Evaluation of Progress in Achieving TMDL Mandated Nitrogen Reductions in the Neuse River Basin, North Carolina |
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Authors: | Martin E Lebo Hans W Paerl Benjamin L Peierls |
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Institution: | (1) Weyerhaeuser Company, 1785 Weyerhaeuser Road, Vanceboro, NC 28586, USA;(2) Institute of Marine Sciences, University of North Carolina at Chapel Hill, 3431 Arendell Street, Morehead City, NC 28557, USA |
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Abstract: | Management efforts to control excess algal growth in the Neuse River and Estuary, North Carolina began in the 1980s, with
an initial focus on phosphorus (P) input reduction. However, continued water quality problems in the 1990s led to development
of a Total Maximum Daily Load (TMDL) for nitrogen (N) in 1999 to improve conditions in N-sensitive estuarine waters. Evaluation
of the effectiveness of management actions implemented in the Neuse River basin is a challenging endeavor due to natural variations
in N export associated with climate. A simplified approach is presented that allows evaluation of trends in flow-normalized
nutrient loading to provide feedback on effectiveness of implemented actions to reduce N loading to estuarine waters. The
approach is applied to five watershed locations, including the headwaters of the Neuse Estuary. Decreases in nitrate + nitrite
(NO3–N) concentrations occurred throughout the basin and were largest just downstream of the Raleigh metropolitan area. Conversely,
concentrations of total Kjeldahl N (TKN) increased at many stations, particularly under high flow conditions. This indicates
a relative increase in organic N (Org-N) inputs since the mid-1990s. Overall, patterns in different N fractions at watershed
stations indicate both partial success in reducing N inputs and ongoing challenges for N loading under high flow conditions.
In downstream waters, NO3–N concentrations decreased concurrent with TMDL implementation in the upper portion of the estuary but not in the middle
and lower reaches. The lack of progress in the middle and lower reaches of the estuary may, at least in part, be affected
by remineralization of settled particle-bound N deposited under high river flows. |
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