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Exogenous and endogenous origins of DOC in lakes: Surprising results from whole lake 13C additions.
Bade, Darren*,1, Carpenter, Stephen1, Cole, Jonathan2, Pace, Michael2, 1 University of Wisconsin-Madison, Madison, Wisconsin, USA2 Institute of Ecosystem Studies, Millbrook, New York, USA
ABSTRACT- Dissolved organic carbon (DOC) is the largest reservoir of organic carbon in the water column of many lakes. DOC can originate from primary production within the lake itself (endogenous) or in the terrestrial watershed (exogenous), and both sources are often thought to have distinctive dynamics and roles in lakes. Because DOC consists of a highly complex mixture of organic compounds, little is known about the sources of the standing stock of DOC. To determine the sources and fates of DOC in lakes we conducted four whole ecosystem inorganic carbon stable isotope additions. In one of these manipulations (Peter Lake, 2002) we also added inorganic N and P to increase primary production; in the other experiments, no nutrients were added (Peter and Paul lakes, 2001; Tuesday Lake 2002). Prior to 13C enrichment, background isotope signatures of DOC were similar to terrestrial carbon and consequently it was difficult to discern the contribution of each source of DOC. Following 13C addition, the DOC isotope signature increased between 1.5 and 9.5 per mil, due to DOC inputs from 13C labeled primary producers. The largest change in the DOC isotope signature was observed in Peter Lake (2002; the year of nutrient enrichment) and the smallest change was observed in Tuesday Lake, which had the highest concentration of DOC. Using carbon isotope results to constrain models of carbon fluxes, algal inputs of DOC was 10 to 30 % relative to terrestrial carbon inputs in the unfertilized lakes, and higher in the fertilized lake. Despite the fact that terrigenous DOC is considered to be fairly recalcitrant to microbial respiration and algal DOC quite labile, the loss rates of each form of DOC were found to be similar.
Key words: stable isotopes, dissolved organic carbon, allochthonous inputs, excosystem experiment