Pesticides can cause harmful effects to aquatic communities, even at concentrations below the threshold limit established as guidelines for the water bodies by environmental agencies. In this research, an input of the herbicide 2,4-dichlorophenoxyacetic acid (i.e., 2,4-D) was simulated under controlled conditions in a 500-m-long reach of a first-order tropical stream in Southeastern Brazil. Two water samplings at eight stations investigated the stream longitudinal contamination profile. The ecotoxicological effects were analyzed using Eruca sativa L. seed germination assays and the acute and chronic toxicity tests with the neotropical cladoceran Ceriodaphnia silvestrii. Physicochemical parameters of water quality were evaluated to characterize the study area and quantify 2,4-D concentrations along the stream to assess pesticide retention. The 2,4-D concentration was reduced by approximately 50% downstream in the samplings, indicating that the herbicide was retained along the stream. Moreover, C. silvestrii reproduction in long-term assays decreased approximately 50% in the stations with higher concentrations of 2,4-D than the laboratory control. After contamination, E. sativa L. showed a lower average root growth (1.0 cm), statistically different from the control (2.2 cm). On the other hand, similar growth values were obtained among the background and the most downstream stations. Our study highlighted the relevance of reviewing and updating herbicide guidelines and criteria to prevent possible ecological risks.

即使浓度低于环境机构为水体制定的指导方针，农药也会对水生群落造成有害影响。在这项研究中，在巴西东南部一级热带河流的 500 米长范围内，在受控条件下模拟了除草剂 2,4-二氯苯氧乙酸（即 2,4-D）的输入。八个站点的两次水样调查了河流纵向污染概况。使用Eruca sativa L. 种子萌发试验和新热带枝角类Ceriodaphnia silvestrii的急性和慢性毒性试验分析生态毒理学效应. 评估水质的物理化学参数以表征研究区域并量化沿溪流的 2,4-D 浓度以评估农药保留。采样中下游的 2,4-D 浓度降低了约 50%，表明除草剂沿溪流保留。此外，与实验室对照相比，在 2,4-D 浓度较高的站点中，C. silvestrii在长期测定中的繁殖减少了约 50%。污染后，E. sativaL. 显示较低的平均根生长（1.0 厘米），与对照（2.2 厘米）有统计学差异。另一方面，在背景和最下游站点之间获得了相似的增长值。我们的研究强调了审查和更新除草剂指南和标准以防止可能的生态风险的相关性。