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 - 2015 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, a combined total of 506 fingerling largemouth bass were stocked in October 2012-2014, and post-manipulation data were collected from May-September 2013-2015. A winter kill event that dramatically reduced fish population densities occurred in late winter/early spring 2014. Secchi depth, chlorophyll aand zooplankton were obtained weekly. Zooplankton samples were counted, measured, and identified to species.
Results/Conclusions
Although water transparency improved in the warm-water period post manipulation in 2013-2014 by 11.23% ± 10.95 and 35.9% ± 8.71, respectively, water transparency in 2015 was 10.18% ± 8.71 lower than pre-manipulation conditions. Chlorophyll a varied between years (p ≤ 0.001), and was significantly lower from late July through mid-August (p ≤ 0.017) post manipulation. Zooplankton biomass was dominated by small bodied species in all four years. Total zooplankton biomass, and biomass of Bosmina longirostris, cyclopoid copepods and nauplii differed significantly from 2012-2015 (p < 0.001). Total zooplankton biomass and cyclopoid copepod biomass were significantly greater from mid-June to end of August (p ≤ 0.020; p ≤ 0.018, respectively), B. longirostris biomass was significantly greater throughout the summer (p ≤ 0.02), and nauplii biomass was significantly greater in August (p ≤ 0.018), post manipulation. Average length of B. longirostris differed from 2012-2015 (p < 0.001), with greater length in June and August in 2013-2014 (p ≤ 0.023), but reduced length throughout most of the summer in 2015 (p ≤ 0.046) post manipulation. Changes in water transparency, decreased algal biomass, increases in zooplankton biomass, and changes in zooplankton length for portions of the warm-water period post manipulation indicate that Dream Lake may be responding to the combined top-down and bottom-up manipulations and the 2014 winter kill event.