Forest Harvest Effects on Dissolved Organic Carbon (DOC): Illuminating Flowpaths and Flux Through DOC Chemistry and Composition

Abstract:

Studies on dissolved organic matter (DOM) dynamics have been recently advanced by the advent of deployable, in-situ sensors capable of measuring both dissolved organic carbon (DOC) concentration and DOC characteristics. These methods, combined with improvements in fluorescence spectroscopy analysis, allow for high temporal resolution investigation of DOC flux into aquatic systems, as well as DOC flowpaths within coupled terrestrial-aquatic systems. This study utilizes a combination of in-situ UV-Vis absorbance spectrophotometry, as well as laboratory-based analysis by fluorescence spectrophotometry of corresponding water samples, to investigate how forest harvest affects organic matter flux in a small headwater stream located on Vancouver Island, British Columbia. The majority of DOC flux occurring during the wet winter months during both pre- and post-harvest periods, while clear-cut harvesting significantly increased the concentration and flux of DOC export. The concentration-discharge relationship for both periods showed transport-limited behavior. This was not significantly altered upon harvest; however the examination of specific events showed that DOC response greatly increased in the post-harvest period for specific storm events. Concentration-discharge relationships evaluated using segmented regression analysis demonstrated a large amount of inter-annual variability in streamwater DOC concentrations. However, when DOC concentration is modeled with a least-squares linear regression model, the pre- vs. post harvest period remains the best predictor of DOC concentration, followed by year (reflecting the large degree of inter-annual variability). Precipitation is a greater predictor of DOC concentration rather than discharge, groundwater height, baseflow or quickflow. The fluorescence characteristics of streamwater DOC show that the DOC fraction is dominated by humic-like structures within both periods. Examining DOC quality parameters obtained through in situ absorbance spectrophotometry supports this shift towards DOC originating from shallower soil horizons. This was confirmed by investigating DOC quality metrics of mobile DOC from various soil profile depths (through analysis of lysimeter and soil extract samples).