"Human activities, such as CO2 emissions are altering aquatic ecosystems in ways that are not fully understood. Because phytoplankton are essential organisms, forming the base of pelagic aquatic food webs, I focus on this group to help us understand how lake ecosystems respond to anthropogenic change. Specifically, I focus on the response of total phytoplankton biomass and community composition to increasing pCO2 in concert with (1) nutrient enrichment, (2) increasing temperatures, and (3) organismal evolution.In the first chapter, I investigated whether CO2 can act as a co-limiting resource that can promote phytoplankton growth and alter community composition across different times of the year. I conducted experiments using mesocosms suspended in a temperate mesotrophic lake, and designed them to evaluate the interactive effects of nitrogen, phosphorus, and CO2 enrichment in the months of July, August, October, April and June. I found that, in some seasons, CO2 acted as a co-limiting factor with phosphorus when nitrogen was also added. The phytoplankton community was affected by all three resources in diverse ways at different times of the year. I concluded that CO2 can affect the community composition and be a co-limiting factor for freshwater phytoplankton communities, especially when other resources are abundant, as is typical in eutrophic lakes.In chapter two, I investigated the interactive effect of CO2 and temperature on phytoplankton and zooplankton communities, two highly inter-related factors in the context of climate change. In the same lake as Chapter 1, I ran a single mesocom experiment in late Fall over four weeks. I did not detect an interactive effect between CO2 and temperature, although both factors had independent and additive effects on the phytoplankton community, and temperature altered zooplankton community composition. Additionally, CO2 altered the stoichiometry of the seston, which has been shown in other studies to affect zooplankton food quality. I concluded that, although no evidence for interactive effects was found, CO2 and temperature can have independent and additive effects across and multiple trophic levels in freshwater ecosystems.The third chapter deals with the evolutionary potential of phytoplankton species responding to changing atmospheric CO2 concentrations. I developed an eco-evolutionary model where phytoplankton growth depends on the influx of atmospheric CO2 and carbon uptake kinetics can evolve to trade off maximum carbon flux for affinity. At equilibrium, I found that populations adapted by optimizing carbon uptake to environmental conditions, which, in modelled monocultures, allowed populations to reach higher biomass, and in multi-species communities, allowed certain species to gain an unexpected advantage over others. The biomass increases depended on the species-specific parameters and concentrations of atmospheric CO2 and initial HCO3. I concluded that evolution in the context of changing pCO2 can affect community composition and generate greater biomass increases than expected from CO2 co-limitation alone.In sum, I found that biomass and composition of freshwater phytoplankton communities can be affected by increases in pCO2, by co-limitation, potentially in concert with factors like temperature, and evolution. One key observation and conclusion across all chapters of this thesis is the ecological and evolutionary effects of CO2 are generally small (compared to eutrophication) and may be involved in complex interactions. Such small effect sizes may seem to make it unnecessary to study the effects of enriched CO2. However, the fact that pCO2 concentrations are increasing worldwide, that even a small but large-scale effect can be significant, and that freshwaters are fragile but essential ecosystems, at the mercy of countless potentially interacting human activities, emphasizes the importance of understanding the impact of high pCO2 on freshwater communities"--