Spatio-temporal variation in the characteristics of dissolved organic matter in the streams of boreal forests: Impacts on modelled copper speciation

Dissolved organic matter (DOM) is an important constituent of natural waters that controls numerous biogeochemical processes such as the toxicity and mobility of metals. In order to predict how metals behave in the presence of DOM, it is necessary to understand its acidic properties. In this study, we report the variations in the acid character of aquatic organic acids using 30 years of sampling data collected from three boreal streams. Based on a charge balance model, significant spatial and temporal variation in carboxylic acid site density was observed. The seasonal average carboxylic group density ranged from 8.01 ± 1.47 to 12.0 ± 1.90 μeq mg C−1 and the overall (multi-site) average site density (winter excluded) was 9.66 ± 0.125 μeq mg C−1 (n = 3193). Both different sources of DOM and seasonal differences in source availability were found to contribute to variations in site density. In the deciduous catchment where wetland contributions to DOM were negligible, the seasonal variability in site density was highly marked with increases of up to 50% observed between spring and fall. Less seasonal variation was noted at the coniferous sites, which had relatively high wetland source contributions. The geochemical equilibrium speciation model MINTEQA2 showed that the increase in site density observed from winter to fall coincided with a decrease in free copper concentration. We conclude that some source-based differences in DOM that may result from variation in both catchment characteristics and seasonal DOC loadings necessitate the determination of location-specific and/or seasonal site density values due to the resulting variability in metal speciation that has been predicted through modelling.