Increasing fluxes of terrestrial dissolved organic carbon (DOC) to aquatic systems have been documented throughout the northern hemisphere. In the boreal zone, they are ascribed to a combination of increasing precipitation due to climate change and increasing soil pH following the recovery from acid deposition, while in the arctic to the ongoing thawing of permafrost soils. Increased DOC is a concern for water quality, as it can reduce the efficacy of UV- and chlorine-based water treatment, and global climate, as DOC can be converted into carbon dioxide (CO<sub>2</sub>), thus providing a positive and accelerating feedback to global warming over decades. In this context, we present a new module for the fate of DOC in vertically stratified water bodies that captures microbial metabolism, photo-processing, and flocculation. We present a review of past formulations for these 3 categories of DOC processing and the results of the implementation of the new module in the daily timestep lake model MyLake, linking DOC to existing state variables in the model via new sinks and sources. The performance of the module was evaluated against 6 years of biweekly DOC field data from Lake Langtjern in Norway. The model successfully reproduced water temperature, dissolved oxygen, and the variability of DOC at the lake outlet. Results suggest that DOC retention depends on residence time, and thus precipitation, with about 10% of inflowing DOC being processed in the lake. The integrated rates predicted by the model suggest that DOC in the lake is primarily degraded via microbial metabolism (68%), with flocculation (30%) as a secondary sink, and negligible photo-processing (&lt;2%). Further, microbial degradation of DOC to CO<sub>2</sub> is likely to increase in response to ongoing climate change via two synergistic mechanisms: i) enhanced water column ventilation due to earlier ice-out that favors aerobic DOC degradation, and ii) warming of epilimnetic waters later in the season that favors faster metabolic rates.

Disentangling the contributions of metabolism, light, and flocculation to removing dissolved organic carbon from vertically stratified aquatic environments

2019 Fall National Meeting

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