Perhaps the best known example of hypoxia is the so-called dead zone that occurs in the Gulf of Mexico each summer over an area of up to 6,000 square miles. An overload of nutrients, notably nitrogen and phosphorus, can trigger excess algae growth and decrease the amount of oxygen dissolved in the water. "The severity and spatial scale of hypoxia have increased in the last few decades," says Rudolf Wu of the City University of Hong Kong, the lead author of the new report. "Due to rapid human population growth, increases in nutrient input and global warming, the problem is likely to become worse in the coming years."
Wu and his colleagues tested the effects of low oxygen levels on sexually immature carp. Half of the fish were raised in waters containing normal levels of oxygen while the other half inhabited waters with just 15 percent of the normal oxygen supply. For 12 weeks, the researchers monitored the development of the fish. They found that oxygen-starved fish had lower hormone levels--and significantly smaller sexual organs--than the fish raised under normal oxygen conditions did. In addition, less than 5 percent of the larvae hatched by the hypoxic fish survived, as compared with approximately 90 percent for the normal fish. Wu warns that this drastic reduction of reproductive output could contribute to major population declines in all sorts of aquatic creatures. "There is every good reason," he says, "to believe that hypoxia may also cause endocrine disruption in other fish and amphibian species, as well as in other animal groups."