Linking dynamics of dissolved organic carbon in a forested lake with environmental factors

Dynamics of dissolved organic carbon concentration (DOC) and capacity to absorb light (color) are determined by in-lake and external properties and processes. In this study, the influence of external factors such as rainfall and solar radiation on DOC and color dynamics was assessed for a small forested lake. DOC and absorption coefficients at 440 nm (a440) ranged 4-fold from 0.46 to 1.62 mM and from 3.4 to 14.8 m, respectively. DOC and a440 varied synchronously, but an important percentage of the variability (26%) in a440 was not explained by DOC. The resulting twofold variation in the molar absorption coefficient of DOC suggested significant seasonal changes in chromophoric content. Both DOC and a440 were positive and significantly related to cumulative rainfall. Solar radiation, however, only appeared to influence a440 dynamics. This influence was mediated by photobleaching. Photobleaching coefficients (kb) were higher in fall and spring relative to the summer. This seasonal variability in kb values was related to monthly rainfall. The influence of photobleaching on a440 dynamics was evaluated by comparing the half life of a440 in the water column with water residence time (WRT). For the study lake, photobleaching contributed notably to a440 dynamics during the dry periods when WRT was longer than the a440 half life. DOC dynamics, however, were not related to solar radiation because photomineralization was considerably slower than photobleaching.