Dissolved organic carbon concentration and character in northern hardwood-dominated headwater catchments: A paired-catchment investigation of legacy harvesting impacts

Abstract

The water quality of forested source water regions can be degraded by natural and anthropogenic landscape disturbances such as wildfires and forest harvesting, the latter an economically important primary industry in Canada and a proposed wildfire mitigation strategy. Harvesting practices can alter the chemistry and hydrologic connectivity of hillslope solute pools, thereby enhancing hillslope-stream transport and the downstream propagation of sediments and solutes, including those relevant to drinking water treatment operations such as dissolved organic carbon (DOC). Although many studies have evaluated the sub-decadal impacts of forest harvesting on the concentration, export, and character of stream DOC, less is known about the legacy (decadal-scale) impacts. The purpose of this thesis was to evaluate the legacy impacts of clearcut harvesting on the variability of stream DOC concentrations, export, and character at the Turkey Lakes experimental Watershed (TLW). Using a paired-catchment approach (unharvested reference vs. legacy (24 years post-) clearcut), inter- and intra-catchment variability in stream DOC concentrations and export was evaluated under a range of flow conditions. Stream DOC variability was related to the concentrations, spatial distribution, and hydrologic connectivity of hillslope solute pool DOC. Additionally, a subset of event-scale stream and hillslope solute pool samples were analyzed for DOC character using Liquid-Chromatography Organic Carbon Detection (LC-OCD). DOC character was expressed in terms of the specific UV absorbance at 254 nm (SUVA) and the relative contributions of LC-OCD-defined DOC fractions. Whereas stream DOC concentrations in the legacy clearcut catchment exceeded (+1.21 mg L-1) and differed significantly (p ≤ 0.05) from the unharvested reference catchment, inter-catchment differences in stream DOC export were inconsistent. No inter-catchment differences were observed in the DOC concentrations or hydrologic connectivity of the hillslope solute pools, despite the common association of these mechanisms with post-harvest increases in stream DOC concentrations. Significant (p ≤ 0.05) inter-catchment differences in the fractional composition of stream DOC were observed at the event-scale but may be related to the presence of a wetland near the outlet of the unharvested reference catchment, rather than a harvesting impact. Wetland position was identified as a key factor in the variability of both DOC concentration and character in the unharvested reference catchment. Overall, the results of this thesis suggest that while forest harvesting practices may result in long-term increases in stream DOC concentration in northern hardwood-dominated headwater catchments, the effects may be limited at decadal-scales and likely do not pose a reasonable threat to downstream drinking water treatment operations.