Incorporating native South American aquatic species into acute Species Sensitivity Distributions: implications for pesticide risk assessment of lambda-cyhalothrin and ammonium glufosinate.

Agricultural intensification has increased pesticide inputs into freshwater ecosystems worldwide, yet pesticide risk assessments still rely largely on toxicity data from standard Northern Hemisphere model organisms. This may limit the biogeographical representativeness of acute SSD-derived hazard benchmarks. We evaluated the acute toxicity of lambda-cyhalothrin and ammonium glufosinate using three native Pampean freshwater species: Hyalella curvispina, Boana pulchella, and Cnesterodon decemmaculatus. Acute toxicity values (96-h LC50) were integrated into Species Sensitivity Distributions (SSDs) to estimate acute HC5 values, assess regional sensitivity, and characterize hazard under reported exposure scenarios. Lambda-cyhalothrin was markedly more toxic than ammonium glufosinate. Hyalella curvispina was the most sensitive species, with 96-h LC50 values of 0.25 μg/L for lambda-cyhalothrin and 1.07 mg/L for ammonium glufosinate. For lambda-cyhalothrin, the acute SSD yielded an HC5 of 0.0052 μg/L, and the highest reported concentration for Pampean streams (0.56 μg/L) exceeded both the LC50 of H. curvispina and the community-level acute hazard benchmark, resulting in an HQcommunity of 107.0. In contrast, reported ammonium glufosinate concentrations remained below acute lethal thresholds and yielded HQ values below unity, although the SSD showed greater uncertainty due to limited taxonomic coverage. Sensitivity analyses showed that adding Pampean taxa changed HC5 point estimates in a compound-specific manner, highlighting that the influence of native species depends on their position within each SSD and on database composition. Overall, these results indicate a robust acute hazard signal for lambda-cyhalothrin under peak exposure scenarios, whereas ammonium glufosinate represents a preliminary screening-level vulnerability signal. The study highlights the need to expand South American toxicity datasets and incorporate chronic, sublethal, and temporally resolved exposure information.