Carbon cycling in Lake Superior

Carbon ( C) cycling in Lake Superior was studied within the Keweenaw Interdisciplinary Transport Experiment in Superior ( KITES) project to assess ( 1) whether the lake is net heterotrophic, ( 2) sources, sinks and residence time for dissolved organic carbon ( DOC), ( 3) importance of terrigenous organic C subsidies, and ( 4) factors limiting C flow through bacteria. During 3 years of fieldwork, measurements were made of spatial and temporal distributions of C pools and rates of photosynthesis, community respiration, and bacterial production. Measurements were made of the composition of dissolved organic matter ( DOM), rates of DOM photolysis, lability of DOM toward microbial consumption, and river inputs of DOM. All measurements suggest the lake is net heterotrophic. The C:N ratios of DOM suggest that it is primarily of terrigenous origin, but other characteristics ( size distribution, UV absorption) point to the presence of autochthonous DOM and to alteration of terrigenous material. The lake mass balance indicates that the residence time ( similar to 8 years) of the DOC pool ( 17 Tg) is short relative to the hydrologic residence time ( 170 years). The known flux of terrigenous DOC ( similar to 1 Tg/yr) is too low to support annual bacterial carbon demand ( 6-38 Tg/yr), but microbial respiration is the major sink for terrigenous DOC. A rapidly cycling, autochthonous DOC pool must exist. Microbial activity was correlated with temperature, phosphorus availability, and DOC concentration but not with photosynthesis rates. Measurements of respiration ( similar to 40 Tg/yr), photosynthesis ( 2-7 Tg/yr), and bacterial production ( 0.5-2 Tg/yr) are not all mutually compatible and result in a discrepancy in the organic carbon budget.