Aquatic controls of inter-annual variability in river phosphorus yields

Knowledge of basin terrestrial characteristics can help predict river nutrient loads, but aquatic characteristics may also be important. To examine river total phosphorus (P) across years and test for upstream aquatic influences, we considered annual river P and water yields for Wisconsin sites. We then weighed the relative influence of catchment land and water cover on P yield magnitude and inter-annual variability. There was a linear relationship between the site-specific coefficient of variation for annual P yield (CVP) and annual water yield (CVQ). However, CVP was larger than CVQ in every instance (mean CVP:CVQ=1.6), and 2x greater at 10 percent of sites. Control of P yields by water bodies was evident at the catchment scale. Percent water had a significant main effect on P yield magnitude (mean, median, and maximum) while CVP was affected by a mix of terrestrial and aquatic cover types, including percent wetland. CVP, CVQ, and CVP:CVQ each decreased with percent water, but not catchment area, suggesting widespread lentic buffering of P and water variability. P measures could not be predicted well from catchment cover for sites near lake or reservoir outlets. We propose that consideration of basin water body composition can improve models of river nutrient loads at broad temporal scales, and a diversity of spatial scales.