Transparency and predation are dynamic drivers of zooplankton community composition in north temperate lakes; as relative light intensity is reduced in aquatic ecosystems, the effectiveness of zooplanktivory by visual predators declines. Our research explores how water column transparency affects zooplankton community characteristics in 14 lakes in the Superior National Forest, MN, that span a transparency gradient. Lakes were sampled over a one week span during June of 2018 and 2019; at each lake, we took abiotic and light measurements, and sampled for zooplankton, TP, SRP, and Chlorophyll a. Lakes were classified to be either high transparency (secchi >3.0m) or low transparency (secchi ≤ 3.0m). We predicted that lakes with reduced transparency will have greater zooplankton biomass, diversity, and density. Lake transparencies were determined using secchi disk measurements. Data were analyzed using the Mann-Whitney U test.
Results/Conclusions
Preliminary data from 2018 and 2019 show that both Holopedium gibberum and Mesocyclops americanus had greater densities in low transparency lakes. Mean lengths of Daphnia pulex, D. navus, H. gibberum, and M. americanus were greater, and biomass of H. gibberum and M. americanus was greater in low transparency lakes (secchi ≤ 3.0m). In contrast, Bosmina longirostris, Diacyclops navus, Epischura lacustris, and D. dubia densities were greater in high transparency lakes (secchi >3.0m). High transparency lakes also exhibited greater overall biomass for D. navus and D. dubia. The greater densities and biomass in high transparency lakes likely reflect greater predation by tactile predators, such as Chaoborus sp., in low transparency lakes. Our study provides insight into how zooplankton communities may respond to changing transparencies resulting from climate change and land use change.