High-frequency analysis of dissolved organic carbon storm responses in headwater streams of contrasting forest harvest history

Abstract

Dissolved organic carbon (DOC) in streams is critically important for aquatic ecosystems and human water use, and is affected by changes in land use and climate. The effects of land use change (e.g. forest harvest) on DOC are not fully understood. Past studies of the effects of forestry on DOC have found mixed results, and covariate effects complicate mechanistic understanding and may underlie regional differences. DOC is typically highest during storm events, and detailed time-resolved measurements of DOC over longer periods are just becoming feasible with advances in in situ sensors. In this paper, we investigated high frequency measurements of DOC concentration in headwater streams of contrasting forest harvest history in the UBC Malcolm Knapp Research Forest near Maple Ridge, British Columbia. We found base flow DOC concentration to be lower in the clear-cut stream (Clear cut: 2.26 ± 0.43 mg/L; Forested: 4.30 ± 0.83 mg/L). This may be due to reductions in carbon inputs like leaf litter, although the difference in drainage area, slope, and the presence of a bog lake in the forested catchment may also account for some of the observed difference. We found the DOC response to storms to be larger and faster at the clear-cut site (mean increase DOC: Clear-cut: 2.42 mg/L, Forested: 1.99 mg/L; mean rate of change DOC: Clear-cut: 0.16 mg/L/h, Forested: 0.11 mg/L/h). Elevated storm responses may be due to changes in flow paths related to forest harvesting. We found low antecedent flow and greater storm intensity to significantly predict elevated DOC storm response. This is important in the context of future climate scenarios, which predict lower summer flows and more intense storms in this region. Overall, this work contributes to our understanding of the intersection between effects of land use change and climate on water quality, with a focus on drinking water treatment implications.