Regional and Long‐Term Analyses of Stable Isotopes of Fish and Invertebrates Show Evidence of the Closure of a Pulp Mill and the Influence of Additional Stressors

A bleached kraft pulp mill discharging effluent to the Mattagami River in northern Ontario, Canada, closed after almost 90 yr of operation. During its operation, effluent from the mill influenced biota in the downstream areas. To assess shifts in the reliance of biota from mill-derived nutrients, the isotopic composition (δ¹³ C and δ¹⁵ N) of white sucker (Catostomus commersoni) muscle and whole mayflies (Hexagenia sp.) were compared before (1990s) and after the pulp mill's closure (2012-2014). To better understand other potential sources of spatial and temporal change, samples from 3 other tributaries in the basin with dams, ongoing pulp mill operations, sites receiving sewage, and at several reference sites were collected and compared. Irrespective of time period, biota collected at sites downstream of both dams and active pulp mills tended to have elevated δ¹³ C values, but variable changes in δ¹⁵ N (negligible in most cases) when compared with upstream samples. The isotopic composition of mayflies varied at reference sites over time, with decreasing values of δ¹³ C and δ¹⁵ N (mayflies only) with increasing depth, and there was evidence of lower δ¹³ C in fish after the pulp mill closure. Overall, these results suggest the importance of long-term, regional-scale measurements for documenting the effects of stressors on nutrient use by aquatic species. Environ Toxicol Chem 2020;39:1207-1218. © 2020 SETAC.