This study aims to model productivity in Cayuga Lake based on aqueous carbon dioxide concentrations. Data was taken in July-September 2010 at varying depth at four locations near the outfall of the Ithaca Area Wastewater Treatment Facility and two reference locations. Carbon dioxide was measured using an OxyGuard Dissolved CO2 meter and dissolved oxygen, chlorophyll, ammonium, and pH data were recorded with a Hydrolab DS 5. Titration techniques were used as a complimentary way to measure carbon dioxide concentration, as well as alkalinity. Three approaches were applied in the productivity analysis: simple carbon dioxide concentration differences over a diel period, dissolved oxygen concentration differences over a diel period, and carbon dioxide estimates based on ammonium ion concentrations. The third method proved most useful, yielding a strong relationship with the gathered CO2 data. This method yielded a simple equation that allows for directly using CO2 values measured by the OxyGuard meter as an estimate of productivity.