Projecting In-stream Dissolved Organic Carbon and Total Mercury Concentrations in Small Watersheds Following Forest Growth and Clearcutting

Forest is an important vegetation type on the globe, and clearcutting is the main forest management method. This paper presents a process-based model developed to project the impact of forest growth and clearcutting on dissolved organic carbon (DOC) and total mercury (THg) export from forest-dominated watersheds over two forest-growing cycles. The modelling of THg is based on the observation that THg export from terrestrial to aquatic eco-systems occurs with the binding and subsequent in-stream transport of THg by DOC. From the results generated with the integrated model, DOC and THg export follows two main trends; (i) a multiple-year trend, associated with forest harvesting and re-growth patterns over the lifetime of the forest, and (ii) an annual trend, associated with the seasonal dynamics in forest litter production and decomposition. During a forest rotation, DOC and THg concentration decreases following clearcutting, reaches a minimum at about 15 years after forest regeneration and then gradually increases with forest ageing. Large debris pools left on site following clearcutting can provide a significant pulse in DOC production and within-watershed THg export during the first 2-3 years after harvest. In a single year, the integrated model predicts that DOC- and THg-concentration peaks after leaf fall in autumn, decreases to a minimum in April, increases to another maximum in June and finally decreases to a second minimum just before leaf fall. This seasonal cycle is repeated every year. Conifer species and wetland-dominated watersheds are anticipated to release a greater amount of DOC and THg to aquatic ecosystems than deciduous and dryland-dominated watersheds. The long-term and seasonal DOC production is consistent with field measurements.