The Temporal Response Surface: A Novel Method for the Assessment of Delayed and Time-Cumulative Aquatic Ecosystem Risk

In Australia, pesticide risk assessments in aquatic environments typically compare measured water concentrations to relevant ecosystem protection guidelines based on species sensitivity distributions (SSDs). These guidelines estimate concentrations that are protective against long-term (chronic) exposure but do not consider extended exposure to chemicals with cumulative toxicity, such as neonicotinoid insecticides. The Australian and New Zealand Guidelines caution against the application of default acute to chronic ratios (ACRs) for such chemicals but lack suitable alternatives. Addressing this gap, we introduce the Temporal Response Surface (TRS) method that incorporates exposure duration into SSDs via taxa-specific regression relationships. The TRS considers both toxicity and exposure duration, setting meaningful guidelines for aquatic ecosystem protection and facilitating probabilistic risk assessments. Applied to imidacloprid, concentrations that are protective of 99, 95, 90, and 80% of the ecosystem (PC99, PC95, PC90, and PC80, respectively) drop quickly within the first 30 days of continuous exposure, then progressively decline over longer exposure periods. This suggests that previous risk assessments may have underestimated the toxic effects of prolonged or repeated imidacloprid exposure. The TRS method offers a holistic solution, addressing policy and risk assessment gaps for chemicals with delayed or cumulative toxicity, further enhancing aquatic ecosystem protection in Australia and internationally.