Impacts of moderate hypoxia on fish and zooplankton prey distributions in a coastal fjord

Hypoxia can cause significant disturbances in aquatic ecosystems, but the impacts of moderately low dissolved oxygen (DO) where physiological tolerance levels vary among organisms and likely have consequences for key food web linkages are not well understood. We hypothesized that the greater sensitivity of fish to DO, compared with their zooplankton prey, would reduce spatial overlap between fish and zooplankton at moderately low DO (2-4 mg l(-1)). We used a combination of multifrequency acoustics and net samples to characterize distributions and abundances of zooplankton and pelagic fish in Hood Canal, Washington, a seasonally hypoxic fjord. We employed a sampling design that included both high and moderately low DO sites sampled prior to, during, and after the onset of seasonally low DO over 2 yr. Contrary to our hypothesis, we found that fish and their zooplankton prey did not change their horizontal or vertical distributions during periods and in locations with moderately low DO levels. Consequently, the vertical overlap between fish and zooplankton did not change with DO concentration. The apparent lack of response of fish to moderately low DO in our system may result from decreased metabolic oxygen demand due to cool temperatures, availability of prey in moderately low DO waters, increased predation risk at shallower depths, and/or phenotypic adaptations to chronic exposure. Stability in distributions of pelagic communities suggests resilience of trophic coupling to moderately low DO in Hood Canal.