Embryotoxicity of Naphthenic Acid Fraction Components to Fathead Minnow (Pimephales promelas)

Abstract

The extraction of bitumen from Alberta’s oil sands region generates large volumes of oil sands process-affected water (OSPW) that are stored in tailings ponds. New policy directives are planning to intentionally release OSPW into surrounding freshwater ecosystems. Presently, no water quality standards exist for the main toxic constituent of OSPW - naphthenic acid fraction components (NAFCs). To aid in the establishment of safe-release thresholds of OSPW, we exposed embryo fathead minnow (Pimephales promelas) to OSPW-derived NAFCs (2.5-54 mg/L) from 1-day post-fertilization to hatch in semi-natural conditions. Observations on embryo heart rate, embryo development, and basal activity at hatch were examined, to assess the developmental and behavioural toxicity of NAFCs. Embryo heart rates declined with increasing exposure concentration. NAFCs caused embryo mortality (LC50=26.76 mg/L) and development impairments at hatch (EC50=14.38 mg/L). Non-viable fish displayed pericardial and yolk sac edemas, craniofacial defects, and spinal curvatures that increased in severity with increasing exposure concentration. Acute narcosis, cardiac dysfunction, oxidative stress, and alteration in gene expression were probable modes of toxic action. At concentrations above 21 mg/L, fish displayed erratic and repetitive twitching patterns indicative of nervous system impairment. Post-hatch mass increased with increasing exposure concentration, potentially as a short term, compensatory-like response. Additionally, a reduction in basal activity was observed for fish exposed to 2.5 and 6.5 mg/L. This result suggests there could be sublethal effects for fish exposed to NAFC concentrations much lower than those known to cause mortality and developmental impairments. Taken together, these results provide important toxicological information to inform future regulatory policies for the management of OSPW in Alberta, Canada.