February 14, 2014

Kyuyeon Lee '14

Abstract:

Lake eutrophication is classified as algal blooms in the aquatic system and is the cause of a reduction in water clarity and the oxygen depletion that can kill fish and other organisms. Typically eutrophication is caused by increased input of nutrients, however, changes in the food web structure also can cause eutrophication. It is well known that size-selective predation by fish affects the zooplankton community composition shift in lakes. Stocking rainbow trout (RBT) in Square Lake resulted in a reduction of large-bodied Daphnia pulicaria, which is correlated to water clarity decline and eutrophication of the lake due to increased algal biomass. The Minnesota Department of Natural Resource finalized a 3-year moratorium of RBT stocking. The result of this study done in the first year of the moratorium to provide the general effect of RBT removal on Daphnia community composition and water clarity in Square Lake. During the pre-moratorium period, D. mendotae was most abundant overall Daphnia community, while D. pulicaria population was minimized with extremely low distribution at the surface water of the lake. In the first year of moratorium, even though, the overall Daphnia community was still dominated by D. mendotae, D. mendotae population significantly declined. D. pulicaria population remarkably increased and they were more abundant at the surface water than they were in the pre-moratorium period. This suggests that D. pulicaria is more susceptible to visual predator than D. mendotae. When the risk of predation is reduced, D. pulicaria appears more at the surface water where algae are most abundant. Water clarity showed a positive effect as a result of recolonized D. pulicaria population. The removal of RBT has a strong impact on Daphnia community in Square Lake. The 3-year period of RBT stocking suspension is expected to show overall zooplankton community composition shift to the former stage as well as water clarity improvement in Square Lake.