Impacts of wastewater on aquatic biota in summer and winter

Abstract

Municipal wastewater treatment plant (WWTP) effluents are a substantial source of aquatic pollution, continuously introducing complex mixtures of contaminants into aquatic environments. While the effects of wastewater effluents have been observed across all levels of biological organization, the impacts on populations and communities remain relatively understudied. Additionally, despite the ubiquity of wastewater effluents and their continuous year-long release, their impacts have rarely been studied during winter. In my thesis, I examined the effects of municipal wastewaters on fish communities (Chapter 2), benthic macroinvertebrate communities (Chapter 3), and zooplankton communities (Chapter 4) along the effluent gradients of the Dundas and Woodward WWTPs in summer and winter. Furthermore, I examined the effects of wastewater exposure on the physiology and behaviour of fathead minnow (Pimephales promelas) under simulated summer (20C) and winter (4C) conditions (Chapter 5). In Chapter 2, I showed that fish abundance, species richness, and species diversity were generally highest at sites closest to the WWTP outfalls, but only during winter. I also showed that fish community compositions differed greatly along the effluent gradients of both WWTPs and in both seasons, with community divergence being highest in sites closets and farthest from the WWTP outfalls. In Chapter 3, I demonstrated that the effects of wastewater are not generalizable across different WWTPs and are likely dependent on the effluent-receiving environments. I found that at the larger WWTP (Woodward) with its more industrial/urban receiving environment, benthic macroinvertebrate abundance was higher and diversity was lower at sites downstream of the outflow compared to upstream sites in both seasons; whereas the opposite was true for the smaller WWTP (Dundas) with its wetland receiving environment. Community composition differed significantly along the effluent gradients of both WWTPs and in both seasons, with sites closest and farthest from the outfalls being the most dissimilar. In Chapter 4, I showed that zooplankton communities were numerically dominated by rotifers in both summer and winter. Furthermore, rotifer abundance was highest near the outfalls of both WWTPs, especially during winter. At the Woodward WWTP, macrozooplankton abundance, richness, and diversity all increased with distance from the outfall in the summer; while in the winter, the opposite trends were observed for abundance and richness. At the Dundas WWTP, only macrozooplankton richness increased with distance from the outfall, and this was observed only during winter. Similar to fish and benthic macroinvertebrate communities, zooplankton community composition differed along the effluent gradients of both WWTPs, with communities closest and farthest from the outfalls being the most distinct. In Chapter 5, I demonstrated that temperature plays a key role in modulating the effects of wastewater exposure on fathead minnow physiology and behaviour. Wastewater exposure at 20C was associated with increased standard metabolic rate, increased haematocrit, and reduced boldness. Whereas exposure to wastewater at 4C was only associated with reduced sociability. Altogether, my findings suggest that wastewater plumes are a significant source of nutrient enrichment and thermal pollution, particularly in winter. These factors likely make wastewater outfalls an ecological trap for fishes and other aquatic organisms in winter, where the deceptively favourable and attractive conditions are quickly outweighed by the costs of exposure to the various contaminants found in wastewater. I believe this thesis highlights the importance of conducting ecotoxicological research in winter, as it can reveal ecological surprises that would have otherwise been left uncovered.