Anthropogenic eutrophication is fundamentally changing the role of lakes in global carbon cycles. Without eutrophication, lakes function as net sources of CO2 to the atmosphere via watershed inputs and degradation of terrestrial organic carbon. In eutrophic lakes, however, this process can be reversed due to sustained phytoplankton blooms and thus high primary producer demand for CO2. Global increases in these blooms over the past two decades suggest the theoretical framework we currently use to explain phytoplankton community assembly may not fully account for the environmental stochasticity associated with climate change processes and anthropogenic landscape manipulation. As human populations increase, so does the proportion of our terrestrial landscape devoted to agriculture and urban areas, and thus the proportion of inland waters subject to eutrophication. Moving forward, it is critical to understand the response of lakes to anthropogenic disturbance, both to better predict harmful blooms and to evaluate the changing role of lakes in global carbon cycles.

My dissertation addressed three primary questions: 1. Can eutrophication render lakes net sinks of atmospheric CO2?, 2. When CO2 is depleted from surface water, what mechanisms sustain cyanobacteria bloom biomass?, and 3. Do blooms act to stabilize or destabilize aquatic primary producer communities? To address these questions, sixteen eutrophic lakes were chosen along orthogonal gradients of interannual variability in watershed hydrologic permeability and Cyanobacteria dominance. My work demonstrated that in these lakes, CO2 and dissolved inorganic carbon (DIC) were derived primarily from internal lake processes, and never from heterotrophic degradation of terrestrial organic carbon. Stable isotopic analyses revealed that DIC came from heterotrophic recycling of autochthonous carbon, atmospheric uptake, or mineral dissolution. Additionally, as production increased and CO2 was depleted from surface waters below atmospheric equilibrium, Cyanobacteria blooms developed that shifted from diffusive uptake of bioavailable CO2 to energetically costly active uptake of scarce CO2 or HCO3-. This mechanism creates a positive feedback loop, where high production is maintained under CO2 depletion, allowing eutrophic lakes to act as net carbon sinks. Finally, I show that long term cyclic fluctuations in cyanobacteria biomass are a mechanism of instability in primary producer communities. These findings suggest that as growing human populations force more nutrient intensive agriculture, lakes will continue to shift to impacted, eutrophic conditions. These processes have the potential to alter the global CO2 budget and cause shifts to harmful algae that can efficiently use non-CO2 DIC.