Ecological Implications

Ecological Analysis of Hypoxia: Altered Energy Flow and Changes in Community Structure

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The 1990 International Conference on Marine Coastal Eutrophication resolved: "It [eutrophication] has increased rapidly worldwide over the last decade; it has become a threat to the functioning of estuarine systems impairing their use for recreation, tourism, fisheries and aqauculture; it has caused substantial economic damage in many parts of the world; it is a potential threat to human health; Governments are urged to develop strategies to bring eutrophication under control (Vollenweider et al., 1992)."

From an ecological perspective the Gulf of Mexico is very unique. Hypoxia has become one of the most significant stressors that is presently influencing and controlling the structure of populations and the flow of energy in the gulf. Seasonal changes in the concentration of dissoloved oxygen affect the habitats of many benthic organisms. A major concern for benthic, epi-benthic and demersal populations is forced migration from habitats that have become unsuitable(Pavela et al., 1983). Hypoxia alters trophic level interactions, affecting flow of energy through the community. This section specifically outlines changes in energy flow which result from the changes in community structure, the alteration of trophic levels, and the overall effect of hypoxia on the benthos.

When bottom waters become hypoxic many motile organisms must migrate out or around the dead zone where they had previously thrived (Lowery, 1998). This causes a loss of the community's secondary productivity and alters energy flow. One of the major primary changes that occurs in these zones is in relation to energy is the amount of the system's energy being transported to microbial decomposition (Pearson and Rosenberg, 1992). Decomposition is an oxygen utilizing process that is the driving force that sustains hypoxia. In the Gulf region affected by the hypoxia, studies have indicated that the energy that was produced was instead biomass in the form of benthic organisms such as worms, snails, and clams (Harper et al., 1991).

There are also potential changes between trophic levels. Many organisms stop feeding when dissolved oxygen levels become hypoxic. The major dilemma under this circumstance is when the hypoxic condition subsides. Typically organisms like demersal fish and crabs can recover quickly however, benthic macorinvertebrates that are prey items can not return to their normal living position within the substrate, thus there is a residual effect on higher trophic levels even after hypoxia is gone (Pihl et al., 1992).

Because the hypoxia in the Gulf of Mexico is generally an annual event most of the dominant benthic species are opportunistic, which means they are tolerant to a wide range of stresses including hypoxia (Diaz and Rosenberg, 1995; Rabalais et al., 1996). Characteristically, opportunists have a high resilience to environmental stress and can recolonize disturbed areas quickly (Boesch and Rosenberg, 1981). Typical benthic communities in the Gulf have developed a late successional stage equilibrium community. The benthic communities that occur in the hypoxic zone tend to exhibit an early successional stage type community that readily adapts to disturbance (but not stress) (Pearson and Rosenberg, 1978). Most of the benthic biomass is comprised of shallow dwelling polychaetes and some mollusks. Most of these organisms are responsible for storing energy that precipitates to the bottom. These organisms also tend to be smaller and have shorter lifespans. Comparatively, long-lived, larger benthic invertebrates tend to store more biomass and can buffer the ecosystem against energy pulses (Odum, 1981). They also have the capability to cycle nutrients more efficiently by burrowing deeper into the substrate (Rhoads, 1986). This process allows for energy to be recycled over longer periods of time whereas, short-lived species can not maintain the energy long enough to be recycled (because of boom and bust population growth and mortality). Therefore, opportunistic organisms are not a good substitute for more longer-living organisms because they cannot withhold large amounts of biomass (energy) and do not live long enough to help cycle nutrients and aid in reoxygenation of sediments.

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