Loiselle, Reane2025-02-142025-02-142024-09https://hdl.handle.net/11299/269956University of Minnesota M.S. thesis. September 2024. Major: Water Resources Science. Advisor: Robert Sterner. 1 computer file (PDF); vi, 89 pages.The Laurentian Great Lakes hold over 20% of our global freshwater. Cyanobacterial harmful algal blooms (CHABs) have been some of the most significant environmental issues to threaten the health of the Great Lakes in recent decades, causing an estimated $65-71 million dollars per year in economic losses for Lake Erie alone (Bingham et al. 2015). The impact of increased temperature and nutrients on HABs has been extensively studied in recent decades. But still, little is known about how changes in temperature, nitrogen and phosphorus impact the relative abundance of specific bloom forming taxa in the Great Lakes or elsewhere. To better understand these shifts, I analyzed identical 28-day nutrient addition bioassays at ambient and plus 3 ℃ temperatures in Mawikwe Bay, Lake Superior and Sandusky Bay, Lake Erie during the summers of 2022 and 2023. This length of experiment was selected to allow for competition to take place over realistic ecological time scales to elucidate the impact of nutrients and temperature on phytoplankton competition. A FluoroProbe was used to track broad changes in phytoplankton groups every 3 days and microscopy was used to obtain detailed community information on days 0, 14, and 28. Further microscopy analyses were used to determine changes in relative abundance of dominant bloom forming species, shifts in major phytoplankton groups, and implications for associated traits.Lowering Lake Superior N:P ratios resulted in shifts from diatom dominance to increased green algae and cyanobacteria abundance, including the main bloom forming cyanobacterium Dolichospermum. This outcome indicated that the normally diatom-dominated phytoplankton community in Lake Superior is a result of intense competition for P, while P additions promote Dolichospermum abundance. In Lake Erie, Microcystis was significantly more abundant in treatments with higher N:P ratios while Dolichospermum was more abundant in treatments with lower N:P ratios. Low N:P favoring Dolichospermum in both lakes might be explained by the ability of Dolichospermum to fix nitrogen. It also suggests that P-only nutrient management in Lake Erie would likely shift the species composition of HABs from non-diazotrophic species to diazotrophic species, but not eliminate them. In contrast, P-only management in Lake Superior is likely to be effective. A several degree warming was not found to be a prominent factor in steering phytoplankton community composition in either lake, suggesting weak temperature effects on competitive outcomes over expected amounts of warming.enAlgaeBioassayCyanobacteriaLimnologyPhytoplanktonStoichiometryThesis or Dissertation