PS 17-19 - Reducing algal blooms in Dream Lake, WI: Algal and zooplankton seasonal dynamics indicate response to food web manipulation

Wednesday, August 10, 2016
ESA Exhibit Hall, Ft Lauderdale Convention Center
Carrie E.H. Kissman1, 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, 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.