Coastal eutrophication and agriculture: Contributions and solutions

Many coastal waters of developed nations have experienced widespread and ra pid eutrophication (the increase in supply of organic matter) during the la st half of the 20(th) century. This has resulted in increased phytoplankton production, decreased water clarity, often-severe depletion of dissolved o xygen in bottom waters, loss of seagrasses and, in some cases, declines or changes in the quality of fisheries production. Large-scale changes resulti ng from eutrophication have been documented for continental shelf waters in the Gulf of Mexico, Mediterranean, Black and North Seas, relatively confin ed seas such as the Baltic and Seto Inland Sea, large bays such as the Ches apeake Bay and Long Island Sound and numerous smaller estuaries and lagoons . These trends are closely tied to the increased use of chemical fertilizer s in agriculture, human population growth, and increasing atmospheric depos ition of nitrogen resulting from fossil fuel combustion. Although atmospher ic and human waste sources are significant in some heavily populated areas, agricultural inputs of phosphorus and nitrogen are the largest source of n utrients driving the increased production of organic matter in most extensi vely affected areas, including coastal waters receiving drainage from large river basins with extensive agriculture (e.g., Mississippi, Po, and Danube Rivers). Agricultural inputs of nutrients are driven not only by applications of che mical fertilizers, but also by animal wastes, irrigation, drainage, and the conversion of wetlands and riparian zones (important sinks for nutrients) for agricultural land uses. More efficient agronomic practices, use of crop rotation and cover crops, and avoiding the over-application of manure can result in reductions in nutrient losses by 20 to 30%. Reconfiguration of ag ricultural landscapes through reconstruction of strategically placed wetlan ds, riparian forests and flood plains can trap a similar fraction of the re maining nutrient losses, such that total reductions of 50% may be feasible without devastating economic impacts and with numerous local benefits to en vironmental quality. Efforts to restore large coastal ecosystems such as th e Baltic Sea, northern Gulf of Mexico, and Chesapeake Bay through commitmen ts to reduce nutrient loading have been underway or are beginning. They rep resent substantial challenges in working across political jurisdictions and across scientific disciplines and internalizing the external environmental costs of food production.