We examined the organic carbon budget for the Earth's largest lake, Lake Superior, in the Laurentian Great Lakes. This is a unique, ultra-oligotrophic system with many features similar to the oligotrophic oceanic gyres, such as dominance of microbial biomass and dissolved organic carbon in biogeochemical processes. Photo-autotrophy is the dominant source of reduced organic matter in the lake. Areal rates of primary production are among the lowest measured in any aquatic system, and are likely a result of cold water temperatures and low nutrient concentrations in the lake. Allochthonous riverine organic carbon inputs were estimated at about 10 percent of photo-autotrophic production. Atmospheric carbon deposition has not been measured to any significant extent but we estimate it at 0.16 to 0.41 Tg yr1. All together, allochthonous carbon sources provide 13 to 19 percent of photo-autotrophic production. The main loss of organic matter in the lake is through respiration in the water column. Respiration is double all estimated organic carbon sources combined and therefore sources are likely underestimated. Few measurements of photo-autotrophic carbon production have been made and none recently. Nonetheless, most of the production and fluxes in this system pass through the large dissolved organic carbon pool (more than 10 times as large as the particulate organic carbon pool), which is mediated by heterotrophic and autotrophic picoplanktonic microbial flora. Improved understanding of dissolved organic carbon pools and dynamics is critical for constraining carbon flux in ultra-oligotrophic Lake Superior.

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