Water quality impacts of wetland drainage and changing climate in the Prairie Pothole Region

Abstract

The Canadian Prairies contains a high density of hydrologically isolated wetlands, termed “potholes”, which have been identified as important to hydrology and water quality. These wetlands have increasingly been drained for expanding agriculture with some evidence of ecohydrological impacts. In addition there is evidence the hydrology of the Prairies is being impacted by climate change including shorter durations of ice and snow cover and shifts in precipitation drivers of runoff (i.e. snowmelt vs. rainfall). The aim of this thesis was to determine, on a wetland pond scale, how a shorter duration of ice-cover alters nutrient cycling, and on a basin scale, how wetland drainage in the Canadian Prairies is impacting water quality under changing hydrologic conditions. Results from this study suggest wetland drainage may have important water quality impacts, specifically an increase in the proportion of wetlands fully drained was correlated with higher flow-weighted mean concentrations of nutrients (phosphorus and nitrogen) at the basin outlet. Observations of nutrient and salt exports in the study basin suggest that climate changes affecting the timing and magnitude of discharge in the Prairies is likely to impact annual nutrient and salt transport. Although for most years snowmelt transport dominated nutrient transport, as is typical for this region, a large summer storm with wet antecedent conditions generated rain-fall driven nutrient transport greater than had been observed for snowmelt. On an intra-storm basis nutrient concentration response to discharge was found to be highly variable, especially during snowmelt, suggesting a need for more frequent (> weekly) measurements during high flow periods or the incorporation of biogeochemical processes affecting nutrient concentrations in load estimation methods. Results from a winter sediment incubation for three prairie wetland ponds suggest that phosphorus loading is significant over winter in pothole ponds and found to be driven mainly by sediment release with exclusion of phosphate from ice representing a minor role. Winter phosphorus release from sediment was highly variable with the very high rates of P release (9 to 274 mg m-2 day-1) occurring in the more saline discharge pond studied. The combined impact of less winter phosphorus accumulation in pothole ponds due to shorter ice-cover and greater rain-fall driven nutrient export may be a shift in timing of nutrient export, impacting eutrophication and risk of harmful algal blooms, and with the potential to be further exacerbated by impacts of ongoing wetland drainage.