Distribution, Behavior And Trophic Interactions Of Pelagic Planktivores And Copepods In An Intermittently Stratified Estuary
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Date
2005-10-18
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Abstract
Hypoxia causes heterogeneity in estuarine habitats and affects mortality in sessile benthic organisms, distribution and abundance of mobile, demersal organisms, and trophic interactions in estuarine food webs. Using a combination of field sampling, spatial statistics and bioenergetics modeling, I examined the effects of intermittent hypoxia on juvenile bay anchovies (Anchoa mitchilli) and copepods in the pelagic habitats of the Neuse River Estuary (NRE). Hypoxia restricted anchovy vertical distribution to water with dissolved oxygen (DO) >2 mg O₂ l⁻¹. In contrast, copepod vertical distribution was not affected by intermittent hypoxia and anoxia. This spatial separation of predators and prey has implications for feeding and growth of individual planktivores and trophic interactions between secondary producers and planktivorous fishes in the NRE. Early in the summer, copepod biomass is sufficient to support anchovy grazing, even during intermittent hypoxic events. During periods of severe hypoxia, however, anchovies that occupied hypoxic regions of the NRE with low copepod availability had lower body condition than anchovies that inhabited a region that contained higher copepod biomass. Interactions between hypoxia, prey availability and competition for resources can also alter anchovy behavior and individual energetic costs. Under conditions of low fish density or a mixed water column, in situ swimming rates were in line with expectations of a diurnal activity pattern with swim speeds very close to expected theoretical values that would maximize net energy gain. In contrast, under high fish density and hypoxia, anchovies maintained high activity rates at night. Furthermore, when the water column was severely hypoxic (>70% of the water column with <2 mg O₂ l⁻¹), activity rates were nearly twice those predicted from a bioenergetics model. I hypothesized that this sustained nocturnal activity was a result of continued foraging at night. Although daily energetic costs were higher when compared to an individual that adopts a diurnal activity pattern, this behavior may afford an advantage by compensating for a reduced ration achieved by anchovies during the day under limiting prey conditions caused by hypoxia. During summers of extensive hypoxia, reductions in prey availability may negatively effect anchovy populations in estuaries by reducing individual feeding and growth rates.
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grazing demand, bioenergetics, hydroacoustics, swim speed, hypoxia, spatial scale
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Degree
PhD
Discipline
Zoology
