Many freshwater ecosystems are experiencing increased algal bloom formation due to cultural eutrophication. Dream Lake, a small water body located in Brown Co., WI, USA, has frequent algal blooms and decreased recreational and aesthetic value. From 2012 - 2016 we implemented a combined top-down trophic cascade by adding piscivores, i.e. largemouth bass (Micropterus salmoides), and a bottom-up approach by reducing fertilizer inputs 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, a combined total of 606 fingerling largemouth bass were stocked in October 2012-2015, and post-manipulation data were collected from May-September 2013-2016. 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 2012-2016 was 5.84% ± 8.12 (N = 14). Chlorophyll a varied between years (p < 0.001), and was significantly lower from late-July through late-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 from early-July to late-August (p ≤ 0.013), B. longirostris biomass was significantly greater from late-May to mid-June (p ≤ 0.036), cyclopoid copepod biomass was significantly greater from mid-June through late-July (p ≤ 0.04), and nauplii biomass was significantly greater in mid-June and late-August (p ≤ 0.004), post-manipulation. Average length of B. longirostris significantly differed between years (p < 0.001), with greater length in June and August in 2013-2014 (p ≤ 0.023), but reduced length throughout most of 2015 and 2016 (p ≤ 0.046; p ≤ 0.022, respectively), post-manipulation. Increased water transparency, decreased algal biomass, increases in zooplankton biomass, and changes in 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.