Pulse‐Exposure Toxicity of Ammonia and Propoxur to the Tropical Copepod Acartia sinjiensis

Abstract

Toxicity risk assessments of short-term discharges of contaminated waters to the aquatic environment have shown that receptor organisms can tolerate higher pulse-exposure than continuous-exposure concentrations of some contaminants. However, these observations are influenced by the mode of toxicity of the contaminants present and the concentration-time profile of the exposure. For common metal contaminants, the time-weighted average concentration (TAC) of the exposure has been useful for predicting risk of toxicity to multiple species, including the tropical, euryhaline copepod Acartia sinjiensis. To increase our understanding of the application and limitations of the TAC approach, the present study examined how varied pulse-exposure durations affect the toxicity of fast-acting contaminants, ammonia, and the common pesticide propoxur to this copepod species. Copepod larvae were exposed under continuous-exposure conditions (all life stages from eggs to nauplii to copepodites exposed) and as 6-and 18-h pulse exposures applied during the most sensitive life stage only (24-h-old nauplii) within 78-h tests. Larval development ratio and population size were assessed as test endpoints. Generally, increased exposure duration resulted in increased toxicity. Trends observed for ammonia and propoxur were slightly different for larval development and population size. Larvae tolerated greater concentrations of contaminants in a 6-h pulse (higher 10% effect concentration) than in an 18-h pulse, or a continuous 78-h exposure, whereas toxicity responses converged for the 18-and 78-h exposures. Continuous toxicity thresholds were always protective of pulse exposures, providing a conservative toxicity threshold for all durations of pulse exposures. Although generalizations for predictions of risk based on TACs are frequently effective for common metal contaminants, the TAC approach was not effective for ammonia and propoxur.

Does toxicity test variability support bioavailability model predictions being within a factor of 2?

“…There were seven datasets based on EC10 values, which is much less than the 22 datasets based on EC50 values reflecting that EC50 values are more common acceptability criteria in reference toxicant testing (see examples in Stone et al (2022) and Price et al (2022)). The seven available EC10 datasets were comprised of 92 toxicity tests across four species (two invertebrates and two microalgae) and two contaminants (copper and nickel) with both acute and chronic data (Supplementary Table S1).…”

“…Test species consist of both freshwater and marine organisms. Additional data was sourced from published reference toxicant data in Stone et al (2022) and Meyer et al (2018).…”