As a result of global change, lakes and reservoirs worldwide are increasingly experiencing low dissolved oxygen concentrations (hypoxia) in their bottom waters. Although the effects of hypoxia on internal nutrient loading have been well-studied, less is known about how hypoxia impacts plankton communities, especially zooplankton, the dominant grazers of phytoplankton in lakes and reservoirs. Typically, zooplankton migrate to the dark bottom waters (the hypolimnion) during the day to escape visual fish predation in the well-lit surface waters (the epilimnion). However, due to the physiologically-stressful conditions of hypoxic hypolimnia, zooplankton may remain in the epilimnion during daylight, trading oxic stress for increased predation risk. We sampled five reservoirs weekly to biweekly during the daytime in southwestern Virginia, USA over three summers to examine how hypolimnetic oxygen concentrations impact the vertical distribution, density, biomass, and community composition of macrozooplankton and rotifers. These reservoirs varied on a gradient of hypolimnetic oxygen concentrations, from no oxygen to high oxygen during the summer stratified period. In addition, we also conducted ten 24-h sampling campaigns on reservoirs across this same oxygen gradient to examine how zooplankton were vertically distributed over day-night periods.
Results/Conclusions
Under hypoxic conditions, zooplankton were predominately found in the epilimnion during the day and night, did not exhibit diel vertical migration, and had overall lower densities and biomass than in reservoirs that exhibited oxic hypolimnia. Only two out of sixteen zooplankton taxa were found predominately in hypoxic zones. Moreover, our data suggest that zooplankton may change their migration patterns under oxic vs. hypoxic hypolimnetic conditions: zooplankton might exhibit greater vertical migration in reservoirs with oxic hypolimnia, but greater horizontal migration (migrating from the shallow littoral zone to the deep pelagic zone) in reservoirs with hypoxic conditions. Consequently, our results suggest that hypolimnetic hypoxia may alter zooplankton vertical distribution, biomass, and behavior, which may in turn exacerbate water quality degradation because of the critical role zooplankton play in freshwater food webs.