Climate Variation Overwhelms Efforts to Reduce Nitrogen Delivery to Coastal Waters
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We calculated watershed nitrogen (N) retention (inputs–outputs)/inputs) each year from 1999–2013 for nine sub-watersheds along an urban–rural gradient near Baltimore MD to determine how land use and climate influence watershed N flux. Retention is critical to efforts to control coastal eutrophication through regulatory efforts that mandate reductions in the total maximum daily load (TMDL) of N that specific water bodies can receive. Retention decreased with urbanization as well as with increases in precipitation with retention decreasing from an average of 91% in the forested sub-watershed to 16% in the most urban sub-watershed. Export was 23% higher, and retention was 7% lower in winter (November–April) than during the growing season. Total N delivery to Baltimore Harbor varied almost threefold between wet and dry years, which is significant relative to the total annual export allowed for all non-point sources to the harbor under the TMDL. These results suggest that expectations for TMDLs should consider watershed land use and climate variability, and their potential for change if they are to result in improvements in receiving water quality.
KeywordsNitrogen cycling Urban systems Watershed Total maximum daily load
The data for this paper are available at the Baltimore Ecosystem Web site (http://www.beslter.org/). This work was funded by the U.S. National Science Foundation: NSF-EAR award #0847838 to N.D.B and NSF-DEB award #1027188 (Baltimore LTER). Special thanks to Dan Dillon, who provided valuable assistance and advice in the field, Lisa Martel, Robin Schmidt, Kate Shepard, and Amanda Irish who processed and analyzed samples in the laboratory and two anonymous reviewers who provided two rounds of valuable and constructive comments and suggestions.
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