Reactivity continuum of dissolved organic carbon decomposition in lake water

We determined microbial decomposition of dissolved organic carbon (DOC) over 3.7 year long dark bioassays of six Swedish lake waters. The overall lost DOC fraction was similar in clearwater lakes (34.8 ± 2.4%) and in brownwater lakes (37.8 ± 1.9%). Reactivity continuum modeling revealed that the most labile DOC fraction, degrading at rates >0.01 d −1 , was larger in the clearwater lakes (11.1 ± 1.2%) than in the brownwater lakes (0.8 ± 0.1%). The initial apparent first‐order decay coefficient k was fivefold larger in the clearwater lakes (0.0043 ± 0.0012 d −1 ) than in the brownwater lakes (0.0009 ± 0.0003 d −1 ). Over time, k decreased more steeply in the clearwater lakes than in the brownwater lakes, reaching the k of the brownwater lakes within 5 months. Finally, k averaged 0.0001 d −1 in both lake categories. In the brownwater lakes, colored dissolved organic matter (CDOM) absorption decayed with an initial k twice as large (0.0018 ± 0.0008 d −1 ) as that of DOC. The initial k was inversely correlated with initial specific UV absorption and CDOM absorption and positively correlated with initial tryptophan‐like fluorescence as proxy for autochthonous DOC. Exposure to simulated sunlight at the end of the incubations caused loss of color in the clearwater lakes and loss of DOC in the brownwater lakes, where subsequent mineralization was also stimulated. The DOC lost in the absence of photochemical processes fell below previously reported watershed‐scale losses in Sweden by 25% at most. This suggests that a major part of the in situ DOC loss could potentially be attributed to dark reactions alone.