Linking water quality to larval survival:predation mortality of fish larvae in an oxygen-stratified water column

Low dissolved oxygen concentrations can affect growth rates, distribut~ons and predatorprey interactions of aquatic organisms. Each of these potential effects is generally examined separately in laboratory and field studies. As a result, it is often difficult to determine the net influence of low oxygen on survival and which individual effect of low oxygen contributes most to mortality. We used a spatially explicit individual-based predation model to predict how effects of low dissolved oxygen on vertical distributions, predation rates, and larval growth combine to influence survival of estuarine fish larvae in a water colun~n where the subpycnocline (bottom) and pycnocline layers are subject to oxygen depletion We analyzed simulations involving 3 predators (scyphomedusae based on Chrysaora quinquecirrha, and fish that were relatively sensitive to, or tolerant of, low dissolved oxygen), water columns that dlffered in the relative thickness of the subpycnocline layer, and bottom dissolved oxygen concentrations ranging from < l mg 1-' to no-effect concentrations. The effect of dissolved oxygen on larval and predator vertical hlghest survival generally occurred with hypoxic bottom layers (1 to 2 mg 1-' dissolved oxygen). Bottom dissolved oxygen concentration also strongly affected the relative importance of fish and sea nettle predation in simulations that included both types of predators. Differences among predator types had important consequences for the magnitude and location of predation. Bottom-layer oxygen depletion shifted the focus of trophic interactions into the pycnocline and surface layers. Additional simulations indicated that distributional and capture success effects on larval survival were more important than growth rate effects, and that the direction of effects depended on the predator type and dissolved oxygen concentrations. Limitations of the model as well as implications of results for efforts to reduce nutrient loadings into estuaries are discussed.