Articles | Volume 15, issue 2
Research article
24 Jan 2018
Research article |  | 24 Jan 2018

Quality transformation of dissolved organic carbon during water transit through lakes: contrasting controls by photochemical and biological processes

Martin Berggren, Marcus Klaus, Balathandayuthabani Panneer Selvam, Lena Ström, Hjalmar Laudon, Mats Jansson, and Jan Karlsson

Abstract. Dissolved organic carbon (DOC) may be removed, transformed, or added during water transit through lakes, resulting in 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 noncolored DOC, while photochemical processing removes colored fractions, the overall changes in DOC color upon water passage through a lake depend on the relative importance of these two processes, 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 photochemical) and the loss of color during water transit time (WTT) through the lakes. We found that influence from photo-decay dominated changes in DOC quality in the epilimnia of relatively clear headwater lakes, resulting in systematic and selective net losses of colored DOC. However, in highly pigmented brown-water lakes (absorbance at 420 nm  > 7 m−1) biological processes dominated, and there was no systematic relationship between color loss and WTT. Moreover, in situ data and dark experiments supported our hypothesis on the selective microbial removal of nonpigmented DOC, mainly of low molecular weight, leading to persistent water color in these highly colored lakes. Our study shows that brown headwater lakes may not conform to the commonly reported pattern of the selective removal of colored constituents in freshwaters, as DOC can show a sustained degree of pigmentation upon transit through these lakes.

Short summary
The quality of dissolved organic carbon (DOC), especially its color, is a defining feature of freshwater ecosystems. We found that colored DOC fractions are surprisingly resistant to natural degradation during water transit through many brown-water lakes. This is explained by the dominance of microbial processes that appear to selectively remove noncolored DOC. However, in lakes where sunlight degradation plays a relatively larger role, significant DOC bleaching occurs.
Final-revised paper