Diel cycles in the fluorescence of dissolved organic matter in dystrophic Wisconsin seepage lakes: Implications for carbon turnover

We monitored the dynamics of chromophoric dissolved organic matter (CDOM) fluorescence in two Wisconsin bog lakes over timescales ranging from hours to months. Peatland-derived dissolved organic matter (DOM) was the major solute in both bog lakes, and diel cycles were dominant features of the CDOM fluorescence time series. Two distinct diel cycles that differed in amplitude and timing were observed: one in oxic epilimnia and a second in anoxic, hypolimnetic waters. These cycles were not attributable to instrumental artifacts (i.e., daily oscillations of temperature, ambient light, or battery voltage), hydrologic forcing, or the effects of inner filtering, pH, or redox conditions. High light extinction coefficients, especially in the ultraviolet region (∼ 10 m−1 to 30 m−1), suggest that DOM photolysis was negligible at the depths of the CDOM fluorescence probes in these dark-water lakes (dissolved carbon concentration: 10 mg C L−1 to 20 mg C L−1). The diel cycles were apparently governed primarily by biological activities that mediate DOM production (release) and destruction (uptake). Rates of carbon turnover derived from properties of the epilimnetic CDOM fluorescence cycle (0.28 mg C L−1 d−1) were similar to rates of net ecosystem production based on daily CO2 dynamics (0.32 mg C L−1 d−1). It appears that a small, secondary pool of labile organic carbon turns over at relatively high rates in these bog lakes, consistent with the two-compartment view of DOM stability.