New Synthesis Describes Current Understanding of Factors Driving Nutrient Trends in Streams of the Chesapeake Bay Watershed

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Issue: Excessive nitrogen and phosphorus in Chesapeake Bay since the 1950s have contributed to low dissolved oxygen leading to fish kills, and poor water clarity and associated loss of submerged aquatic vegetation. The Chesapeake Bay Program partnership has been working to improve aquatic conditions in the Bay and its tidal tributaries, and streams in the watershed, by reducing inputs of nutrients and sediment.

Under the Clean Water Act, the bay has been listed as impaired‖ and a total maximum daily load (TMDL) was established in 2010 to provide a framework for limiting nitrogen, phosphorus, and suspended sediment flux from the watershed. The TMDL requires management practices be implemented by 2025 to reduce nitrogen and phosphorus input.

USGS Study: The USGS conducted new research, and synthesized results from other studies, to provide useful insights on multiple factors complicating the understanding of nutrient trends in bay watershed. This paper summarizes the current understand of factors driving nutrient trends.

Major findings: Despite decades of effort toward reducing nitrogen and phosphorus flux to Chesapeake Bay, water-quality and ecological responses in surface waters have been mixed. Recent research provides insight on multiple factors driving complex nutrient trends, including:

U.S. Geological Survey hydrologic technicians collect a stream sample from Hallocks Mill Brook

  • Improvements in water quality in many streams are attributable to management actions that reduced point sources and atmospheric nitrogen deposition and to changes in climate.
  • Nitrogen from urban non-point sources has declined, although water-quality responses to urbanization in individual streams vary depending on pre-development land use.
  • Evolving agriculture, the largest watershed source of nutrients, has likely contributed to local nutrient trends but has not affected substantial changes in flux to the bay. Changing average nitrogen yields from farmland underlain by carbonate rocks, however, may suggest future trends in other areas under similar management, climatic, or other influences, although drivers of these changes remain unclear.
  • Regardless of upstream trends, phosphorus flux to the bay from its largest tributary has increased due to sediment infill in Conowingo Reservoir.

The supporting science for major findings are further described in the article.

Management Applications: The findings will help managers understand water-quality response to nutrient reduction activities and other factors as the CBP approaches the 2025 TMDL management deadline.

  • In general, recent research emphasizes the utility of reducing the input of nutrients to the lands over attempts to manage nutrient transport from lands to surface waters.
  • Ongoing research opportunities include evaluating effects of climate change and conservation practices over time and space and developing tools to disentangle and evaluate multiple influences on regional water quality.

For more information:

Read: Ator, S.W., Blomquist, J.D., Webber, J.S. and Chanat, J.G. (2020), Factors driving nutrient trends in streams of the Chesapeake bay watershed. J. Environ. Qual.. doi:10.1002/jeq2.20101.

Contact: Scott Ator, swator@usgs.gov

 

Science Summary was released July 13, 2020.

 

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