Lake Minnewaska, located in a state park in New York, has been recovering over the last 30 years from acidification; concurrently, water clarity has decreased raising concern amongst park managers and visitors. As lake acidity decreased Notemigonus crysoleucas (golden shiner), a zooplanktivore, and Micropterus salmoides (largemouth bass), a piscivorous species, were unintentionally introduced, resulting in a trophic cascade with increasing algal growth and decreasing water clarity. Previous research used the concept of top-down control to evaluate the effect of the introduced top predator on algal growth; however, in this study we investigated the possibility of bottom up controls on algal biomass by performing a nutrient limitation experiment. We focused on Lake Minnewaska, a mesotrophic lake, and predicted the co-limitation of both nitrogen and phosphorus on phytoplankton biomass. The experiment consisted of introducing treatments of an unamended lake water control (C), and three treatments amended with nitrogen (N), phosphorus (P), and a combination of nitrogen and phosphorus (N+P). The samples were suspended on the surface of Lake Minnewaska and incubated in-situ for one week during June 2016, and one week in September 2016. After retrieving the samples, we measured chlorophyll a (chl a) concentrations as a proxy for algal biomass.
Results/Conclusions
For the June 2016 experiment, chl a concentration of the N+P treatments in Lake Minnewaska were highest at 10.8 ± 1.4 µg/L (mean ± standard deviation). The chl a concentration in the N treatment was the lowest at 5.43 ± 5.0 µg/L (mean ± standard deviation). However, a one-way ANOVA was not significant (p = 0.17) indicating the means were not significantly different. In the September 2016 experiment, the chl a concentration of the N+P treatments were highest at 5.4 ± 1.1 µg/L (mean ± standard deviation) and similar across all three other treatments with a significant ANOVA (F3,12=12, p<0.001). We saw co-limitation of nitrogen and phosphorus leading to more substantial algal growth. Lowering phosphorus and nitrogen concentration levels would lead to an increase in lake water clarity. We will work with the state park managers to determine external and internal sources of nitrogen and phosphorus to the lake phytoplankton and evaluate steps for mitigating the effects of the excess nutrients.