In a third of the estuaries studied, the amount of nitrogen in rain and airborne particles was as large as those carried into estuaries by streams, according to the findings of the study carried out by the US Geological Survey (USGS) (see related story), the National Oceanographic and Atmospheric Administration (NOAA) and the Blackland Research Centre at Texas A&M University. The watersheds of 40 major US estuaries were investigated, and of these, the highest atmospheric contributions of nitrogen were in streams along the north-eastern and Mid-Atlantic coasts, where a quarter to over a third of the nitrogen came from the air. Atmospheric contributions along the Louisiana Gulf coast were almost as high.

“This study provides important new information about how coastal watersheds process atmospheric nitrogen and about the amounts of atmospheric nitrogen that enter estuaries,” said Richard Alexander, a USGS hydrologist and expert on nutrients, and co-author of the study. “There are important scientific and policy questions about the role of atmospheric nitrogen in causing coastal water-quality problems. This information can help local resource managers determine sources of nitrogen entering estuaries. This improved understanding of the links between air deposition of nitrogen and coastal water quality also leads to a better scientific basis for steps to minimise coastal pollution sources.”

Though the oxygen-depleting effects of nitrogen on estuaries are well known, until now, scientists have had incomplete information on the sources of nitrogen entering them, and scientists have been unclear as to how much came from other sources such as vehicles and power plants (see related story).

The investigation also confirmed that estuaries receive much of their nitrogen from non-atmospheric sources, such as farms, pastureland, and wastewater treatment plants. Agricultural runoff contributed the largest share, more than one-third in most of the coastal watersheds studied. The contributions from municipal and industrial wastewater are similar to those from the atmosphere in many watersheds, but provide the largest share in a number of densely populated watersheds along the North Atlantic and Gulf coasts.

The study, Atmospheric Nitrogen Flux from the Watersheds of Major Estuaries of the United States: An Application of the SPARROW Watershed Model, is part of a larger collaborative effort, including scientists from more than 15 federal, state, and academic institutions, and is published by the American Geophysical Union.

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