Benthic Macroinvertebrate Recovery After Wastewater Treatment Plant Upgrades: Implications for Biomonitoring

ABSTRACT During the past decade, major upgrades have been made to the Kitchener and Waterloo wastewater treatment plants (WWTPs) in the Grand River watershed in Ontario, Canada. As part of efforts to evaluate the effectiveness of these investments (>$450 M CDN), a benthic macroinvertebrate (BMI) monitoring program was initiated by sampling upstream and downstream (near and far field) of effluent outfalls at approximately 3‐year intervals from 2009 to 2022 using a Before‐After/Control‐Impact (BACI) design. The data were evaluated using multivariate analysis on the entire assemblage and univariate analysis on several metrics (family richness, modified family biotic index, % scrapers, % filterers, Bray–Curtis dissimilarity) to assess if treatment plant upgrades restored BMI assemblages. Results indicated effects of effluent from the WWTPs have reduced over time, with multivariate analysis and family richness indicating upstream and downstream assemblages becoming more similar following upgrades at both WWTPs. Although there was not complete agreement between multivariate and univariate analyses, this study highlights the benefit of using both in biomonitoring programs to describe the effects of remediation efforts as different metrics may respond differently to the same environmental factors. Ultimately, the multi‐year BACI experimental design was effective in tracking the recovery of BMI assemblages associated with effluent quality improvements following WWTP upgrades by considering spatial and interannual variation, while also highlighting the need for long‐term study to allow effects to manifest and be detectable despite the influence of other spatial and temporal factors.