Municipal wastewater treatment plant effluent-induced effects on freshwater mussel populations and the role of mussel refugia in recolonizing an extirpated reach

Global human population and urbanization continually increase the volume of wastewater entering aquatic environments. Despite efforts to treat these effluents, they contribute a diverse suite of substances that enter watersheds at concentrations that have the potential to elicit adverse effects on aquatic organisms. The relationship between wastewater treatment plant (WWTP) effluent exposure and biological responses within aquatic ecosystems remains poorly understood, especially at the population level. To examine the effect of WWTP effluents on sentinel invertebrates, freshwater mussels were assessed in the Grand River, Ontario, in populations associated with the outfall of a major WWTP. This watershed, within the Laurentian Great Lakes basin, has a diverse community of twenty-five species of mussels, including nine Species at Risk, and is representative of many habitats that receive WWTP effluents regionally as well as globally. Surveys were conducted to assess the presence and species richness of freshwater mussels. In total, 55 sites downstream of the WWTP were examined using timed visual searches with one or 2 h of effort spent searching 100 m segments. Although seven species of mussels were found in moderate abundance (mean of 8 mussels per hour of searching across 2 sites) upstream of the WWTP outfall, no live mussels were observed for 7.0 km downstream of the WWTP. Long-term water quality monitoring data indicate that ammonia and nitrite concentrations along with large seasonal declines in diel dissolved oxygen were associated with the extirpation of mussels downstream of the WWTP. The first live mussels found downstream were below the confluence with a major tributary indicating that in addition to an improvement in water quality to a state that enables mussels (and/or their fish hosts) to survive, a nearby mussel refuge may have facilitated the recolonization of the depauperate WWTP-impacted zone.