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, distributions and predator-prey interactions of aquatic organisms. Each of these potentia l effects is generally examined separately in laboratory and field studies. As a result, it is often difficult to determine the net influence of low o xygen on survival and which individual effect of low oxygen contributes mos t to mortality. We used a spatially explicit individual-based predation mod el to predict how effects of low dissolved oxygen on vertical distributions , predation rates, and larval growth combine to influence survival of estua rine fish larvae in a water column where the subpycnocline (bottom) and pyc nocline layers are subject to oxygen depletion, We analyzed simulations inv olving 3 predators (scyphomedusae based on Chrysaora quinquecirrha, and fis h that were relatively sensitive to, or tolerant of, low dissolved oxygen), water columns that differed in the relative thickness of the subpycnocline layer, and bottom dissolved oxygen concentrations ranging from cl mg l(-1) to no-effect concentrations. The effect of dissolved oxygen on larval and predator Vertical distributions, predator capture success and larval growth rates in simulations were based on previous experiments and field sampling in the Patuxent River. a tributary of the Chesapeake Bay, USA. Simulations indicated that bottom dissolved oxygen can strongly affect predation morta lity of fish larvae. Thus, there is the potential for eutrophication to hav e a large effect on larval fish survival, and possibly recruitment, even in the absence of direct effects of low oxygen-induced mortality of larvae or the effects of nutrient enrichment on the abundance of larval prey or pred ators. Depending on predator characteristics and water column depth, lowest larval survival occurred when oxygen concentrations were either <1 mg l(-1 ) or sufficiently high that oxygen concentration had no effect on distribut ions or capture rates; highest survival generally occurred with hypoxic bot tom layers (1 to 2 mg l(-1) dissolved oxygen). Bottom dissolved oxygen conc entration also strongly affected the relative importance of fish and sea ne ttle predation in simulations that included both types of predators. Differ ences 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 sim ulations indicated that distributional and capture success effects on larva l survival were more important than growth rate effects, and that the direc tion of effects depended on the predator type and dissolved oxygen concentr ations. Limitations of the model as well as implications of results for eff orts to reduce nutrient loadings into estuaries are discussed.