Effects of agriculture on discharges of nutrients from coastal plain watersheds of Chesapeake Bay

We measured annual discharges of water, sediments, and nutrients from 17 Chesapeake Bay watersheds with differing proportions of agricultural lands on the inner, central, and outer Coastal Plain. In all regions of the Coastal Plain, the flow-weighted mean concentrations of N species in watershed discharge increased as the proportion of cropland in the watershed increased. In contrast, the concentrations of P species did not correlate with any land use. Instead, P concentrations correlated with the concentration of suspended particles, which differed greatly among watersheds in different regions of the Coastal Plain. Consequently, the ratio of N/P in discharges differed widely among watersheds, potentially affecting N or P limitation of phytoplankton growth in the receiving waters. Concentrations of dissolved silicate, organic C, pH, and alkalinity in discharges did not differ greatly among watersheds or correlate with land use. Nitrogen discharge correlated with net anthropogenic inputs of N to the watershed, but usually less than one-third of the net anthropogenic inputs were discharged.