Investigating the recovery of stable isotope values in an aquatic ecosystem after the closure of a pulp mill

Abstract

The closure of a bleached kraft mill on the Mattagami River in northern Ontario presented an opportunity to study the recovery of two previously studied indicator species in response to the removal of pulp mill effluent. The study used stable isotopes of carbon (δ13C) and nitrogen (δ15N) in the burrowing mayfly (Hexagenia sp.) and one of its predators, the white sucker (Catastomus commersoni). Historical data from the 1990s were used to follow temporal changes in δ13C values for both species; changes in δ13C values were found in white sucker between mill operation and closure while values for mayflies were similar over time. No differences were found in isotope bi-plots or comparisons of food chain length for food webs between upstream and downstream sites in the Mattagami River, or at the regional reference site. Body size and development for Hexagenia sp. were also used to test for residual nutrient enrichment. There was no difference between the previously exposed site and the reference sites for either endpoint. These results suggest that recovery in the Mattagami River was complete in terms of δ13C values within 5 years following the mill’s closure.