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The effects of low dissolved oxygen on predation interactions between Mnemiopsis leidyi ctenophores and larval fish in the Chesapeake Bay ecosystem

Title of Document: THE EFFECTS OF LOW DISSOLVED OXYGEN ON PREDATION INTERACTIONS BETWEEN MNEMIOPSIS LEIDYI CTENOPHORES AND LARVAL FISH IN THE CHESAPEAKE BAY ECOSYSTEM Sarah Elizabeth Kolesar, Doctor of Philosophy, 2006 Directed By: Senior Scientist Denise L. Breitburg, Smithsonian Environmental Research Center Professor Walter R. Boynton, University of Maryland, Chesapeake Biological Laboratory Differences in predator and prey tolerances to low dissolved oxygen (DO) concentrations are important to planktonic food webs in seasonally hypoxic environments like Chesapeake Bay. Hypoxia alters field distributions, encounter rates, and predatorprey interactions between hypoxia-tolerant ctenophores, Mnemiopsis leidyi, and less tolerant ichthyoplankton and zooplankton prey. To examine the effect of hypoxia on estuarine food web species’ interactions, I conducted medium and small-scale experiments, field sampling, and collaborated on individual-based model development, focusing on ctenophore-larval fish dynamics. Laboratory estimates of clearance rates for ctenophores on bay anchovy (Anchoa mitchilli) eggs and yolk sac larvae, and naked goby (Gobiosoma bosc) feeding larvae were the same at low and high DO. Field sampling for M. leidyi, ichthyoplankton, mesozooplankton, and scyphomedusae (Chrysaora quinquecirrha) during day and night at two sites in the Patuxent River indicated increased abundance of most species in the bottom layer with increasing bottom DO. Vertical overlap between predator and prey pairs also increased with higher bottom DO, increasing potential encounters and predation. Larval fish swimming speeds did not differ significantly with DO, but ctenophores swam significantly faster at intermediate DO (2.5 mg L) than at either low or high DO. DO did not significantly affect ingestion. Greater ingestion of fish larvae by ctenophores followed multiple encounters (56%) than initial encounters (10%) at all DO concentrations, highlighting the potential importance of repeated predator-prey interactions. DO did not significantly affect encounter model estimates of ingestion rates. Ingestions averaged 0.4 fish larvae d m for first encounters and 2 fish larvae d m for multiple encounters. Results from laboratory and field studies parameterized a spatially-explicit individual based model of a ctenophoreichthyoplankton-copepod intraguild predation food web. Ctenophore predation had a bigger effect on survival of modeled ichthyoplankton than did competition between ctenophores and fish larvae for shared zooplankton prey, but competition more strongly affected larval fish growth rates. DO did not alter the relative importance of ctenophore predation and competition, but low DO did decrease larval fish survival and increase growth rates. Results suggest that effects of DO on vertical distribution and species overlap are more important to predation than direct DO effects. THE EFFECTS OF LOW DISSOLVED OXYGEN ON PREDATION INTERACTIONS BETWEEN MNEMIOPSIS LEIDYI CTENOPHORES AND LARVAL FISH IN THE CHESAPEAKE BAY ECOSYSTEM