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

Stable isotopes (δ15N and δ13C) were used to assess the changes in exposure and assimilation of sewage-derived nutrients in an aquatic food web following changes in effluent quality over an 8 year period at two municipal wastewater treatment plants (WWTPs) that discharge to the Grand River, in southern Ontario. Upgrades at the Kitchener WWTP started in late 2012 to enhance nitrification, while the Waterloo WWTP had a series of construction issues at the plant that resulted in a deterioration of its effluent quality over the study period (2007-2014). Fish (rainbow darter, Etheostoma caeruleum) and primary consumers (benthic invertebrates) were sampled in the receiving waters associated with each outfall. Upgrades at the Kitchener WWTP resulted in improved effluent quality with total annual ammonia output dropping by nearly sixfold (583-100 t), while the Waterloo WWTP increased its total annual ammonia output by nearly fourfold (135-500 t) over the duration of the study. Downstream of the Kitchener WWTP, the reduction in total ammonia output negatively correlated with changes in δ15N of rainbow darter from being depleted (prior to the upgrade) to reflecting signatures similar to those at the upstream reference site. The biota downstream of the Waterloo WWTP showed the opposite trend, going from slightly enriched, to being depleted relative to the upstream reference sites. δ13C was consistently higher downstream of both WWTPs regardless of changing effluent quality, and annual variability in δ13C was associated with annual river discharge. In a laboratory based dietary switch study conducted with rainbow darter, the isotope half-life in muscle (29 days for δ15N and 33 days for δ13C) were determined and these rapid changes were consistent with responses in muscle of wild fish. This is a unique study that was able to contrast two WWTPs in the same watershed as they underwent major changes in treatment processes. Stable isotopes were very effective as a tool to trace the changes in aquatic biota due to changes in wastewater effluent quality, both improvements and deterioration over time.