δ15N tracks changes in the assimilation of sewage-derived nutrients into a riverine food web before and after major process alterations at two municipal wastewater treatment plants
Introduction
Municipal wastewater treatment plants (WWTPs) discharge among the highest volumes of effluent compared to other industries in Canada (Chambers et al., 1997). WWTP effluents contain a mixture of chemicals including total suspended solids (TSS), nutrients (phosphorous and nitrogen products), metals, and pharmaceuticals and personal care products (Chambers et al., 1997, Daughton and Ternes, 1999, Metcalfe et al., 2003, Lishman et al., 2006). Environmental impacts associated with municipal WWTP effluents released into aquatic environments have been associated with eutrophication and oxygen depletion (Gücker et al., 2006, Carey and Migliaccio, 2009, Kiedrzyńska et al., 2014), endocrine disruption (Jobling et al., 1998, Tetreault et al., 2011), impacts on fish assemblages (Tetreault et al., 2013), and alterations of food webs (deBruyn et al., 2003).
Stable isotope ratios of carbon (δ13C) and more commonly nitrogen (δ15N) have successfully been used to track the exposure and assimilation of sewage-derived nutrients into aquatic food webs (deBruyn and Rasmussen, 2002, Morrissey et al., 2013, Loomer et al., 2015). Wastewater constituents enter the aquatic food web through ingestion of particulate organic matter by consumers or through the uptake of sewage-derived inorganic nutrients by primary producers (Tucker et al., 1999). δ15N measured in organisms exposed to WWTP effluent will depend on the treatment processes utilized at the plant, final effluent quality, and the characteristics of the receiving environment. Organisms exposed to secondary or greater treated effluent typically results in enriched δ15N values (Gaston et al., 2004, Morrissey et al., 2013, Robinson et al., 2016). This is because nitrification and denitrification processes associated with secondary treatment tend to result in the accumulation of the heavier nitrogen isotope, 15N (Heaton, 1986). A lack of nitrification and denitrification processes (e.g. in raw sewage or primary treated effluent) usually result in an accumulating pool of ammonia depleted in 15N and when released into the receiving environment, and primary producers will preferentially take up 14NH4 over 15NH4 (Birgand et al., 2007), resulting in organisms depleted in 15N (deBruyn and Rasmussen, 2002, Gaston and Suthers, 2004, Daskin et al., 2008). The carbon discharged from municipal WWTP effluent is primarily terrestrial in origin which has a relatively constant δ13C value of about −28‰, hence it is possibly discriminated from aquatically derived (autotrophic) sources which can range between −40 and −20‰ (France, 1995).
The Grand River watershed is the largest drainage basin in southern Ontario, Canada, which flows into the northeastern part of Lake Erie. This watershed assimilates effluent from 30 municipal WWTPs serving almost one million people. The largest WWTPs, Kitchener and Waterloo, (collectively serving >370,000 people in 2014), both use secondary conventional activated sludge processes, and discharge into the central reaches of the Grand River. The effluents from these WWTPs have been historically associated with poor water quality in the receiving environment including hypoxic river conditions (Venkiteswaran et al., 2015), unionized ammonia concentrations above the provincial water quality objective (>0.0165 mg/L) (Loomer and Cooke, 2011), and the presence of elevated levels of selected pharmaceuticals (Arlos et al., 2015). Impacts on fish downstream of these WWTPs include the feminization (Tetreault et al., 2011, Tanna et al., 2013, Bahamonde et al., 2015) and reduced reproductive success (Fuzzen et al., 2015) of male rainbow darter (Etheostoma caeruleum). A study conducted in 2007 by Loomer et al. (2015), documented changes in δ13C and δ15N, in rainbow darter and primary consumers exposed to these WWTP effluents in the Grand River. Exposure to the poorly treated (non-nitrifying) Kitchener effluent resulted in a decrease in δ15N, while exposure to the effluent at the Waterloo WWTP (higher quality effluent with partial nitrification at the time) resulted in little to no change (Loomer et al., 2015). Major planned upgrades at both the Kitchener and Waterloo WWTPs created a unique opportunity to examine how changes in effluent quality impacted the stable isotope ratios of fish (rainbow darter) and primary consumers (benthic invertebrates).
The major planned upgrades at the Waterloo and Kitchener WWTPs were to convert them from carbonaceous activated sludge treatment (primarily for BOD removal) to fully nitrifying activated sludge. In August 2012, the Kitchener WWTP had initiated its upgrades for nitrification, and by January 2013 it achieved full nitrification. Nitrification was achieved by retrofitting the current WWTP with return activated sludge (RAS) reaeration and replacing the old aeration system with more efficient fine bubblers (Table 1) (Bicudo et al., 2016). At the same time, the Waterloo WWTP initiated upgrades, but a number of changes and construction issues led to a decrease in effluent quality (e.g. increasing total ammonia) over several years. Similar to the Kitchener WWTP, the Waterloo WWTP was retrofit with RAS reaeration in 2014; however, fine bubblers had not been installed to achieve full nitrification (Table 1) (Region of Waterloo, 2016).
The primary objective of the present study was to assess how the changing effluent quality at two WWTPs altered the stable isotope ratios (δ15N and δ13C) throughout an aquatic food web, using two trophic levels, primary consumers (benthic invertebrates) and a secondary consumer (rainbow darter). The rainbow darter was selected for this study since it had been used as a sentinel species in a variety of recent biomonitoring studies in the Grand River (Tetreault et al., 2011, Tanna et al., 2013, Bahamonde et al., 2015). Using new collections, archived samples, and previously published data, the patterns of stable isotopes in rainbow darter and selected primary consumers collected adjacent to the Waterloo and Kitchener WWTPs were assessed before and after the process changes (2007–2014). There were two specific research questions addressed in this study. The first question was to test whether a difference could be detected in δ15N and δ13C in fish and primary consumers before and after the Kitchener WWTP upgrade and in the years the Waterloo WWTP had deteriorating effluent quality. The second question was to test whether any changes in δ15N and δ13C could be linked to changing effluent quality. To help with the interpretation of the isotope data, a laboratory-based diet switch experiment was conducted with rainbow darter to estimate the relative isotopic turnover rate in muscle and liver tissues. The contrasting changes in effluent quality at the Kitchener and Waterloo WWTPs, with either improvements or deteriorations over time, provided a unique opportunity to follow these changes, and how they may alter the flow of nutrients in a riverine food web.
Section snippets
Sampling sites
Sampling sites selected for this study were based on previously published or unpublished studies related to the impacts of municipal WWTPs on rainbow darter in the Grand River, Ontario, Canada (Tetreault et al., 2011, Bahamonde et al., 2015, Fuzzen et al., 2015, Loomer et al., 2015). These sites were selected due to their proximity to the Kitchener and Waterloo WWTP outfalls and to represent similar riffle/run habitats (Fig. 1). This study comprised a total of nine sites all located on the
Effluent quality
Annual tonnage of total ammonia and nitrate released from the Kitchener and Waterloo WWTPs between 2007 and 2014 indicated that effluent quality (based on ammonia) had changed during these years (Fig. 2). Nitrate was inversely related to the ammonia and is a strong indicator of the degree of nitrification occurring at the WWTPs. In pre-upgrade years (2007–2011), the Kitchener WWTP released 500–600 t/year of total ammonia. Total ammonia levels began to drop in 2012 (beginning of upgrades) and by
Change in δ15N
The upgrades at the Kitchener WWTP, which resulted in increased nitrification, considerably reduced the amount of ammonia (and increased nitrate) in the final effluent. This change in effluent quality was associated with higher δ15N values of both fish (rainbow darter) and primary consumers in the receiving environment relative to years prior to the upgrades. This is also consistent with higher δ15N values often associated with secondary or greater treatment plant outfalls (Wayland and Hobson,
Conclusion
This was a unique study that assessed the changes in exposure and assimilation of sewage-derived nutrients into a riverine aquatic food web following either improved or deteriorated effluent quality from two WWTPs. There was a direct link between effluent quality and the assimilation of nutrients from primary consumers to fish. The improved effluent quality at the Kitchener WWTP was associated with changes in δ15N from being depleted to reflecting reference conditions in primary consumers and
Acknowledgements
The authors would like to thank all the assistance received from the University of Waterloo and Environment Canada field crews and research technicians who assisted in sample collection and processing. The treatment plant process and effluent data have been supplied under an agreement with the Regional Municipality of Waterloo. We would also like to thank them for their review of the manuscript. The authors also acknowledge Mark Anderson from the Grand River Conservation Authority (GRCA) for
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