PS 16-20 - Reducing algal blooms in Dream Lake, WI: Algal and zooplankton seasonal dynamics indicate response to food web manipulation

Tuesday, August 8, 2017
Exhibit Hall, Oregon Convention Center
Carrie E.H. Kissman1, Tyler J. Butts1, Cole Brennan2, Kristin L. Kniech2 and James R. Hodgson1, (1)Biology and Environmental Science, St. Norbert College, De Pere, WI, (2)Biology, St. Norbert College, De Pere, WI
Background/Question/Methods

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.