Abstract
AbstractLakes 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.
Highlights
Climate change is expected to significantly alter the distribution and community interactions of organisms in a diverse suite of ecosystems (MEA 2005, IPCC 2013)
We found that crustacean zooplankton predominantly avoid hypoxic and anoxic hypolimnia and remain in the epilimnion during the daytime in reservoirs with planktivore communities (Fig. 6; Virginia Department of Game and Inland Fisheries, unpublished data; Western Virginia Water Authority, unpublished data)
Because both natural lakes and human-made reservoirs are increasingly experiencing hypolimnetic anoxia (e.g., Posch et al 2012, Jenny et al 2014, 2016), our data suggest that it is possible that zooplankton diel vertical migration (DVM) behavior may change in response to lower dissolved oxygen (DO) conditions in inland waterbodies
Summary
Climate change is expected to significantly alter the distribution and community interactions of organisms in a diverse suite of ecosystems (MEA 2005, IPCC 2013). Many species are moving poleward or to higher elevations (Parmesan and Yohe 2003, Hijmans and Graham 2006, Deutsch et al 2008, Beever et al 2017). In freshwater lakes and reservoirs (hereafter, together referred to as waterbodies), where organisms cannot migrate poleward or to higher elevations, species migrations and community interactions may be responding to global change. Hypolimnetic hypoxia and anoxia are increasing in intensity and duration globally in many waterbodies due to land use and climate change (Posch et al 2012, Jenny et al 2014, 2016), but little is known about the consequences of this forcing on freshwater community interactions and species migrations
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