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Periodic hypoxia in estuarine ecosystems is a natural phenomenon. The temporal extent of hypoxia and whether a period of anoxia follows is a critical determinant of secondary production. Pihl, Baden, and Diaz (1991) studied the effects of periodic hypoxia on the distribution of demersal fish and crustaceans in the Chesapeake Bay. They found that demersal fish and crustaceans were affected by hypoxia, and that many species were forced to migrate from deeper to shallower waters during the extent of severe hypoxia. Once oxygen levels increased, organisms returned. This forced migration was a potential added stress to a community that is already exposed to significant changes in salinity, temperature, turbidity, and tides (Pihl et al., 1991). A similar study conducted by Baden et al. (1990) on benthic communities off the Swedish west coast found that when oxygen saturation declined below 10 to 15 %, infaunal benthic organisms emerged from sediment and Norway lobsters Nephrops norvegicus became immobilized on the sediment surface. Many organisms were left for predation on the surface and when conditions worsened, many species died. It was also observed that the total biomass, mean abundance, and annual production of flatfish in the Kattegat region decreased (Baden et al., 1991). Hypoxia is not causing a loss in species composition or abundance in this region but it is stressing the system and causing proximal regions like Laholm Bay to maintain the losses in Kattegat. As eutrophication increases with a subsequent increase in the temporal and spatial breadth of hypoxia, the western coast of Sweden may become significantly impacted. Both of these studies support evidence of the benthic community impact caused by excessive nutrient loading from the Mississippi River.
Is secondary productivity affected by hypoxia? The answer to this question depends on the length of time that benthic communities are exposed to low oxygen levels. The amount of productivity in a stressed ecosystem depends on the resilience of the organisms and whether they can recruit and mature during normoxic conditions (Rainer and Fitzehardinge, 1981). Based on evidence from studies on diatom production, sediments, and carbon accumulation there has been a decrease in benthic diversity (Rabalais et. al.,1999). However, quantitative analysis of hypoxia on specific benthic fauna in this region over time is still yet to be determined. There does seem to be a correlation between decreased secondary productivity and the increase in catch-per-unit effort, and ultimately, a decrease in economic success of the Gulf fishery.