Many freshwater ecosystems are experiencing increased algal bloom formation due to cultural eutrophication. Dream Lake, a small water body located in Brown Co, WI, has frequent algal blooms and decreased recreational and aesthetic value. From 2012 - 2018 we implemented a combined top-down trophic cascade by adding piscivores, i.e. largemouth bass (Micropterus salmoides), and a bottom-up reduction of fertilizer inputs approach to reduce algal blooms. We hypothesized that by combining both top-down and bottom-up remediation techniques, algal bloom frequency would decline, recreational and aesthetic value would increase and sport fishing would be enhanced. Baseline pre-manipulation data were collected from May - September 2012. The manipulation consisted of stocking ~200 fingerling largemouth bass each October in 2012-2014, and asking homeowners to restrict usage of fertilizers on their properties. During manipulation data were taken from May – September 2013-2015. Post-manipulation data were collected from May-September 2016-2018. A winter kill event that dramatically reduced fish population densities occurred in late winter/early spring 2014. Secchi depth, chlorophyll a and zooplankton were obtained weekly. Zooplankton samples were counted, measured, and identified to species. Data were analyzed using repeated measures ANOVA in SYSTAT.
Results/Conclusions
Percent increase in water transparency from May-September during the manipulation (2013-2015) was 11.8% ± 13.0, but was -2.8% ± 14.4, post-manipulation (2016-2018). Chlorophyll a varied between project time periods (p ≤ 0.037) and was significantly lower in mid-July through early-August (p ≤ 0.049) during the manipulation. Total zooplankton biomass, and biomass of cyclopoid copepods and nauplii differed significantly between project time periods (p ≤ 0.026). Total zooplankton biomass was significantly greater in mid-June, and from mid-July to early-August (p ≤ 0.037) post-manipulation; cyclopoid copepod biomass was significantly greater from mid-June to early-August (p ≤ 0.049) post-manipulation; and nauplii biomass was significantly greater in late-June and mid-August (p ≤ 0.048) during the manipulation. Increased water transparency and decreased algal biomass during the manipulation, and increases in zooplankton biomass during and post-manipulation, indicate that Dream Lake may be responding to the combined top-down and bottom-up manipulations and the 2014 winter kill event.