A comparison of independent N-loading estimates for U.S. estuaries



We assembled 27 recent and independently derived Total Nitrogen (TN) loading estimates for watersheds of east coast and Gulf of Mexico estuaries (watershed size range: 27 to 90,672 km2; yield range: 44 to 2,722 kg N km−2 y−1). These results were compared to those from two other recent studies that used national water quality data bases and computer modeling or statistical analyses to estimate loadings. The loadings from these other studies average about 50 to 60% of the 27 independently derived TN loading estimates. In many cases the individual estimates are more likely to achieve fuller inclusion of all nitrogen sources to the estuary than these other two efforts, because of the inclusion of groundwater, sewerage, or site-specific point and non-point discharges.

The nitrogen yield from these coastal estuarine watersheds is strongly related to population density, and the per capita yield (kg N person−1) is lower than for the large watersheds draining into the Northern Atlantic, including the Mississippi River. The variability in TN yield per capita is strongly related to the percentage of the landscape that is harvested cropland, which is generally higher in the larger watersheds. A simple statistical model of population density and % of the watershed that is harvested cropland describes 79% of the variation in TN yield (kg N km−2) for 11 basins, including the Mississippi River basin.

The amount of direct atmospheric deposition to the estuarine surface rises to about 25% of the TN loading to the estuary, when the estuarine surface: watershed area ratio is 0.2. About 20% of all estuarine surface area in the US, distributed in 12 estuaries, has a ratio of 0.2, or higher. The significance of direct atmospheric nitrogen loadings to the estuarine surface is thus responsive to several geomorphic and socio-economic factors that range greatly across U.S. estuaries.

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