An IFYGL Carbon Budget for Lake Ontario

A carbon budget was produced for each month of the International Field Year for the Great Lakes (IFYGL) year (April 1972 to March 1973) to determine the importance of the various sources and sinks of carbon. Major sources were found to be CO₂ which was fixed in organic matter during primary production and inorganic carbon in tributary streams, especially the Niagara River. The major sinks were found to be inorganic carbon outflow at the St. Lawrence River and net CO₂ gas exchange between the inorganic carbon pool and the atmosphere. Inflow and outflow of organic matter in rivers, sedimentation of organic and inorganic matter, ground water transport, and municipal and industrial perturbations accounted in total for less than 10% of the annual budget.The lake had an inventory of approximately 4.0 × 10¹⁰ kg of inorganic carbon and approximately an order of magnitude less organic carbon. The riverborne flux of inorganic carbon of 0.5 × 10¹⁰ was 13% of the lake's inventory, assuming complete mixing; a minimum mean residence time of 8 years can be calculated from that inventory.The seasonal cycle inherent in the fixation of carbon in primary production was primarily balanced by a complementary seasonal cycle in the air-lake CO₂ gas exchange system. The lake acts as a sink for CO₂ gas in the warm months when primary productivity is highest and as a source of CO₂ in the colder part of the year.The IFYGL year had higher than normal rates of water flow, but this does not appear to have perturbed the inorganic carbon system. A comparison of IFYGL carbon budget results with corresponding estimates calculated for a typical year from historical data shows no major differences.