Cultural eutrophication reduces water quality by increasing algal bloom formation in freshwater systems. Dream Lake, a small water body in Wisconsin, suffers from frequent algal blooms and decreased recreational and aesthetic value. We implemented a short-term, three-year top-down trophic cascade by adding piscivores, i.e. largemouth bass (Micropterus salmoides), and a bottom-up reduction in fertilizer inputs to reduce algal blooms. The effectiveness of the three-year manipulation is compared to the four-year post-manipulation period. We hypothesized that water quality decreases following short-term top-down and bottom-up remediation techniques such that algal bloom biomass and the Hilsenhoff Biotic Index increase, and zooplankton biomass and macroinvertebrates abundance decrease post-manipulation. The manipulation consisted of stocking ~200 fingerling largemouth bass each October, 2012-2014, and asking homeowners to restrict fertilizer use. During-manipulation data were taken late-May through August 2013 and 2015. Post-manipulation data were collected late-May through August 2016-2019, with the exception of chlorophyll a data taken in 2016-2018. A winter kill event dramatically reduced fish population densities in spring 2014. Secchi depth, chlorophyll a, total phosphorus, zooplankton, and macroinvertebrates were obtained every other week. Zooplankton samples were counted, measured, and identified to species. Macroinvertebrate samples were counted and identified to order. Data were analyzed using Repeated Measures ANOVA.
Results/Conclusions
Soluble reactive phosphorus (SRP) was significantly lower post-manipulation (p ≤ 0.007). Bosmina sp. biomass was significantly lower post-manipulation (p ≤ 0.034). Diaphanosoma birgei (p ≤ 0.001) and calanoid copepod (p ≤ 0.022) biomass were significantly greater post-manipulation. Snail abundance (p ≤ 0.033) and percent snails (p ≤ 0.001) were significantly greater post-manipulation. Percent caddisflies (p ≤ 0.005) and percent other category (i.e. clams, aquatic worm, water mites; p ≤ 0.001) were significantly lower post-manipulation. Decreased SRP, and changes in zooplankton and macroinvertebrate biomass indicate that Dream Lake declined in water quality after the conclusion of the top-down and bottom-up remediation techniques. Studying the responses of Dream Lake will build understanding of management strategies to maintain water quality in shallow freshwater systems as eutrophication becomes increasingly prevalent due to changes in climate and land use.