Effects of hypoxia on Mnemiopsis leidyi, ichthyoplankton and copepods: clearance rates and vertical habitat overlap

Abstract

Differences in predator and prey tolerances to low dissolved oxygen (DO) concentra- tions influence planktonic food web interactions in seasonally hypoxic environments. We examined low-DO effects on predation by hypoxia-tolerant ctenophores, Mnemiopsis leidyi, on less hypoxia- tolerant ichthyoplankton prey in laboratory experiments. We also examined the relationship between bottom DO concentrations and vertical distributions, and considered potential effects of biotic (pre- dators and competitors) versus abiotic factors (low DO concentration, temperature, salinity, time of day) on vertical habitat overlaps between gelatinous zooplankton and their prey during summer in a Chesapeake Bay subestuary. Laboratory clearance rates of M. leidyi feeding on bay anchovy (Anchoa mitchilli) eggs and yolk sac larvae, and naked goby (Gobiosoma bosc) larvae were as high at low DO (1.5 mg l -1 ) as at high DO concentrations (7 mg l -1 ). Multiple years of field sampling at 2 sites revealed that the proportions of motile species in the bottom layer of the water column, espe- cially fish larvae and copepods, increased with increasing bottom DO concentrations during both day and night. Ctenophore densities remained high in the bottom layer even at low DO concentrations. Vertical habitat overlaps among 4 motile predator-prey pairs (M. leidyi-copepods, M. leidyi-naked goby larvae, bay anchovy larvae-copepods, and naked goby larvae-copepods) increased with increasing bottom DO concentrations (1st- or 2nd-order regression models). Vertical overlap between M. leidyi and fish eggs was significantly higher during the day than at night. Statistical models sug- gest that both bottom DO concentration and scyphomedusan predators influenced M. leidyi-fish egg nighttime and M. leidyi-copepod combined day-night overlaps. Hypoxia influences food web inter- actions more through altered habitat use and encounter rates than by directly affecting predation. Strong behavioral responses of larval fish and copepods to low bottom DO concentrations may decrease habitat overlap between M. leidyi predators and their prey.