AN EVALUATION OF THE SURVIVAL OF EXPERIMENTAL POPULATIONS EXPOSED TO HYPOXIA IN THE SAVANNA RIVER ESTUARY

The occurrence of hypoxia (< 2 mg l-1) is being reported with increasing frequency in estuarine ecosystems; however the impacts of this environmental stress are often difficult to evaluate, Since changes induced by episodes of hypoxia or anoxia may be impossible to distinguish from the natural variability of biological communities, often it is only possible to infer effects based on measurements of low dissolved oxygen that coincide with events of catastrophic mortality. In the present study, alteration in the operational schedule of a tide gate in the Back River estuary (Savannah, Georgia, USA) enabled an examination of the responses of 3 species of estuarine organisms to hypoxia; survival was measured for caged amphipods Gammarus tigrinus and bivalves Mulinia lateralis and Mercenaria mercenaria deployed near the tide gate during phases of its operation (reduced ebb flow) and nonoperation (normal tidal flow). Water quality parameters showed that while the tide gate was operated during periods of seasonally elevated water temperature, bottom waters downstream of the gate became hypoxic. The interruption of normal ebb flow was clearly linked to the onset of increased stratification and resultant dissolved oxygen depletion. G. tigrinus was more sensitive to hypoxia than the bivalves, and laboratory tests suggested that mortality can result after as little as 3 h of exposure to oxygen saturation levels between 12 and 18 %. The possible ecological consequences of hypoxia in the Savannah River estuary are discussed.