Infrequent, episodic stress can have long-term effects on ecological communities. In shallow ponds or lakes, episodic winter hypoxia (low oxygen) can affect benthic invertebrate communities, either through direct mortality, or by promoting pulses of fish predation. In such waters, benthic areas become hypoxic while adequate oxygen often remains near the ice-water interface. To persist, benthic taxa must tolerate low oxygen or migrate to regions with adequate oxygen. However, behaviors such as increased activity or movement toward the ice-water interface that are adaptive in avoiding low oxygen conditions would be maladaptive in avoiding fish predation. Although avoidance of low oxygen and behavioral changes in the presence of fish have been documented separately, no one has examined how invertebrates balance these conflicting demands. This research will examine the interaction of an abiotic stress (winter hypoxia) and a biotic factor (fish predation) in structuring benthic invertebrate assemblages. In laboratory experiments, the ability of benthic invertebrates to balance the conflicting demands of avoiding both hypoxia and fish predation will be examined. Taxa will be ranked according to their vulnerability to fish predation as the benthic environment becomes hypoxic. These predictions will then be tested in outdoor ponds where hypoxia of the benthic region will be manipulated. The behavioral response of benthic invertebrates to developing hypoxia in ponds with and without fish will be monitored using an underwater video camera. To determine if laboratory rankings of species' vulnerability are observed in the field, fish diets will be be quantified before, during, and after the benthic zone becomes hypoxic. Abundance of invertebrate taxa before and after benthic hypoxia will allow us to determine the effect of winter hypoxia alone and in combination with fish predation in structuring benthic invertebrate assemblages.
