Differential recovery of δ13C in multiple tissues of white sucker across age classes after the closure of a pulp mill

Techniques to document recovery after the closure of pulp mills that discharge enriching effluents are not well established, but delta C-13 may be a useful tool. In the 1990s, the muscle tissue of white sucker (Catostomus commersonii) collected downstream of two pulp and paper mills discharging into separate streams (Mattagami and Kapuskasing rivers) was enriched in C-13 compared with upstream fish, suggesting uptake of pulp-derived C. The Mattagami River mill was closed in 2006, and analysis of muscle and gonad for delta C-13 was performed in 2011. As expected, fish captured in 2011 downstream of the operational Kapuskasing mill still showed the influence of the pulp-derived C in muscle and gonad tissue. After the closure of the Mattagami River mill, muscle tissue of white sucker was still enriched in C-13 compared with upstream fish, while gonad tissue was not. The patterns observed in the Mattagami River were, however, related to age; the oldest fish showed enrichment of delta C-13 in both muscle and gonad tissue, suggesting the residual occurrence of pulp-derived C. This study suggests that measurements of stable isotopes in fish across a broad age range may indicate ecosystem improvements. These techniques may also be useful where no data prior to the upgrade or closure are available for comparison.