VARYING EFFECTS OF LOW DISSOLVED OXYGEN ON TROPHIC INTERACTIONS IN AN ESTUARINE FOOD WEB

Ecological studies, including those focusing on coastal eutrophication, vary in the emphasis they place on species-level vs. ecosystem-level processes. The degree of variation among interacting species in their response to perturbations to the physical environment is likely to be important in determining when species- or population-level processes will strongly affect attributes measured at higher levels of ecological organization. We conducted mesocosm and small-scale laboratory experiments to determine how low oxygen affects predation rates in a zooplankton–fish larvae–larval predator food web typical of mesohaline areas in the Chesapeake Bay. Dissolved oxygen concentrations in bottom waters of the Chesapeake Bay decline during summer to levels that can be physiologically stressful or lethal to animals dependent on aerobic respiration. Our results indicate that the effects of low oxygen on trophic interactions vary among interacting pairs of species in the food web studied. Low but nonlethal dissolved oxygen concentrations greatly increased predation on fish larvae (mostly naked goby Gobiosoma bosc) by sea nettles (the scyphomedusan jellyfish Chrysaora quinquecirrha) but decreased predation by juvenile striped bass (Morone saxatilis). Predation by a single predator, the sea nettle, increased for fish larvae, decreased for fish eggs (Anchoa mitchilli), and was significantly but not strongly affected for copepods (mostly Acartia tonsa) at low dissolved oxygen concentrations. Changes in predator–prey interactions reflected variation among species in their physiological tolerance to low oxygen and the effects of low oxygen on the escape behavior of prey, as well as on swimming and feeding behaviors of predators. Because of the variation in effects on trophic interactions, low dissolved oxygen has the potential to cause major alterations in the relative importance of different pathways of energy flow in the Chesapeake Bay and in other estuarine systems.