We developed a series of models using landuse and water-quality variables to predict sediment P release rates under anoxic conditions in reservoirs. We collected sediment cores from 17 reservoirs in the Central Plains region of the USA, and we measured nutrient release rates under anoxic conditions in laboratory incubation studies. We used corresponding landuse and water-quality data from the reservoirs to develop regression models for predicting P release rates. We used variables that relate directly to trophic state, including % cropland in the watershed, which explained the greatest amount of variation in release rates. P release rates tended to be higher in reservoirs that had greater % cropland in the watersheds. We developed additional predictive models using surface total P concentrations and Secchi disk depths. Trophic state was also a good predictor of release rates because more P was released from hypereutrophic reservoirs than from mesotrophic or eutrophic reservoirs. The median release rates for reservoirs representing different trophic state classes (e.g., mesotrophic, eutrophic, and hypereutrophic) were very similar to those previously reported for natural lakes. Our models can be used to predict sediment release rates in individual reservoirs of concern or to screen a large number of reservoirs to help direct resources to those systems that are most vulnerable to internal loading. Models based on landuse characteristics are particularly valuable because these data can be obtained from computer-based assessments and do not require labor-intensive field sampling.
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