Physical and mathematical models were employed to determine the effects of atrazine on pH, dissolved oxygen, and dissolved nutrients in aquatic systems. The data show that changes in measured variables are observed in the presence of toxicant but that systems recover rapidly when toxicant input ceases. Simple linear donor-controlled mathematical models of nutrient cycles are capable of simulating the effect, although sufficient model detail should be incorporated to account for direct and indirect effects and to improve verification. System measurements are responsive in terms of disruption and recovery. Changes in nutrient cycling patterns should be incorporated into the hazard evaluation process.