Mentor: Isabelle Anderson, Ph.D.

The availability and relative abundance of nitrogen (N) and phosphorus (P) in lake ecosystems can have a high degree of seasonal variability, which can influence the nutrient limitation dynamics of phytoplankton communities. Along with nutrient inputs, temperature plays a key role in promoting phytoplankton growth, further intensifying eutrophication in water bodies. In this study, we combined classic N and P nutrient limitation bioassays with an elevated temperature treatment to investigate seasonal limitation patterns and the extent to which warming alters nutrient effects on the phytoplankton community in a hypereutrophic reservoir. From May to October, we quantified biomass responses of the total phytoplankton assemblage and the three dominant divisions present: chlorophytes, cyanobacteria, and diatoms. The total phytoplankton community was P-limited from spring through August, then shifted to co-limitation for the remainder of the study. Chlorophytes and diatoms showed similar late-summer co-limitation patterns, whereas cyanobacteria instead shifted to strict N limitation. Temperature did not directly influence nutrient limitation status, but during periods of N limitation, elevated temperatures significantly increased the phytoplankton biomass response of the +N and +NP treatments. Therefore, our results suggest that elevated temperatures may magnify cyanobacteria blooms under N-limiting conditions. These findings indicate that reductions in both N and P are necessary for effective eutrophication management, especially as global temperatures continue to rise.

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