Love Handles in Aquatic Ecosystems: The Role of Dissolved Organic Carbon Drawdown, Resuspended Sediments, and Terrigenous Inputs in the Carbon Balance of Lake Michigan

During the unstratified (winter) and stratified (summer) periods of 1999 and 2000, we examined carbon (C) dynamics in the upper water column of southern Lake Michigan. We found that (a) bacte-rial respiration (BR) and planktonic respiration (PR) were major sinks for C, (b) C flux through bacteria (CFTB) was diminished in winter because of reduced bacterial production (BP) and increased bacterial growth efficiency (BGE) at colder temper-atures, and (c) PR exceeded primary production (PP) during the spring–summer transition. Draw-down of dissolved organic C (DOC), resuspended organic matter from the lake floor, and riverine organic matter likely provided organic C to com-pensate for this temporal deficit. DOC in the water column decreased between winter and summer (29–91 mg C m2 d1) and accounted for 20%–53% of CFTB and 11%–33 % of PR. Sediment resuspen-sion events supported elevated winter heterotrophy in the years that they occurred with greatest inten-sities (1998 and 2000) and may be important to interannual variability in C dynamics. Further, riv-erine discharge, containing elevated DOC (5) and dissolved P (10) relative to lake water, peaked in the winter–spring season in southern Lake Michi-gan. Collectively, terrigenous inputs (river, stream, and groundwater discharges; storm water runoff; and atmospheric precipitation) may support ap-proximately 10%–20 % of annual in-lake heterot-rophy as well as autotrophy. Terrestrial subsidies likely play a key role in the C balance of even very large lakes, representing a critical linkage between terrestrial and aquatic ecosystems. Key words: land–lake linkages; carbon balance; respiration; bacterial growth efficiency; dissolved organic carbon; sediment resuspension; terrigenous matter; southern Lake Michigan