Biogeosciences Discuss., https://doi.org/10.5194/bg-2017-279 Manuscript under review for journal Biogeosciences Discussion started: 14 July 2017 c Author(s) 2017. CC BY 4.0 License. Quality transformation of dissolved organic carbon during water transit through lakes: contrasting controls by photochemical and biological processes Martin Berggren1, Marcus Klaus2, Balathandayuthabani Panneer Selvam1, Lena Ström1, Hjalmar 5 Laudon3, Mats Jansson2, Jan Karlsson2 1Department of Physical Geography and Ecosystem Science, Lund University, SE-223 62, Lund, Sweden 2Department of Ecology and Environmental Science, Umeå University, SE-90187, Umeå, Sweden 3Department of Forest Ecology and Management, Swedish University of Agricultural Sciences, SE-90183, Umeå, Sweden Correspondence to: Martin Berggren (
[email protected]) 10 Abstract. Dissolved organic carbon (DOC) may be removed, transformed or added during water transit through lakes, resulting in qualitative changes in DOC composition and pigmentation (color). However, the process-based understanding of these changes is incomplete, especially for headwater lakes. We hypothesized that because heterotrophic bacteria preferentially consume non-colored DOC, while photochemical processing remove colored fractions, the overall changes in DOC quality and color (absorbance) upon water passage through a lake depends on the relative importance of these two processes, 15 accordingly. To test this hypothesis we combined laboratory experiments with field studies in nine boreal lakes, assessing both the relative importance of different DOC decay processes (biological or photo-chemical) and the loss of color during water transit time (WTT) through the lakes. We found that photo-chemistry qualitatively dominated the DOC transformation in the epilimnia of relatively clear headwater lakes, resulting in selective losses of colored DOC.