Alum Treatment Improves Water Quality and Overwintering Potential in Stocked Ponds in Alberta, 2025–2026
Interim Report
Author(s)
Scott Seward, B.A., M.Sc.
Kevin Fitzsimmons, B.Sc.
Lindsay Marley, B.Sc.
Summary
Alberta Conservation Association (ACA) stocks trout into over 60 ponds in Alberta, many of which, including Westlock Pond, are prone to high nutrient loading, algal blooms, and summer/winter hypoxia. To address these water quality challenges, we conducted a five-year (2020–2025) Before-After Control-Impact (BACI) study at Westlock Pond, a small eutrophic waterbody in central Alberta. In 2023, we applied a total of 250 mg/L of aluminum sulphate (alum), split across two 125 mg/L applications, to determine if alum treatment could successfully suppress internal phosphorus loading and improve overwintering dissolved oxygen (DO) for stocked trout.
Alum treatment proved effective at reducing nutrient concentrations in Westlock Pond. BACI analysis confirmed a statistically significant treatment effect, demonstrating a sustained three-fold decline in mean phosphorus concentrations despite persistent high nutrient concentrations in control ponds. Mean total phosphorus decreased from 0.09 mg/L pre-treatment to 0.03 mg/L post-treatment, while dissolved phosphorus decreased from 0.03 mg/L to 0.01 mg/L. Furthermore, Interrupted Time-Series Analysis (ITSA) and Comparative ITSA revealed a partial improvement in winter DO concentrations. Alum treatment successfully delayed the onset of lethal winter hypoxia, maintaining survivable DO concentrations (>3 mg/L) at 1 m and 2 m depths for one month longer (extending from early December to early to mid-January) compared to pre-treatment conditions. Conversely, while summer (July–September) daily minimum DO concentrations fluctuated across years, comparative analysis revealed these shifts during the open-water season mirrored natural regional variability and could not be statistically attributed to alum treatment.
This study demonstrates that alum was an effective management tool for suppressing internal phosphorus loading and extending winter habitat duration in Westlock Pond. Following the sustained three-fold reduction in nutrient concentrations, Westlock Pond DO concentrations remained suitable for trout survival for a full month longer than pre-treatment conditions. While these results are drawn from a single waterbody, they suggest that targeted phosphorus reduction can practically extend the viable overwintering window of a stocked fishery, serving to maximize angling opportunity even if late-winter hypoxia is not eliminated.