoxia. Motile fish and crustaceans (e.g., crabs, shrimp and mantis shrimp) are generally absent from bottom habitats when the oxygen falls below 1.5-2 mg l-1. Less motile invertebrates die at oxygen levels below 1.5 mg l-1. The organisms that live in the sediments display stress behavior below 1.0 mg l-1. In the community that typically lives in the sediments, the smaller worms, snails, bivalves and crustaceans, there is a fairly linear decrease in benthic diversity and abundance as oxygen concentrations fall from 0.5 mg l-1 to anoxia. Oxygen stressed macroinfaunal communities are characterized by limited taxa (none with direct development, e.g., amphipods), characteristic resistant fauna (e.g., a few polychaetes and sipunculans), a reduced species richness, severely reduced abundances (but never azoic), low biomass, and limited recovery following the abatement of oxygen stress. Meiofaunal communities become reduced in abundance and diversity as the oxygen levels approach zero, but selected nematodes maintain populations. The long-term secondary productivity of the benthos is not known. Differences in benthic foraminiferans demonstrate historic and extant conditons of oxygen stress on the shelf.
Penaeid shrimp avoid hypoxic bottom waters and are concentrated on the inshore and western and eastern margins of the zone. Analysis of long-term data from the northern Gulf of Mexico associated with the by-catch of shrimp trawls indicate that there has been a shift in dominance of the some abundant fishes from those that are associated with the bottom (habitat and food resources) to those that are planktivorous in the upper water column. A bell-shaped curve models a continuum of fishery yield in response to increasing nutrients as ecosystems become eutrophic then dystrophic. In waters with low nutrients, the fishery yield is low. As the quantity of nutrients increases, the fishery yield increases. As the ecosystem becomes increasingly eutrophied, there is a drop in fishery yield but the decreases are variable. The benthos are the first resources to be reduced by increasing frequency of seasonal hypoxia and eventually anoxia; bottom-feeding fishes then decline. Loss of a planktivorous fishery follows as eutrophication increases, with eventually a change in the zooplankton community composition. Where the current Gulf of Mexico fisheries lie along the this model of increasing eutrophication is not known.
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