Fish living near two wastewater treatment plants have unaltered thermal tolerance but show changes in organ and tissue traits

Municipal wastewater treatment plants (WWTPs) are a significant source of anthropogenic pollutants and are a serious environmental stressor in Laurentian Great Lakes ecosystems. In this study, we examined whether three freshwater fish species (bluegill sunfish Lepomis macrochirus, green sunfish Lepomis cyanellus, and round goby Neogobius melanostomus) collected near two wastewater effluent outflows in Lake Ontario showed altered measures of somatic investment and thermal tolerance. Fish of all three species collected near the WWTPs were larger with 50–60% heavier body masses compared to those collected at reference sites. Green sunfish had higher body condition and increased haematocrit at wastewater-contaminated sites, and both round goby and bluegill sunfish had larger livers (controlling for body mass) at wastewater-contaminated sites. Thermal tolerance (critical thermal maximum, CTmax) differed between species (green sunfish > bluegill sunfish > round goby), but was similar in fish collected at wastewater-contaminated sites compared to cleaner reference sites. Wastewater-contaminated sites had poorer water quality, higher nutrient loadings, and higher concentrations of anthropogenic contaminants (measured via polar organic chemical integrative samplers, POCIS) than reference sites. Our results suggest that fish in the wild may have some capacity to cope with WWTP effluent and avoid any potential impairments in thermal tolerance. Our findings also suggest that treated wastewater is changing water quality locally in Great Lakes watersheds, and that many fish species may be able to access extra nutrients provided by such effluent outflows. However, if outflow areas become preferred foraging areas this will inadvertently increase exposure to anthropogenic stressors and pollutants.