A comprehensive analysis of external sources of nutrients is presented including an impact, comparison of external loadings, and the resulting scale of eutrophication. The major emphasis of the analysis is on nitrogen and phosphorus. The relative contribution of various external sources for five major ecosystems is delineated. Discussion of seasonal and long-term trends of external sources is presented. The impact of external sources on the eutrophication process is evaluated. A detailed comparison of the five estuaries of the Chesapeake Bay is presented.