Greatest Opportunities for Future Nitrogen Reductions to the Chesapeake Bay Watershed are in Developed and Agricultural Areas

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Issue: As human population has increased, land-use changes have led to increases in nutrients (nitrogen and phosphorus) and sediment into the Bay. The excess nutrients cause algal blooms which contribute to water-quality impairments such as low oxygen or hypoxia (dead zones), and poor water clarity in the Chesapeake Bay. Management efforts to improve water quality focus on dissolved oxygen needed for fisheries, and water clarity needed for submerged aquatic grasses, which add oxygen into the Bay, provide habitat for fish, and food for waterfowl. Recreational and commercial fisheries in the Bay and its watershed are valued at more than $20 billion annually.

USGS Study

The USGS conducted a comprehensive synthesis of nitrogen in the Chesapeake Bay watershed. The report provides a unique, long-term perspective of the major drivers of nitrogen change from 1950 up to the present, and forecasts how those drivers may affect nitrogen loading in the Chesapeake Bay out to 2050. The report examines the major nitrogen sources including the application of crop fertilizers and livestock waste (manure), wastewater discharged to streams, atmospheric deposition (primarily fossil fuel emissions and agricultural sources) and runoff from developed areas.

Major Findings

This report examines several scenarios out to 2050 for nitrogen changes in the watershed and delivery to the Bay. “The scenarios suggest reductions of nitrogen in agricultural and developed areas have the greatest potential in decreasing nitrogen to the Chesapeake Bay in the coming decades.” said John Clune, a USGS scientist and lead editor on the new report.

Some of the findings for changes in nitrogen from the 1950’s until present include:

  • The sum of all the sources of nitrogen to the Bay increased substantially from 1950 until the 1980s, but varied afterwards (see figure on changes and forecasts, which is from the Circular).
  • Changes in nitrogen from wastewater treatment and atmospheric deposition since 1950 reflect influences from population growth and management efforts. Nitrogen from atmospheric deposition increased substantially from 1950 but started to decrease in the 1980s owing to the effects of the Clean Air Act and is projected to continue to moderately decrease for the next few decades. Wastewater also increased nitrogen to the Bay from 1950 but started to decrease around 1990 with the implementation of enhanced nitrogen removal technologies in many developed areas.
  • Nitrogen load from agriculture peaked in 2000 and decreased through 2012. Examination of primary agricultural sources of nitrogen shows manure has continually increased from 1950 to 2012 due to the intensification of animal agriculture. Nitrogen from chemical fertilizer has increased over this same time period, but there is substantial variation during the past few decades. The variety and quantity of conservation practices to reduce nitrogen has also increased substantially over this time period and across the watershed. These counteracting influences have mixed results on nitrogen trends in the watershed, with 41 percent of monitoring sites showing improvement, but a similar percent still degrading, according to another USGS study (Moyer and Blomquist, 2020).
Annual nitrogen loads exported to the Chesapeake Bay by source

Annual nitrogen loads exported to Chesapeake Bay by source, 1950-2050
(From Clune and Capel, 2021, figure OV.4; see Circular for more explanation)

This report examines several scenarios out to 2050 for nitrogen changes in the watershed and delivery to the Bay, with some of the major findings:

  • Future scenarios for agriculture, which is the largest source of nitrogen in the watershed, included both increasing and decreasing amounts of fertilizer and manure. All the scenarios suggest the export of nitrogen to the Bay presents a challenge to nitrogen load targets. The load targets are part of a Total Maximum Daily Load for the Bay established by the Environmental Protection Agency, which is maximum amount of a pollutant allowed to enter a waterbody to meet water quality standards.
  • Developed areas have increased along with population growth and this trend is projected to continue. Developed areas, although a smaller fraction of overall land use in the Chesapeake Bay watershed, can have increased nitrogen inputs as development expands in the coming decades.

Management Implications

For decades the USGS has provided critical science to understand and improve the health of the Chesapeake Bay ecosystem. The new report will provide further science to inform the complex decisions toward the challenge of improving water quality in the Bay and its watershed, including:

  • The report provides insights into the different types on management controls being considered by the states to reduce nutrients from agricultural lands, including decreases in fertilizer application, placement of conservation practices. new technology to reduce nitrogen exported from fields and decreasing the number of animals and animal waste on farms.
  • The report suggests managers will have the greatest ability to control nitrogen from wastewater discharge and urban runoff in developing areas.

For Further Information

Clune, J.W., and Capel, P.D., eds., 2021, Nitrogen in the Chesapeake Bay watershed—A century of change, 1950–2050: U.S. Geological Survey Circular 1486, 168 p.,
Moyer and Blomquist, USGS updates trends for nutrients and sediment in the Chesapeake Bay Watershed (2020)

Primary Contact:
Hydrologist, Pennsylvania Water Science Center


Posted November 12, 2021


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