ABSTRACT:

Horticultural peat extraction can affect downstream mobilization of dissolved organic matter (DOM) and inorganic nutrients (nitrogen and phosphorous), harming aquatic ecosystems and water quality. However, it is unknown how regional climate and catchment land use influence the biogeochemical vulnerability of peatlands to extraction. Over two years, we used eight repeat synoptic surveys of 55 nested subcatchments to quantify the impacts of peat extraction on DOM and nutrient concentrations and transport through the stream networks of two mixed land-use catchments on the subhumid interior plains of western Canada. We used random forest models to disentangle the effects of land cover, hydrology, and temperature on water chemistry. Peatlands were the dominant source of DOM to streams, but we detected no significant effect of peat extraction on DOM concentration or composition. Stream hydrology was the most important predictor of DOM composition, with generally aromatic DOM becoming more aliphatic during snowmelt and summer base flow. We detected no effect from peat extraction on soluble reactive phosphorous (SRP) or nitrate (NO3-). However, total ammonia nitrogen (TAN) was an order of magnitude higher in subcatchments with >40% extracted peatland cover (median: 1.5 mg TAN L-1). Mass balance analysis suggested that DOM and inorganic nutrients were attenuated during low flows, and these rates covaried, likely due to biological uptake. During high flows, DOM, NO3-, and TAN were conservatively transported downstream, while SRP continued to attenuate, likely due to sorption to suspended particles. Our study suggests that excess TAN mobilized by peat extraction is utilized in headwaters during low flow but propagates downstream during high flow, with implications for eutrophication that land managers should consider.