Citation:

Detenbeck, N., M. You, AND D. Torre. Recent Changes in Nitrogen Sources and Load Components to Estuaries of the Contiguous United States. Estuaries and Coasts. Estuarine Research Federation, Port Republic, MD, 42(8):2096-2113, (2019). https://doi.org/10.1007/s12237-019-00614-1

Impact/Purpose:

The condition of estuaries, areas where fresh and salt water meet, is assessed on regional and national scales every five years. Nutrients, such as nitrogen, are necessary for plant and animal growth and are released into estuaries through human activities such as application of fertilizers and the treatment of human waste. There is currently no corresponding assessment of how much of these nutrients enter estuaries (called the load) in the 48 adjoining U.S. states in North America. Calculations of the load of these nutrients has been incomplete because of lack of data and incomplete models. This report presents an evaluation of the sources and trends of nitrogen entering these estuaries based on application of regional models. This information is useful to control the sources of nitrogen entering the estuary, provide data to link with estuary condition, and prioritize efforts to reduce nitrogen loads.

Description:

Regional Spatially Referenced Regressions on Watershed models were used to update 2002 delivered nitrogen (N) loads to estuaries of the contiguous US for 2011, supplemented by direct estuarine atmospheric deposition from the Community Multiscale Air Quality Model. Median 2011 watershed N yields were greatest for the Puget Trough, Virginian, and Oregon–Washington–Vancouver Coast marine ecoregions (MEs; 13.7, 11.0, and 9.9 kg N/ha watershed/year, respectively); intermediate for the Floridian, Southern California Bight, and Northern California MEs (4.4–6.3 kg N/ha watershed/year); and lowest for the Northern Gulf of Mexico, Carolinian, and Gulf of Maine MEs (2.4–3.2 kg N/ha watershed/year). Dominant sources varied across marine ecoregions, with direct atmospheric deposition as the dominant source only in the far northern Gulf of Maine ME. Delivered N loads from atmospheric deposition have significantly decreased (p < 0.05) for most estuaries on the Atlantic and Gulf coasts for 2002–2012. Estimated point source delivered N loads for 2002–2012 increased for most estuaries with upstream treatment plants, with estimated loads to only seven estuaries decreasing by more than 50%. Urban runoff increased for most estuaries in the Puget Trough and Carolinian MEs and either increased or had no significant trend for the remaining marine ecoregions. The magnitude of change in total N delivered loads is uncertain due to incomplete monitoring for most minor dischargers. In areas with increased population growth and decreases in agricultural land, decreasing agricultural fertilizer inputs have been insufficient to offset increases in urban runoff.

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