The effects of hypolimnetic anoxia on the diel vertical migration of freshwater crustacean zooplankton

Abstract

Lakes and reservoirs worldwide are increasingly experiencing depletion of dissolved oxygen (anoxia) in their bottom waters (the hypolimnion) because of climate change and eutrophication, which is altering the dynamics of many freshwater ecological communities. Hypolimnetic anoxia may substantially alter the daily migration and distribution of zooplankton, the dominant grazers of phytoplankton in aquatic food webs. In waterbodies with oxic hypolimnia, zooplankton exhibit diel vertical migration (DVM), in which they migrate to the dark hypolimnion during the day to escape fish predation or ultraviolet (UV) radiation damage in the well‐lit surface waters (the epilimnion). However, due to the physiologically stressful conditions of anoxic hypolimnia, we hypothesized that zooplankton may be forced to remain in the epilimnion during daylight, trading oxic stress for increased predation risk or UV radiation damage. To examine how anoxia impacts zooplankton vertical migration, distribution, biomass, and community composition over day–night periods, we conducted multiple diel sampling campaigns on reservoirs that spanned oxic, hypoxic, and anoxic hypolimnetic conditions. In addition, we sampled the same reservoirs fortnightly during the daytime to examine the vertical position of zooplankton throughout the summer stratified season. Under anoxic conditions, most zooplankton taxa were predominantly found in the epilimnion during the day and night, did not exhibit DVM, and had lower seasonal biomass than in reservoirs with oxic hypolimnia. Only the phantom midge larva, Chaoborus spp., was consistently anoxia‐tolerant. Consequently, our results suggest that hypolimnetic anoxia may alter zooplankton migration, biomass, and behavior, which may in turn exacerbate water quality degradation due to the critical role zooplankton play in freshwater ecosystems.

Day-night differences in crustacean zooplankton in Lake Glubokoe and Mozhaysk Reservoir (European Russia)

“…Some studies suggest that zooplankton might favor diel horizontal migration (instead of vertical) in lakes with hypolimnetic hypoxia (Doubek et al, 2018), but we did not find enough evidence of major horizontal migration in Lake Glubokoe, where the hypoxic layer thickness extended to over 20 m. In Mozhaysk Reservoir, the hypoxic layer at the bottom was 3 m thick, but the day-night differences were much greater than Lake Glubokoe. The tendency of the Glubokoe's plankton to stay in the pelagic zone during the day might be better explained by relatively low littoral area, insufficient littoral vegetation cover for zooplankton refuge, or low predatory pressure.…”

Zooplankton Community Responses to Oxygen Stress

“…The first response of zooplankton to oxygen depletion that we observed was reduced biomass in lower water layers, but generally not in the layer where oxygen deficits began, i.e., in the anoxic metalimnion. We observed even higher biomass of zooplankton in anoxic metalimnia, although warmer waters where the oxygen solubility is lower and the oxygen demand (respiration) is higher [9].…”

Hypolimnetic Hypoxia Increases the Biomass Variability and Compositional Variability of Crustacean Zooplankton Communities

“…At nighttime, a large portion of the zooplankton population migrates back to the epilimnion, where they are closer to phytoplankton as a food resource, with a decreased risk of being eaten, or ending up damaged by UV radiation. However, because zooplankton also need oxygen for respiration [20,21], they may be forced out of the safety of the hypolimnion during the daytime and instead remain in the epilimnion, trading oxic stress for increased risk of predation or UV radiation damage [35]. Subsequently, the increase in the biomass variability of the zooplankton community during hypolimnetic hypoxic periods may be related to sporadic pulses of zooplankton taxa that suddenly increase due to emergence from sediment resting stages, and then shortly thereafter exhibit population crashes due to fish predation in the epilimnion or the deleterious effects of UV radiation.…”