Water transparency plays a key role in the predation of zooplankton. Lakes that are darker and less transparent protect zooplankton from visual predators, such as fish. It is hypothesized that transparency of water will affect species richness and biomass of zooplankton. In more transparent water, it is predicted that the biomass of zooplankton will decrease and species composition will change as predation by visual predators increases. Lakes in the Superior National Forest, bordering the Boundary Waters Canoe Area Wilderness of Northern Minnesota were sampled in late June of 2016 (n=12) and 2017 (n=13). Triplicate zooplankton samples, secchi disk measurements, and basic limnological parameters were collected from the deepest part of each lake. Zooplankton were counted, measured and identified to species. High transparency lakes had secchi values >2.5m while low transparency lakes had secchi values <2.5m. Data were analyzed using a Mann-Whitney U-Test in Systat 13.
Results/Conclusions
Total zooplankton biomass was significantly greater in high transparency lakes (p=0.012). Cladoceran (Bosmina longirostris, Daphnia dubia, D. pulex, and D pulicaria), calanoid (Leptodiaptomus minutus and Epischura lacustris), and cyclopoid (Mesocyclops edax and M. americanus) biomass and density were significantly greater in high transparency lakes (p≤0.039). Species richness was significantly greater in high transparency lakes (p=0.026). Holopedium gibberum had significantly greater density, biomass, and length in low transparency lakes (p=0.024, p<0.001, p=0.08, respectively). Water column dissolved oxygen and pH was significantly lower in low transparency lakes (p≤0.012). Preliminary analyses suggest that zooplankton species assemblage, richness and biomass in lakes in the Superior National Forest may be partially affected by water transparency. Lower density and biomass of zooplankton species in low transparency systems may result from enhanced predation by non-visual predators, Chaoborus sp., that are likely found throughout the water column due to protection from visual fish predators. However, greater density and biomass of H. gibberum in low transparency waters may be explained by their gelatinous mantle preventing predation by Chaoborus sp. and tolerance to lower pH and dissolved oxygen concentrations.