Bioretention for fish habitat protection: treatment performance and spatial prioritization in a cold climate

Abstract

Urban stormwater and meltwater mobilize pollutants and transport them into urban streams, where they pose ecotoxicological risks to aquatic life. Bioretention systems (a.k.a. bioretention cells or rain gardens) are a form of green stormwater infrastructure (GSI; a.k.a. low impact development practice or LID) that retain and filter runoff, reducing volumes and improving water quality before it enters receiving watercourses. This dissertation evaluates the potential for bioretention systems to support urban fish habitat protection by reducing runoff volumes and improving water quality before discharging into urban streams. It also examines whether event-scale stormwater and meltwater treatment improvements are detectable through short-term downstream water quality monitoring and identifies where stormwater interventions should be prioritized to support the protection of sensitive fish habitats. A perspective chapter outlines an integrated approach to urban stream restoration that combines physical habitat improvements with green stormwater infrastructure practices, such as bioretention systems, within urban land use planning and renewal. The chapter emphasizes post-restoration monitoring, incorporating water quality guidelines into GSI performance evaluations, strategically locating restoration and stormwater interventions where they are most likely to support ecological protection, and increasing stormwater management on private property through public engagement. Field investigations evaluated three bioretention systems discharging into trout-sensitive urban tributaries in Thunder Bay. Water quality was monitored at all three systems, while water quantity was monitored only at Bioretention Systems 1 and 2 because site constraints at Bioretention System 3 prevented reliable inflow and outflow discharge measurements. During rainfall events, Bioretention Systems 1 and 2 fully retained runoff during 43 and 70 of 87 monitored events, respectively. When effluent occurred, suspended solids concentrations decreased by 51-64% across the three bioretention systems and turbidity was reduced at two of the three systems. These reductions in runoff volumes, turbidity, and suspended solids suggest a reduced potential for particulate pollutant delivery and fine sediment inputs into fish-bearing streams. Additional analyses examined pollutant accumulation in the winter snowpack and bioretention performance during the spring freshet. Roadside snowbanks contained significantly higher concentrations of chloride, suspended solids, and dissolved organic carbon than open field and bioretention sites. During spring melt, peak and total meltwater volumes were reduced at Bioretention Systems 1 and 2, where hydraulic monitoring was feasible, while water quality was evaluated across all three systems. Across the three systems, pH, turbidity, suspended solids, and dissolved organic carbon concentrations decreased in meltwater before discharging to receiving waters. A rapid assessment framework integrating stormwater impairment data and habitat surveys was developed to identify priority locations for green stormwater infrastructure. Applied to a trout-sensitive tributary in Thunder Bay, Ontario, this framework provides municipalities with a practical tool to prioritize stormwater interventions in locations where they are most likely to support the protection of sensitive fish habitats. This dissertation makes several novel contributions to stormwater management and fish habitat protection in a cold climate. First, it provides a critical perspectives-based synthesis that identifies why urban stream restoration, stormwater management, and land use planning can fail to protect urban streams when implemented independently. Rather than simply arguing that these practices should be integrated, this chapter clarifies specific management disconnects between these practices. It is argued that stream habitat restoration focusses on improving physical fish habitat, but does not adequately address impacts from untreated stormwater runoff, that GSI may reduce runoff quantity and improve runoff quality at the site level, without producing detectable ecological recovery, and that land use planning may miss opportunities to reduce future pollutant loading, protect sensitive areas, or support GSI implementation on private land. The contribution of this chapter is a critical synthesis that reframes urban stream revitalization as an integrated planning challenge rather than a separate set of stream restoration, stormwater and land use practices. Stream habitat restoration projects and green stormwater infrastructure (GSI) aim to protect urban streams, but are often implemented independently. This chapter provides a critical perspectives-based synthesis that reframes urban stream revitalization as an integrated planning challenge. It highlights how isolated approaches can limit ecological recovery and identifies strategies to coordinate habitat restoration, GSI performance evaluation, winter snow monitoring, stormwater controls on private land, and riparian protection in cold-climate urban watersheds. Recommendations include post-restoration monitoring, aligning restoration with local degradation, strategic placement of GSI and habitat improvements, and incorporating land-use planning and zoning to protect sensitive areas. These recommendations provide the conceptual framework for the field-based and applied chapters that follow. Second, it provides empirical evidence from rainfall and snowmelt events demonstrating that bioretention systems can substantially reduce runoff volumes and particulate-associated pollutants before discharging into trout-bearing waters. Third, it demonstrates that treatment performance differs between particulate and dissolved contaminants, with high reductions in turbidity and suspended solids concentrations, but limited or inconsistent reductions in chloride and nutrients, emphasizing the need to reduce pollutant sources at the source through changes in land use practices to complement bioretention treatment performance. Fourth, it identifies roadside snowpack as an important seasonal reservoir of sediment, chloride, and organic carbon, highlighting the influence of winter road maintenance, vehicular activity and spring freshet processes on cold climate stormwater quality. Lastly, it develops a spatial prioritization framework that combines stormwater impairment identification with downstream fish habitat sensitivity analysis to guide green stormwater infrastructure placement where the ecological benefit to fish habitats is most likely.