Reducing algal blooms in Dream Lake: Algal and zooplankton seasonal dynamics indicate combined response to food web manipulation and winter kill event
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 - 2014 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 306 fingerling largemouth bass were stocked in October 2012 and 2013, and post-manipulation data were collected from May-September 2013 and 2014. 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.
Percent increase in water transparency during the warm-water period from 2012-2014 was 35.9% ± 8.71 (N = 15). Chlorophyll a varied between years (p = 0.001), and was significantly lower from June-early August (p ≤ 0.007) post manipulation. Zooplankton biomass was dominated by small bodied species in all three years. Total zooplankton biomass and biomass of Bosmina longirostris differed significantly from 2012-2014 (p < 0.001 and p = 0.001, respectively). Total zooplankton biomass was significantly higher in June and August (p ≤ 0.024) and B. longirostris biomass was significantly higher in 11 of the 14 sampling dates (p ≤ 0.029), post manipulation. Average length of B. longirostris significantly differed from 2012-2014 (p < 0.001), with greater length post manipulation in June and August (p ≤ 0.023). Increased transparency, decreased algal biomass, increases in zooplankton biomass and length for portions of the warm-water period post manipulation indicate that Dream Lake may be responding to the combined top-down manipulation and the 2014 winter kill event.