Background/Question/Methods Elevated atmospheric carbon dioxide (CO₂) changes the carbonate chemistry in water, increasing total dissolved inorganic carbon in water and shifting dissolved inorganic carbon away from carbonate (CO₃^2-) to bicarbonate (HCO₃^-). Since most phytoplankton species use HCO₃^- as their primary carbon source, increasing HCO₃^- has the potential to increase primary production of phytoplankton. However, primary production may be constrained by nitrogen availability. How phytoplankton respond to elevated CO₂ in the long term and how N and C interact to affect primary production remain an open question. We grew natural freshwater phytoplankton assemblages in growth chambers to steady state at two different CO₂ and N concentrations to determine how C and N availability interact to limit primary production. Our goal is to 1) examine how biomass accumulation responds to different CO₂ levels, 2) understand how N availability modifies the response of phytoplankton to elevated CO₂ and 3) examine the pattern of C allocation in inorganic and organic pools to understand how N and CO₂ level affect carbon dynamics in fresh water ecosystem. Results/Conclusions Carbon dynamics in fresh water ecosystem is affected by CO₂ levels and N availability. High CO₂ levels stimulate greater carbon consumption. Increased N availability stimulates biomass accumulation. Furthermore, phytoplankton community composition changes in response to C and N availability. These results suggest that C dynamics is closely coupled with N dynamics in fresh water ecosystems. It also contributes to our understanding of C dynamics in freshwater ecosystem in the context of increasing atmospheric CO₂ and anthropogenic N deposition.