Shifts in the stable carbon and nitrogen isotope signatures of rainbow and greenside darters exposed to a municipal wastewater effluent in the Speed River, Ontario

The Speed River is a relatively small 5^th order stream flowing through the City of Guelph in southern Ontario. The city’s population is expected to grow by 35% over the next 20 years, resulting in an increase in effluent released into the river from the Guelph Wastewater Treatment Plant (GWWTP). The assimilative capacity of the Speed River could be challenged by the projected increases in nutrient and contaminant discharges associated with such urban growth. To evaluate impacts on aquatic organisms in the receiving environment, shoreline riffle fish communities were compared upstream and downstream of the GWWTP. The two most prominent riffle-dwelling species, Rainbow Darter (Etheostoma caeruleum) and Greenside Darter (Etheostoma blennioides), and common riffle benthic invertebrates were collected and analyzed for δ¹³C and δ¹⁵N. The diets of the darter species were examined by stomach content analysis. In summers of both 2008 and 2009, the abundance, condition factor, and δ¹⁵N signatures of Rainbow Darters significantly increased downstream of the treatment plant while these responses were absent in Greenside Darter. Collectively, these data suggest Rainbow Darters have exploited the nutrient-rich conditions of the GWWTP receiving environment to gain an ecological competitive advantage.