Reducing algal blooms in Dream Lake: Pre-manipulation algal and zooplankton seasonal dynamics indicate weak top-down control by largemouth bass (Micropterus salmoides)

Background/Question/Methods

Many freshwater ecosystems are experiencing increased algal bloom formation due to cultural eutrophication, or increased nutrient inputs from the surrounding watershed. Dream Lake, a small water body located in Brown Co, WI, has frequent algal blooms and decreased recreational and aesthetic value. In 2012-2014, we are implementing a combined top-down trophic cascade by adding piscivores; i.e. largemouth bass (Micropterus salmoides) and bottom-up reduction of fertilizer inputs approach to reduce algal blooms. We hypothesize that by combining both top-down and bottom-up remediation techniques, algal bloom frequency will decline, recreational and aesthetic value will increase and sport fishing will be enhanced. Baseline pre-manipulation data were collected from April - October 2012. Secchi depth and zooplankton were obtained weekly. Largemouth bass diet contents were taken monthly. Zooplankton samples and largemouth bass diet contents were counted, measured, and identified to species. 

Results/Conclusions

Algal blooms in the warm-water period reduced water transparency by 1.5 m ± 0.07 (N = 15). Zooplankton biomass was dominated by small bodied species throughout the entire summer and the dominant species changed over time. Bosmina longirostris contributed 40.5% ± 9.3 (N = 7) of the relative biomass from late April-June, Diaphanosoma birgei contributed 54.5% ± 8.3 (N = 9) from late June – end of August, and calanoid copepods contributed 36.2% ± 5.2 (N = 16) throughout the entire warm-water period. Zooplankton length changed significantly among species (F_2,60 = 116.5, p = 0.003) during the warm-water period. Length of the cladocerans B. longirostris, D. birgei and Daphnia ambigua decreased by 13.8% ± 1.3, 5.4% ± 1.4 and 21.0% ± 1.9, respectively, while length of the calanoid copepod, Leptodiaptomus minutus, increased by 17.5% ± 4.6. Largemouth bass diet composition contained 43.5% ± 3.1 fish. Despite confirmation of fish consumption by largemouth bass, persistent algal blooms and dominance by small-bodied zooplankton indicate size selective predation by zooplanktivorous fish and weak top-down control by largemouth bass. Thus, Dream Lake is a system likely to respond to the proposed top-down and bottom-up manipulations to reduce algal bloom frequency.