| Summary |
Recent reestablishment of beaver populations on low order streams of the southeastern USA has dramatically altered these functionally important systems. This study quantified the natural variation in the plant community of 15 beaver impoundments and examined the effects of 13 of these on water quality. A paired study design was used to determine impoundment effects on agriculturally influenced stream water concentrations of total suspended solids (TSS), nitrate, ammonium, and soluble reactive phosphorus (SRP) during the fall and winter. The influence of sample date, discharge, watershed position, stream channelization, impoundment dimensions, and impoundment age were also examined. Collectively, impoundments decreased nitrate and TSS concentrations, increased ammonium concentrations, and had no effect on SRP relative to control reaches. Impoundments higher in drainage basins and those with greater slopes reduced nitrate concentrations by greater amounts. Older impoundments reduced TSS concentrations by greater amounts. Significant differences were found between the effects of impoundments on SRP concentrations at the lowest and highest discharges. It is concluded that, collectively, beaver impoundments ameliorate agricultural impacts to water quality and that they vary in their individual capacity to do so. This variation should be considered when developing strategies to integrate beaver populations into water quality management. The plant community was analyzed through principal components analysis and detrended correspondence analyses. Drainage basin area, slope, beaver dam height, impoundment surface area, channelization, impoundment age, and stand age were among the variables tested to determine potential effects on structure and the composition of five vegetative strata. Dam height and impoundment age were the most important variables influencing a structural transition from impoundments dominated by forest cover to those dominated by herbs and dead wood. Dam height and drainage basin area interacted to create a complex hydrologic gradient affecting species composition. Channelization and stand age secondarily affected composition. Herbaceous species richness increased significantly with impoundment age. Composition of the dominant species relative to unimpounded floodplains shifted to those with greater flood tolerance and showed greater coverage of the exotic-invasive Murdannia keisak. A conceptual model of the factors influencing plant community dynamics is presented. |
