Background/Question/Methods Dissolved organic carbon (DOC) is a fundamental driver of multiple aquatic processes, and may play a key role in structuring aquatic food webs: bacterial production (BP) fueled by DOC can supply an alternate energy source to higher trophic levels. The importance of DOC to food webs, however, likely depends on its quality and source. Despite this, most research on DOC and foodwebs has occurred in temperate lakes dominated by terrestrially-derived DOC. The Mackenzie Delta provides a unique site to study DOC in aquatic systems. Lakes in this arctic delta differ significantly in their DOC signature. Lakes with short connection times to nearby river channels have elevated DOC concentrations resulting from abundant macrophyte growth, while those with long connection times receive regular inputs of terrestrially-derived riverwater C. Permafrost melting (thermokarst) adjacent to some Delta lake shorelines is a further potential DOC source. Here, we assessed the efficacy of these DOC sources as a substrate for bacterial growth, as part of a broader study examining how variation in DOC affects its incorporation into aquatic food webs. We used a multi-pronged approach that examined BP, bacterial respiration and growth efficiency on short-connection, long-connection and thermokarst lakewater, riverwater, and macrophyte and local permafrost extracts.

Results/Conclusions BP was consistently greater on DOC leachates derived from C that had been most recently fixed (macrophytes) that on aged (permafrost) C. Although much less pronounced, this trend also occurred within Delta lakes: Lakes that had an abundant source of newer, macrophyte C had greater rates of BP than lakes where the DOC source was mainly older, riverwater or thermokarst, C. However, laboratory incubations assessing bacterial growth and respiration on different sources of lakewater DOC gave equivocal results. Stable C isotope measurements suggest that new, macrophyte-derived DOC may be quickly taken up by bacteria in this system, thus fueling in-situ growth, but leaving little of this C in the residual DOC pool. Future work in this system will assess whether substrate-driven differences in bacterial C processing affect the relative importance of bacteria to higher trophic levels. Understanding these processes is particularly relevant in northern environments, where future climate-induced changes in DOC may be intense.