The influence of forestry activity on the structure of dissolved organic matter in lakes: Implications for mercury photoreactions

It is well known that dissolved organic matter (DOM) increases in lakes associated with forestry activity but characterization of the DOM structure is incomplete. Twenty-three lakes with a wide range of forestry activities located in central Quebec, Canada were sampled and analyzed for dissolved organic carbon (DOC) concentration, DOC fluorescence, and ultra violet–visible (UV– VIS) absorption spectra. The results show that DOC increases (as does the associated DOC fluorescence) with increased logging (slope=0.122, r2=0.581, p b0.001; and slope=0.283, r2=0.308, p b0.01, respectively) in the 23 lakes sampled however, the aromaticity of the DOM does not change with changes in logging (as found by UV–VIS ratios, absorbance slope in the UV region, and DOC normalized fluorescence (slope=1.42 10 2, r2=0.331, p b0.01). The DOM from four of these lakes was concentrated using reverse osmosis (RO) followed by freeze-drying. The structures of the concentrated dissolved organic matter (DOM) samples were analyzed using X-ray analysis of near edge structures (XANES), X-ray diffraction (XRD), and 13C solid-state nuclear magnetic resonance (13C NMR) analysis. XANES analysis of functional groups in the four concentrated samples shows that there are significant differences in reduced sulphur between the samples, however there was no clear relationship with forestry activity in the associated catchment. XRD data showed the presence of amorphous sulphide minerals associated with the DOM concentrate that may be important sites for mercury binding. The 13C NMR spectra of these samples show that the percentage of carbon present in carboxylic functional groups increases with increasing logging. Such structures are important for binding photo-reducible mercury and their presence may limit mercury photo-reduction and volatilization. We propose a mechanism by which increased logging leads to increased carboxylic groups in DOM and thereby increased weak binding of photo-reducible mercury. These results, in part, explain the decrease in dissolved gaseous mercury (DGM) production rates with increased logging found in our previous work.