Factors influencing phosphorus in midcontinent impoundments (USA) and challenges for detecting abatement

Impoundments in the midcontinent region of the United States provide the beneficial services of lentic waters. These impoundments were constructed long after intensified agriculture was established, and most are eutrophic. Total phosphorus (TP) increases in regional streams and reservoirs, concurrent with the proportion of cropland in the watershed, serve as a surrogate for nutrient loading from agriculture. The potential impact of external loading of TP to these reservoirs is moderated by sedimentation of particulate P and plunging interflows during summer stratification. Hydrology has a strong influence on TP; cross-system analyses show TP more than doubles across the range of flushing rates found in Missouri reservoirs when other factors are held constant. This link between hydrology and TP suggests reservoir placement on the landscape and design criteria strongly influence trophic state. Temporal variation is such that summer mean TP typically ranges 2- or 3-fold in individual reservoirs, suggesting benefits of best management practices to reduce external loading may be masked by inherent variation. Eutrophication of a midcontinent reservoir from poultry litter has been documented. Expansion of industrialized animal production throughout the region suggests legacy P, and resulting eutrophication, will likely increase over time without regulation.