Hypoxia and Anoxia

A common manifestation of eutrophication is hypoxia (dissolved oxygen concentration DO < 2 mg l ) and anoxia (DO = 0), that is, the depletion of dissolved oxy gen in coastal waters, leading to 'dead zones'. When the DO is less than a critical value (typically 2 mg l 1), mobile animals such as demersal fish, crabs, and shrimp migrate away from the area. Resident animals die when the DO < 1 mg l 1. Fisheries have collapsed, notably in the Baltic and Black seas.

Hypoxia occurs naturally in many parts of the world's marine environments, such as fjords, deep basins, open ocean oxygen minimum zones, and oxygen minimum zones associated with upwelling systems. Hypoxic and anoxic waters have existed throughout geologic time, but their occurrence in shallow coastal and estuarine areas is increasing. The severity of hypoxia (either dura tion, intensity, or size) has increased where hypoxia occurred historically, and hypoxia exists now when it did not occur before. The severity of hypoxia increased in the northern Gulf of Mexico, primarily since the 1960s. Evidence comes from paleo indicators in accumulated sediments, long term hydrographic data, and scenarios based on empirical models. The size and frequency of hypoxia in the Gulf of Mexico have increased as the flux of nitrate increased, and there is a direct correlation between nitrate flux to the Gulf of Mexico from the Mississippi River and the mid summer size ofthe hypoxic zone.

Aerobic bacteria consume oxygen during decomposi tion of the excess carbon that sinks from the upper water column to the seabed. There will be a net loss of oxygen in the lower water column, if the consumption rate is faster than the diffusion of oxygen from surface waters to bottom waters. Hypoxia is more likely when stratifica tion of the water column occurs and will persist as long as oxygen consumption rates exceed those of resupply.

Some of the largest hypoxic zones are in the coastal areas of the Baltic Sea, the northern Gulf of Mexico, and the northwestern shelf of the Black Sea (reaching 84 000 km2, 22 000 km2, and 40 000 km2 (until recently), respectively). Hypoxia existed on the northwestern Black Sea shelf historically, but anoxic events became more frequent and widespread in the 1970s and 1980s reaching over areas of the seafloor up to 40 000 km2 in depths of 8-40 m. Recent reductions in nutrient loads to the north western Black Sea resulted in a minimization of the hypoxic zone there. There is also evidence that the sub oxic zone of the open Black Sea enlarged toward the surface by about 10 m since 1970. Similar declines in bottom water dissolved oxygen have occurred elsewhere as a result of increasing nutrient loads and anthropogenic eutrophication, for example, the northern Adriatic Sea, the Kattegat and Skaggerak, Chesapeake Bay, the German Bight and the North Sea, and Long Island Sound. The number of estuaries with hypoxia or anoxia continues to rise.

The obvious effects of hypoxia/anoxia include the displacement of pelagic organisms and selective loss of demersal and benthic organisms. These impacts may be aperiodic if recovery occurs, may occur on a seasonal basis with differing rates of recovery, or may be perma nent so that there is a long term shift in ecosystem structure and function. As the oxygen concentration falls from saturated or optimal levels toward depletion, a vari ety ofbehavioral and physiological impairments affect the animals that reside in the water column or in the sedi ments or attached to hard substrates. Hypoxia also affects optimal growth rates and reproductive capacity. Mobile animals, such as shrimp, fish, and some crabs, flee waters where the oxygen concentration falls below 2-3 mgl . Movements of animals onshore can result in 'jubilees' where stunned fish and shrimp are easily captured, or result in massive fish kills. As dissolved oxygen concen trations continue to fall, less mobile organisms become stressed and move up out of the sediments, attempting to leave the seabed. As oxygen levels fall from 0.5 mg l 1 toward 0, there is a fairly linear decrease in benthic infaunal diversity, abundance, and biomass.

Entire taxa may be lost in severely stressed seasonal hypoxic/anoxic zones. Larger, longer lived burrowing infauna are replaced by short lived, smaller surface deposit feeding polychaetes, and certain typical marine invertebrates are absent from the fauna, for example, pericaridean crustaceans, bivalves, gastropods, and ophiuroids. Increasing oxygen stress for the Skagerrak coast of western Sweden in semienclosed fjordic areas resulted in declines in the abundance and biomass of macroinfauna, particularly mollusks, suspension feeders, and carnivores. These changes in benthic communities result in an impoverished diet for bottom feeding fish and crustaceans.

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