Effects of agricultural best-management practices on total phosphorus yields in the Johnson Brook and Lovejoy Pond watersheds, Kennebec County, Maine, 1980-84

Abstract

Analysis of daily phosphorus-yield and streamflow data collected before and after implementation of agricultural best-management practices in the Johnson Brook watershed in south-central Maine indicated statistically significant reductions in phosphorus loading in all flow categories. Reduction of median loadings for five flow categories ranged from 26 to 90 percent. The annual total phosphorus yield was reduced 17 percent after implementation of the best-management practices. The observed phosphorus yield reduction is considerable because of two streamflow factors. First, the period after implementation of the best-management practices had eight more storms. Periods of storm runoff in the postimplementation period had 31 days with greater average streamflow and a maximum daily streamflow more than three times greater than those observed in the preimplementation period. Second, the annual streamflow was 128 percent greater in the year after the management practices were implemented. Because the potential for phosphorous transport increases with runoff, and greater yields are possible when the volume of water increases, a higher phosphorous yield would be expected in the postimplementation period then during the preimplementation period, if other factors had remained unchanged. The reductions in phosphorous yield in the study area are not expected to have a significant effect on the eutrophic conditions observed in Lovejoy Pond. Phosphorous concentrations in the pond will continue to be capable of supporting algal blooms. However, the intensity and duration of blooms are expected to be less than those observed before best-management practice implementation. 1 Hydrologist, U.S. Geological Survey, Augusta, Maine 2 Biologist, Maine Department of Environmental Protection, Augusta, Maine