Optimizing Water Retention to Reduce Algal Blooms in Canadian Lakes

This executive summary explains how optimizing the design of water retention projects in the Canadian Prairies can maximize phosphorus reduction to prevent algal blooms. Using data from the De Salaberry project (2019-2023), the authors demonstrate that increasing hydraulic retention time through strategic design changes, such as modifying outlet culvert diameters, significantly improves phosphorus removal. The report recommends integrating quantitative phosphorus reduction estimates into planning, targeting phosphorus hotspots, and developing practical tools for engineers to balance water quality with flood mitigation.

• Water retention projects, including small naturalized dams and conserved or restored wetlands, are essential for climate change adaptation in the Canadian Prairies. These projects provide multiple benefits, such as reducing flood and drought risks, improving biodiversity, and reducing atmospheric carbon. Economically, these investments are estimated to return CAD 2.45 in benefits for every dollar spent.
• Phosphorus is identified as the primary limiting nutrient responsible for algal blooms, making its reduction critical for water quality. While many current water retention projects are expected to reduce phosphorus, these benefits are often only qualitatively assessed and are unlikely to be maximized under standard design practices.
• Research and modelling from the De Salaberry water retention project (monitored from 2019 to 2023) indicate that hydraulic retention time—the duration water remains stored before release—is the most significant factor in phosphorus reduction. Increasing this time allows for more sedimentation and natural treatment. For example, reducing an outlet culvert diameter from 600 mm to 400 mm increases the average flow-weighted retention time from 2.76 days to 4.14 days, increasing total phosphorus load reduction from 271.3 kg (11.15%) to 309.9 kg (12.73%).

APA

Simoes, J., & Grosshans, R. (2026). Optimizing Water Retention to Reduce Algal Blooms in Canadian Lakes. International Institute for Sustainable Development. https://www.iisd.org/system/files/2026-06/water-retention-reduce-algal-blooms-canadian-lakes.pdf

Chicago

Simoes, Joey, and Richard Grosshans. Optimizing Water Retention to Reduce Algal Blooms in Canadian Lakes. International Institute for Sustainable Development, 2026. https://www.iisd.org/system/files/2026-06/water-retention-reduce-algal-blooms-canadian-lakes.pdf.

Wikipedia

{{cite report |last1=Simoes |first1=Joey |last2=Grosshans |first2=Richard |title=Optimizing Water Retention to Reduce Algal Blooms in Canadian Lakes |publisher=International Institute for Sustainable Development |date=June 2026 |url=https://www.iisd.org/system/files/2026-06/water-retention-reduce-algal-blooms-canadian-lakes.pdf |access-date=17 August 2026 |via=Climate Insights Directory}}

BibTeX

@techreport{simoes2026optimizing, author = {Simoes, Joey and Grosshans, Richard}, title = {{Optimizing Water Retention to Reduce Algal Blooms in Canadian Lakes}}, institution = {International Institute for Sustainable Development}, year = {2026}, month = jun, url = {https://www.iisd.org/system/files/2026-06/water-retention-reduce-algal-blooms-canadian-lakes.pdf}, urldate = {2026-08-17}, note = {Indexed by Climate Insights Directory} }