An ecological risk assessment paradigm using the Spatially Integrated model for Phosphorus Loading and Erosion (SIMPLE)

Abstract

Ecological risk assessments provide a probabilitistic approach to analyzing and predicting ecosystem responses to stress. We are evaluating the relationship between nonpoint source (NPS) phosphorus loading and the trophic status of the aquatic ecosystem. We are using SIMPLE (the Spatially Integrated Model for Phosphorus Loading and Erosion) to identify probable phosphorus sources in a watershed, simulate the phosphorus loading to streams, and analyze the relationships between input variables and their ecological impact. The objective of this paper is to describe a risk-based paradigm using SIMPLE to characterize the probability of exceeding a critical phosphorus loading to a lotic ecosystem. We have characterized the risk of exceeding a threshold loading of 0.5 kilogram total phosphorus per hectare per year from a 2238 hectare watershed. Two-hundred-fifty random SIMPLE simulations were performed to estimate annual total phosphorus, dissolved phosphorus, and sediment-bound phosphorus loading to a lotic ecosystem from the watershed. Simulation results were analyzed statistically to determine the probabilities of exceeding the critical loadings. Based on the current land use practices in the Battle Creek watershed, the probability of exceeding the total phosphorus critical loading rate of 0.5 kg/ha/yr was approximately 11 percent, or one year in nine the total annual loading will exceed the critical loading rate. The 95 percent confidence intervals for the total phosphorus loading occurring on average once in nine years were relatively close (0.45 to 0.60 kg/ha/yr), assuming the only variability from year to year was due to natural variability in weather.