Comparative responses of freshwater organisms to exposures of a commercial naphthenic acid

• Fish & invertebrates were relatively sensitive to NAs (7-d LC50 = 1.9–6.5 mg NA L−1).
• Typha latifolia was the least sensitive to NA exposures (7-d EC50 = 56.2 mg NA L−1).
• Based on peer-reviewed data, commercial NAs were more potently toxic than OSPW-NAs.

Comparative toxicity studies using unconfounded exposures can prioritize the selection of sensitive sentinel test species and refine methods for evaluating ecological risks of complex mixtures like naphthenic acids (NAs), a group of organic acids associated with crude oils and energy-derived waters that have been a source of aquatic toxicity. The objectives of this study were to compare responses of freshwater aquatic organisms (vertebrate, invertebrates, and a macrophyte; in terms of acute toxicity) to Fluka commercial NAs and to compare measured toxicity data with peer-reviewed toxicity data for other commercial NA sources and energy-derived NA sources. Exposures were confirmed using high performance liquid chromatography. Responses (7-d LC50s/EC50) ranged from 1.9 mg L−1 for Pimephales promelas to 56.2 mg L−1 for Typha latifolia. Following P. promelas in order of decreasing sensitivity were Ceriodaphnia dubia (7-d LC50 = 2.8 mg L−1), Hyalella azteca (7-d LC50 = 4.1 mg L−1), Chironomus dilutus (7-d LC50 = 6.5 mg L−1), and T. latifolia (7-d EC50 = 56.2 mg L−1), indicating that in terms of sensitivities, fish > invertebrates > plant for Fluka NAs in this study. Factors that affect exposures and measurements of exposures differ among commercial and energy-derived NAs and constrain comparisons. Despite differences in exposures, fish and invertebrates were relatively sensitive to both commercial and energy-derived NA sources (based on laboratory measurements and peer-reviewed data) and could be appropriate sentinel species for risk evaluations.