Avoiding Hypoxia and Escaping Predators: Examining Behavior Trade-Offs with an Individual Based Model

Zooplankton face multiple stressors including low dissolved oxygen (hypoxia) and predation in many coastal systems. This study evaluated whether copepod behaviors to avoid hypoxia affect their risk of predation, using an individual-based behavior model to test how different behavioral responses may alter that risk. The weighted mean depths of the copepods and their time spent in both hypoxia and predation patches were tracked under three different scenarios (no hypoxia, moderate, and severe hypoxia) to test the effects of hypoxia on vertical distribution and the tradeoff between avoiding hypoxia and predation. With similar amounts of predation stress, the predation risk was highest under the severe hypoxia scenario, followed by the moderate hypoxia scenario, and finally by the no hypoxia scenario. Sensitivity analyses indicated that increasing swimming speed when individuals encountered stressors was the most influential behavior change for allowing copepods to quickly leave an undesirable area, but it also increased the risk of predation from ambush predators. Increasing sinking rate was important for aggregating copepods in deeper depths, and increasing turning angle was critical for maintaining vertical position at a specific layer. These findings suggest a trade-off between avoiding hypoxia and predation. In scenarios with the most severe hypoxia and the most predators at the surface, avoiding hypoxic bottom water by swimming faster, jumping more, sinking less, and turning smaller angles resulted in shallower weighted mean depth and more aggregation between the layers of stressors, which also resulted in copepods spending more time in predation patches and potentially increasing predation risk. The simulation results reflected the field observation that copepod predation mortality was higher under hypoxic conditions and supported the hypothesis that copepod behavior changes under hypoxic conditions could contribute to enhancing predatory mortality.