Analysis and experimentation on ecosystems

Eutrophication and climate change have increased the frequency of hypoxic conditions in the hypolimnia of lakes and reservoirs worldwide. Hypoxia can alter zooplankton migration from vertical to horizontal because zooplankton require oxygen for respiration, and thus may be forced to littoral refuge during the day. Fish communities can also affect zooplankton migration as well as zooplankton community composition and size structure. However, the interaction of fish communities and hypoxia on zooplankton migration has not been studied. We tested the hypothesis that zooplankton exhibit more extensive (greater magnitude) vertical and horizontal migration when piscivores are rare than when they are abundant, and also under hypoxic vs. oxic hypolimnetic conditions. We sampled zooplankton and other environmental variables across six consecutive days and nights during summer 2019 in a 16-lake experimental system, as part of AQUACOSM. Half of the artificial lakes received a fertilization treatment with N and P (eutrophic lakes with hypolimnetic hypoxia), and lakes were manipulated in order to enrich half of them with European Perch (potentially piscivorous), and to reduce perch individuals in the other lakes (all lakes had planktivorous Roach). Zooplankton were sampled in the epilimnion and hypolimnion of the open water habitat and just outside and inside of the vegetated littoral zone. We will report on the effects of fish communities and dissolved oxygen conditions on zooplankton density and biomass in littoral vs. pelagic zones, and their diel vertical and horizontal movements.