In partial fulfillment of the requirements for the degree of 

Master of Science in Biology 

in the 

School of Biological Sciences 

Kelton Southard 

Will defend his thesis 

“Nitrate and Phosphate Loads, but not Light Availability, Impact Freshwater Phytoplankton Diversity via Tradeoffs Between Dominant Species” 

28, November, 2022 

3:30PM 

Ford ES&T Room L1116 

https://gatech.zoom.us/j/91321371094 

Thesis Advisor: 

Dr. Lin Jiang 

School of Biological Sciences 

Georgia Institute of Technology 

Committee Members: 

Dr. Julia Kubanek 

School of Biological Sciences 

Georgia Institute of Technology 

  Dr. Yongsheng Chen 

School of Civil and Environmental Engineering 

Georgia Institute of Technology 

Abstract: Eutrophication of freshwater ecosystems, mainly caused by nitrogen (N) and phosphorus (P) pollution, causes significant economic damages every year in the U.S. Excess N and P deposition in lakes can result in harmful algal blooms, reduced biodiversity, and increased greenhouse gas production, but we still do not fully understand how and why phytoplankton communities react to nutrient enrichment under varying conditions. Several theories – including the niche dimension hypothesis, biomass-driven competition hypothesis, nitrogen detriment hypothesis, and benthic model – are currently being explored in both terrestrial and aquatic producer communities in attempt to better understand the biological mechanisms effecting these systems, and the goal of this study was to determine which models are most applicable to freshwater phytoplankton. Using five-species microcosms of green algae, we found that N and P enrichment significantly reduced diversity (independent of light availability), which was likely mediated through nutrient tradeoffs between the two dominant species, Ankistrodesmus falcatus and Selenastrum capricornutum. Additionally, we observed a significant decrease in monoculture carrying capacity across all species with high N addition in low P concentrations, indicating that high N:P ratios may be physiologically harmful to green algae. These findings suggest that the niche dimension and nitrogen detriment hypotheses may be the most applicable to freshwater phytoplankton communities and could be useful for protecting and mitigating economic losses from these systems.