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Effects of Low Dissolved Oxygen in the Chesapeake Bay.
II. Trophic Interactions in a
Zooplankton-Fish Larvae-Nekton Assemblage
Mesocosms and small scale experiments clearly show that the effect of low oxygen on trophic interactions is highly dependent on the behavioral and physiological responses of interacting species. Consumption of larvae by sea nettles greatly increases at oxygen concentrations < 2.5 mg/L, probably because decreased swimming speeds of larvae makes them more likely to become entrained in vortices shed by contractions of the medusae bell. In contrast, predation by juvenile striped bass feeding on larvae decreased at oxygen concentrations of < 4 mg/L in mesocosms. Thus for a single prey type, low oxygen has markedly different effects for different predators. A similar dependence on individual species responses can be seen if one considers a single predator and suite of alternate prey. Although predation by sea nettles under low dissolved oxygen increases for larval prey, it is unaffected for copepod prey, and increases for bay anchovy eggs. When prey have effective escape behavior under high oxygen conditions, the extent to which low oxygen compromises prey behavior can determine the outcome of predator-prey encounters. When prey are entirely passive (e.g. fish eggs), only low oxygen effects on predator behavior alter outcomes of predator-prey interactions. Cruise samples indicate that both fish larvae and gelatinous predators are abundant in subpycnocline waters at oxygen concentrations of < 2 mg/L. Other studies have found that juvenile striped bass occur in waters with >4 mg/L. Therefore, low oxygen has the potential to strongly alter the relative importance of different trophic pathways at oxygen concentrations that are relevant to the interacting species. In addition, low oxygen in a stratified water column such as Chesapeake Bay alters encounter rates between species because of strong avoidance of oxygen concentrations that lead to mortality during relatively short exposures.
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Edward Houde
Chesapeake Biological Laboratory
University of Maryland Center for Environmental Science
Denise Breitburg
Estuarine Research Center
The Academy of Natural Sciences
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