ORGANIC CARBON IN LARGE LAKES OF THE WORLD: ESTIMATES OF PRODUCTION, BURIAL, AND LAKE-ATMOSPHERE EXCHANGE

The carbon cycles of the world's great lakes are poorly understood, making it difficult to rigorously characterize ancient lacustrine deposits in relation to modern processes. We compiled data from the literature on primary production, lake-atmosphere carbon dioxide (CO₂) exchange, and carbon burial to characterize relationships between these important carbon fluxes, latitude, and other environmental parameters. The compilation includes data from 32 lakes, ranging from 16°S to 66°N latitude and accounting for >70% of the world's freshwater volume. A significant exponential relationship exists between carbon fixation through primary production and latitude, with higher and more variable productivity in the tropics. Sedimentary burial of organic carbon also correlates significantly with latitude, and a greater proportion of carbon fixed in large lakes is buried at higher latitudes than in the tropics. In a subset of the database (n=4 lakes), all lakes have accumulated orders of magnitude more organic carbon in their lake sediments than is stored in the biota and soils of their watersheds, with the magnitude of this discrepancy being higher in the tropics than at higher latitudes. Eutrophication appears to increase the overall amount but not necessarily the proportion of organic carbon buried in sediments. Based on temporally limited data available for calculating lake-atmosphere CO₂ exchange, most large lakes appear to function as net carbon sources to the atmosphere, although the magnitude of this flux is smaller in the tropics and probably neutral in some cases. Many temperate large lakes may be greater net emitters of CO₂ than estimated with this database, as water-column measurements of the parameters needed to calculate net CO₂ fluxes (pH, alkalinity) are rarely available for winter months when respiration rates exceed production rates by the widest margin. Understanding and quantifying the relationships among major carbon fluxes in modern large lakes will facilitate the interpretation of paleolacustrine carbon cycling parameters related to paleolimnology and petroleum production.