Widespread, high-volume use and subsequent off-site transport of herbicides, specifically photosystem II inhibitors (PSII), on agricultural and noncultivated lands in south Florida has resulted in frequent detections in freshwater systems. In light of the current restoration efforts as part of the Comprehensive Environmental Restoration Plan (CERP), increased water flows containing detectable herbicide levels into the Everglades ecosystem and adjacent areas may have adverse consequences to the unique plant communities present in the region. The potential impact of individual herbicides to aquatic plant and algae species was examined using a probabilistic risk assessment approach. Risk was characterized for nine PSII herbicides (four triazines: ametryn, atrazine, simazine, and prometryn; two triazinones: hexazinone and metribuzin; two substituted ureas: diuron and linuron; and a uracil: bromacil) and a pyridazinone herbicide (norflurazon) by comparing the overlap of the 90th percentile surface water concentration (exposure benchmark) from the exposure distributions to the 10th percentile of effects (effects benchmark) determined from species sensitivity distributions of acute laboratory toxicity data. Overall, the risk of the individual herbicides was generally low. A Multiple Substance Potentially Affected Fraction (msPAF) risk approach also was utilized to examine the joint toxicity of the herbicide mixtures based on a concentration addition model. The risk associated with the herbicide mixture (mainly bromacil, diuron, and norflurazon) was determined to be high for Lee (site S79 on the Caloosahatchee River), Martin (site S80 on St. Lucie Canal), and St. Lucie (site S99 on C25 emptying into Indian River Lagoon) counties in south Florida. This study highlights the need to consider joint action of chemical mixtures as part of an ecological risk assessment.