Effects of wastewater on phosphorus, nitrogen, and nuisance benthic algae in nearshore regions of a large lake

Upgrading wastewater treatment plants (WWTPs) is a global practice for achieving increasingly stringent nutrient discharge objectives set by governments to accommodate population growth and reduce surface water pollution. However, associated downstream improvements in nutrient conditions are difficult to determine in nearshore regions of large aquatic ecosystems due to complex biophysical processes. We conducted a nine-year water quality study and analyzed the data using linear mixed models (LMMs) within a Before-After-Control-Impact (BACI) framework to assess effects of an upgrade to the Duffin Creek Water Pollution Control Plant (DCWPCP) on surface water nutrient conditions and proliferation of nuisance benthic algae (Cladophora glomerata) in nearshore Lake Ontario. The DCWPCP upgrade resulted in increased effluent concentrations and loads of nitrite+nitrate (NO2+3) due to enhanced nitrification, while reducing total Kjeldahl nitrogen and ammonia+ammonium (NH3+4). However, total phosphorus (TP) in effluent only changed slightly due to operational constraints during plant upgrade. For nearshore nutrient conditions, our LMM-BACI framework revealed that, after upgrade, NH3+4 decreased at impact site relative to control sites. In contrast, following upgrade, an observed decline in NO2+3 concentrations was less pronounced at impact site compared to control sites, suggesting increased NO2+3 inputs into nearshore surface water from the DCWPCP. We could not detect obvious improvement in nearshore TP concentrations, stoichiometric ratios of total inorganic nitrogen to TP and NO2+3 to TP, or phosphorus tissue content of Cladophora, likely due to the only slight reduction in TP from the DCWPCP. Overall, our findings showed that the DCWPCP upgrade increased NO2+3 inputs, which could have important implications for nutrient management and trade-offs associated with WWTP upgrades that reduce one chemical species at the expense of another. Other researchers may find our LMM-BACI framework useful to detect localized impacts of nutrient inputs in nearshore and coastal areas where multiple physical and climate drivers influence water quality. [Display omitted] •Wastewater treatment was upgraded in a major facility on Lake Ontario.•A censored linear mixed model was built in a before/after impact/control framework.•In nearshore surface water, ammonia decreased, but nitrate decreased less obviously.•TP and nuisance algae did not decline, likely due to additional plant refurb