Nitrogen in the Chesapeake Bay watershed: A century of change, 1950–2050

Nitrogen, a critical element in all forms of life, is continuously being passed from nonliving to living matter and then back again, but an excess of this nutrient can have adverse effects on aquatic environments. An understanding of the past, present, and future sources, movement, and fate of nitrogen in the Chesapeake Bay watershed can help inform efforts to bring this cycle back into balance (fig. OV.1). A substantial shift in the nitrogen balance has occurred since the 1940s, when industrially produced nitrogen fertilizer became widely available. Fertilizer application to crops increased substantially in succeeding decades in order to meet the demand for food by a growing population. During this period, the loading of excess nitrogen took a toll on aquatic ecosystems and the related economies that are dependent on the Chesapeake Bay (also referred to as the Bay). Increased algal productivity from nutrient enrichment, both nitrogen and phosphorus, has resulted in the expansion of low oxygen (hypoxia) conditions, also known as “dead zones,” in the Bay. Substantial legislative, management, and grass roots initiatives have been developed over recent decades to combat the increasing impacts of excessive nutrients in the Chesapeake Bay watershed. In 2010, regulatory pollution limits were developed for the Chesapeake Bay creating the largest and most complex total maximum daily loads in the Nation for nitrogen, phosphorus, and sediment. Over this same time period, the monitoring and modeling of water quality has expanded to provide a better understanding of the fate and transport of nitrogen in the watershed. Our ability to improve the nitrogen balance in the Chesapeake Bay is dependent on our understanding of how a changing climate, population, land use, and decisions at all levels of government impact the health of the Bay. The significance and magnitude of the management and scientific efforts to restore the Chesapeake Bay have provided a template for restoration and serve as a model for the Nation and the world.

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