Managing Urban Eutrophication Risks Under Climate Change

Nearly three quarters of Canadians live in large urban areas, a number that is expected to keep increasing. High densities of people, buildings, and impervious surfaces increase emissions of pollutants that degrade the water quality and ecosystem health of nearby lakes and rivers. While stormwater management (SWM) is traditionally designed to prevent flooding, it can also offer opportunities for mitigating the effects of urban development on water quality. Better planning of SWM is especially important as urban communities are facing accelerating changes in weather patterns due to climate change. 

One challenge facing many freshwater ecosystems around the world is eutrophication, meaning they become overly enriched with nutrients. This promotes the growth of algae and aquatic plants that, if left unchecked, can pose serious risk to drinking water supplies, limit recreational opportunities, and cause the die-off of aquatic life. The standard approach to manage eutrophication is to reduce phosphorus (P) loadings because P is generally the nutrient element limiting algae and plant growth in freshwater ecosystems.    

Traditional SWM practices, such as stormwater ponds, and newer low impact development (LID) technologies, such as bioretention cells, have the potential to attenuate the export of P and other nutrients, such as nitrogen (N), from urban landscapes. However, the efficiencies of different SWM practices in reducing P and N loads remain poorly characterized, with some studies even reporting increased concentrations of P and N in the outflow of LIDs.

The project Managing Urban Eutrophication Risks Under Climate Change has been working toward quantitatively assessing nutrient loads in urban stormwater and to identify how these loads are modulated by land use, municipal operations, climate conditions, and SWM practices. By advancing the predictive understanding of urban stormwater P and N loads, the project has been generating actionable research outcomes that can assist municipal planners in the implementation of SWM strategies that alleviate the risks of eutrophication in and around cities. Research activities are focusing on the most densely populated region of Canada – the Golden Horseshoe which surrounds the western end of Lake Ontario and hosts many streams and lakes, as well as key groundwater resources.  

The researchers analyzed the emission concentrations of different chemical forms of P in stormwater runoff originating from urban watersheds with variable land cover. The results showed that, in contrast to rural landscapes, a large fraction of P in urban stormwater runoff consists of reactive particulate P. Because this particulate P can easily become available for uptake by plants and algae, an important implication of this finding is that current nutrient monitoring programs, which typically consider reactive dissolved P as being the bioavailable fraction, may underestimate the total emissions of bioavailable P from urban areas.  

Detailed studies of a stormwater pond and a bioretention cell allowed the team to unravel the processes controlling the fate of P in these SWM systems. They found that the formation of stable calcium phosphate minerals explains the high retention of P by both the traditional stormwater pond and the LID system. The good news is that, in less performing SWM systems, relatively simple technical interventions could stimulate calcium phosphate mineral formation and, therefore, increase P retention efficiencies. 

By analyzing water quality data from the International Stormwater Best Management Practices Database, researchers found that traditional management approaches, in particular stormwater ponds, are generally more effective at capturing and retaining P than newer LID approaches. The data analyses also showed that SWM systems can cause a shift in the P to N ratio of urban stormwater runoff. That is, SWM practices can alter the nutrient limitation patterns in receiving aquatic environments, potentially causing far-reaching ecological shifts. The research further delineated the roles of meteorological and hydrological variables in regulating the nutrient control performance of SWM systems. 

The researchers have also made an important breakthrough by drawing a connection between road salt applications and eutrophication symptoms in urban lakes. The team examined a small lake north of Toronto called Lake Wilcox and discovered that it was experiencing eutrophication not due to higher P inputs from the watershed but instead because of increasing salt levels as a result of road salt applications during winter. Salinization of the lake is leading to an increase in water density, which reduces the mixing of the lake’s waters and limits the amount of oxygen that can reach the deeper parts of the lake. Lower oxygen concentrations then trigger the release of P from the sediment at the bottom of the lake, a process known as internal P loading. The key message is that managing eutrophication and salinization of freshwater ecosystems are intricately linked. 

The project further assessed the use of satellite remote sensing to map the spatial and temporal distributions of chlorophyll-a (a proxy for the abundance of algae) in the nearshore zone of the western basin of Lake Ontario. These waters are the direct recipient of nutrient inputs from the Golden Horseshoe. By optimizing the resolution of the satellite imagery, selecting the best performing spectral bands, and applying proper corrections for atmospheric artefacts, the researchers were able to reconstruct past distributions of chlorophyll-a. The results imply a generally declining trend in the algal abundance of the nearshore zone of Western Lake Ontario, which can be attributed in part to effective P retention by SWM best practices in the surrounding urban areas.     

Within this project, researchers developed and/or enhanced several predictive tools: