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TECHNICAL REPORTS

Organic Compounds in the Environment

Contrasting Pathways of Assimilation

Stable Isotope Assessment of Fish Exposure to Pulp Mill Effluents

^a Univ. of Saskatchewan, Toxicology Centre, 44 Campus Dr., Saskatoon, SK S7N 5B3, Canada
^b National Water Research Institute, Environment Canada, Potato Research Centre, 850 Lincoln Rd., Fredericton, NB E3B 4Z7, Canada
^c National Water Research Institute, Environment Canada, 11 Innovation Blvd., Saskatoon, SK S7N 3H5, Canada

^* Corresponding author (monique.dube{at}usask.ca)

Received for publication April 10, 2005. Fish are commonly used for monitoring the quality of waters receiving pulp mill effluents (PMEs). Isotopic assays of fish tissues have the potential to provide empirical evidence to link an effluent source to exposure. We show in a 45-d factorial laboratory experiment that different exposure pathways lead to isotopic signatures in fish tissue. Rainbow trout (Oncorhynchus mykiss) were exposed to 10% PME in three ways; direct exposure through addition of PME to aquaria, indirect exposure through invertebrate food consumption (Chironomus tentans cultured in 10% PME), and a combination of both exposure pathways. Of the four stable isotopes measured (¹³C, ¹⁵N, ³⁴S, ³⁷Cl), ¹³C, ³⁴S, and ³⁷Cl showed significant differences in exposed animal tissues. ³⁷Cl of fish muscle tissue showed consistent differences across trophic levels and revealed contrasting pathways of PME exposure. Contrasting ³⁷Cl values in C. tentans due to the presence or absence of 10% PME did not translate into ³⁷Cl differences in fish. Rather, ³⁷Cl ratios of fish muscle tissue were specifically related to 10% PME exposure in the aquaria (waterborne exposure pathway). Feasible distributions of ³⁷Cl source contributions for observed mixture ratios confirmed that PME accounted for observed differences in ³⁷Cl among treatments. Direct uptake of chloride ions across gill surfaces is the most likely pathway for assimilation of PME into fish tissues. Considering the variability of PMEs and receiving environments, use of a multi-isotope approach is recommended for tracing exposure of fish. Use of ³⁷Cl should also be considered in light of its alternative assimilation pathway.