In freshwater lakes and reservoirs, zooplankton often exhibit diel vertical migration (DVM) or diel horizontal migration (DHM) to escape from visual predators and ultraviolet radiation in surface waters. While both avoidance behaviors are well documented, the prevalence of DVM vs. DHM within an ecosystem in response to hypolimnetic anoxia (<0.5 mg/L dissolved oxygen) is unknown. In general, DVM is considered to be the more ubiquitous strategy; however, DHM may become more advantageous in the presence of hypolimnetic anoxia, particularly for larger taxa with low oxygen tolerance. In this study, we simultaneously measured both DVM and DHM in a eutrophic reservoir over two 24-hour periods. To quantify DVM, we collected vertical water column tows at the reservoir’s deepest site at four different times throughout the 24-hour period to assess differences in zooplankton density, biomass, and community composition in the hypolimnion (lower layer of water column) vs. epilimnion (upper layer of water column). To quantify DHM, we collected vertical net tows at both a littoral and deep site and compared zooplankton community metrics between each site over time. Although DVM and DHM both result in trade-offs that may influence the prevalence of each migration behavior, this study was the first to our knowledge to simultaneously assess taxon-specific horizontal and vertical migration in the presence of hypolimnetic anoxia.
Results/Conclusions
We found evidence that zooplankton are exhibiting both DHM and DVM; however, the prevalence of each migration behavior varied by taxon. Despite the presence of hypolimnetic anoxia throughout the summer stratified period, we observed both higher zooplankton densities in the hypolimnion compared to the epilimnion during the day, indicative of DVM, and peaks in density outside the littoral site before sunrise and sunset, indicative of DHM. DHM was largely exhibited by rotifers, whereas DVM was largely exhibited by cladocerans and cyclopoids. Differences in migration behavior among taxa are thus likely mediated by functional traits (e.g., oxygen tolerance, body size, and vulnerability to visual predation), as taxa that remain in the hypolimnion during the day must have physiological or behavioral adaptations to cope with anoxic conditions. Ultimately, identifying which migration strategies are favored among crustaceans and rotifers will advance our understanding of how zooplankton communities respond to increased hypolimnetic anoxia in the face of global change.