| Scope and method of study. Ecological risk assessment and watershed-level management are quickly becoming fundamental components of environmental decision making concerning the nation's water bodies. Appropriate tools and methodologies are needed to allow for ecological risk assessment and watershed management while addressing uncertainties in knowledge, data, and ultimately, predictions. The purpose of this research was to develop an ecological risk assessment methodology at the watershed level for freshwater ecosystems.; Findings and conclusions. An ecological risk assessment methodology at the watershed level for freshwater ecosystems was developed and presented. The methodology involves a two-phased Monte Carlo procedure that provides for the evaluation and propagation of natural stochastic variability and knowledge uncertainty separately in a pollutant transport and fate model, EUTROMOD. The model and uncertainty propagation methodology allows for evaluating the risk of eutrophication in lentic ecosystems as a probabilistic description of uncertain phosphorus loadings. The methodology and model were used to perform an ecological risk assessment on Wister Lake in Oklahoma. The model was used to estimate annual watershed phosphorus loads from point and nonpoint sources as well as resulting lake response. The lake was estimated to be eutrophic with the most significant source of phosphorus being agricultural nonpoint sources. The simulation of alternative management scenarios indicated that an average reduction of agricultural nonpoint source loads by 25 to 35 percent would be needed to meet proposed water quality goals with and without point source controls, respectively. |
