Influence of light, nutrients, and temperature on the toxicity of atrazine to the algal species Raphidocelis subcapitata: Implications for the risk assessment of herbicides

• No trend in 96-h EC50s (growth rate) for atrazine with varying light intensity.
• EC50s (PSII yield) increased with decreasing light intensity.
• EC50s (growth rate) decreased with decreases in temperature.
• No trend in EC50s with increase in nitrogen or phosphorus.
• Sensitivity of R. subcapitata not greater under conditions that mimic Midwestern US.

The acute toxicity of herbicides to algae is commonly assessed under conditions (e.g., light intensity, water temperature, concentration of nutrients, pH) prescribed by standard test protocols. However, the observed toxicity may vary with changes in one or more of these parameters. This study examined variation in toxicity of the herbicide atrazine to a representative green algal species Raphidocelis subcapitata (formerly Pseudokirchneriella subcapitata) with changes in light intensity, water temperature, concentrations of nutrients or combinations of these three parameters. Conditions were chosen that could be representative of the intensive corn growing Midwestern region of the United States of America where atrazine is used extensively. Varying light intensity (4–58 µmol/m2 s) resulted in no observable trend in 96-h EC50 values for growth rate. EC50 values for PSII yield generally increased with decreasing light intensity but not significantly in all cases. The 96-h EC50 values for growth rate decreased with decreases in temperature (20–5 °C) from standard conditions (25 °C), but EC50 values for PSII yield at lower temperatures were not significantly different from standard conditions. Finally, there was no clear trend in 96-h EC50 values for both endpoints with increases in nitrogen (4.1–20 mg/L) and phosphorus (0.24–1.2 mg/L). The 96-h EC50 values for both endpoints under combinations of conditions mimicking aquatic systems in the Midwestern U.S. were not significantly different from EC50 values generated under standard test conditions. This combination of decreased light intensity and temperature and increased nutrients relative to standard conditions does not appear to significantly affect the observed toxicity of atrazine to R. subcapitata. For atrazine specifically, and for perhaps other herbicides, this means current laboratory protocols are useful for extrapolating to effects on algae under realistic environmental conditions.