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 - 2017 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, ~200 fingerling largemouth bass were stocked each October in 2012-2016, and post-manipulation data were collected from May-September 2013-2017. 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 2012-2017 was 16.5% ± 8.81 (N = 14). Chlorophyll a varied between years (p ≤ 0.001) and was significantly lower in early August (p < 0.001) post-manipulation. Total zooplankton biomass, and biomass of Bosmina longirostris, cyclopoid copepods, and nauplii differed significantly between years (p < 0.001). Total zooplankton biomass was significantly greater in early-June, and from early-July to late-August (p ≤ 0.01); B. longirostris biomass was significantly greater from late-May to mid-June, mid-July, and late August (p ≤ 0.042); cyclopoid copepod biomass was significantly greater from mid-June to mid-July (p ≤ 0.003); and nauplii biomass was significantly greater in early-June, mid-June to late-July, and mid-August (p ≤ 0.039), post-manipulation. Average length of B. longirostris significantly differed between years (p < 0.001), with greater length in late-May to late-June, and mid-July (p ≤ 0.041), post-manipulation. Changes in water transparency, decreased algal biomass, increases in zooplankton biomass and zooplankton length, post-manipulation, indicate that Dream Lake may be responding to the combined top-down and bottom-up manipulations and the 2014 winter kill event.