Zooplankton Community Structure is More Variable Over Short Timescales than Over Space, with Implications for Water Quality Management
Zooplankton are integral components of freshwater lake and reservoir food webs. Because of their role as a food source for fish and their ability to rapidly graze down algal populations, zooplankton play a critical role in maintaining water quality. To avoid fish predation during the day, zooplankton may migrate both vertically or horizontally (Diel Vertical Migration or DVM and Diel Horizontal Migration or DHM, respectively). In DVM, zooplankton migrate to the bottom waters, whereas in DHM, zooplankton migrate to near-shore shallow habitat during the day to avoid predation. In both cases, zooplankton return to deep surface waters at night to feed where algal food is most abundant. While drivers of these migration behaviors are not fully understood, DHM is thought to be more favorable than DVM when bottom water oxygen concentrations are low, a common occurrence in reservoirs. Additionally, shifts in DVM and DHM behavior may be linked to changes in zooplankton community structure (i.e., community composition and density) and therefore could alter freshwater functioning.
To understand how zooplankton community structure changes over time, we conducted five independent 24-hour sampling campaigns during the summers of 2019-2021 in Beaverdam Reservoir, VA, USA, a secondary drinking water supply reservoir that experiences summer hypolimnetic anoxia. We collected paired vertical tows at deep and shallow sites throughout a 24-hour period to capture patterns in DVM and DHM, respectively. Our results show that zooplankton exhibit greater variability within 24-hours than they do among years or sites. Variability in zooplankton community structure is likely partially explained by zooplankton migration behavior, as spatial distribution changes among taxa as they respond to changing environmental conditions over both short and long-term scales. Our work implies that current zooplankton monitoring approaches which typically occur once a day around noon are likely underestimating much of the variability in zooplankton community structure. Importantly, this short-term variability in zooplankton densities can have implications on fish communities that feed on zooplankton, as well as algal biomass and water quality, as zooplankton grazing efficiency changes over the course of 24-hours. Ultimately, this work will further our understanding of how zooplankton communities control water quality and food web dynamics in the face of global change.