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Whole-Stream Response to Nitrate Loading in Three Streams Draining Agricultural Landscapes

^a U.S. Geological Survey, Water Resources Div., 345 Middlefield Road, MS 439, Menlo Park, CA 94025
^b U.S. Geological Survey, Portland
^c U.S. Geological Survey, La Crosse, WI
^d Univ. of Wisconsin, La Crosse, WI
^e U.S. Geological Survey, Tacoma, WA

^* Corresponding author (jhduff{at}usgs.gov).

Received for publication April 17, 2007. Physical, chemical, hydrologic, and biologic factors affecting nitrate (NO₃^–) removal were evaluated in three agricultural streams draining orchard/dairy and row crop settings. Using 3-d "snapshots" during biotically active periods, we estimated reach-level NO₃^– sources, NO₃^– mass balance, in-stream processing (nitrification, denitrification, and NO₃^– uptake), and NO₃^– retention potential associated with surface water transport and ground water discharge. Ground water contributed 5 to 11% to stream discharge along the study reaches and 8 to 42% of gross NO₃^– input. Streambed processes potentially reduced 45 to 75% of ground water NO₃^– before discharge to surface water. In all streams, transient storage was of little importance for surface water NO₃^– retention. Estimated nitrification (1.6–4.4 mg N m^–2 h^–1) and unamended denitrification rates (2.0–16.3 mg N m^–2 h^–1) in sediment slurries were high relative to pristine streams. Denitrification of NO₃^– was largely independent of nitrification because both stream and ground water were sources of NO₃^–. Unamended denitrification rates extrapolated to the reach-scale accounted for <5% of NO₃^– exported from the reaches minimally reducing downstream loads. Nitrate retention as a percentage of gross NO₃^– inputs was >30% in an organic-poor, autotrophic stream with the lowest denitrification potentials and highest benthic chlorophyll a, photosynthesis/respiration ratio, pH, dissolved oxygen, and diurnal NO₃^– variation. Biotic processing potentially removed 75% of ground water NO₃^– at this site, suggesting an important role for photosynthetic assimilation of ground water NO₃^– relative to subsurface denitrification as water passed directly through benthic diatom beds.

Abbreviations: AFDM, ash-free dry mass • DO, dissolved oxygen • DON, dissolved organic nitrogen • TN, total nitrogen • TP, total phosphorus • PAR, photosynthetically active radiation • P/R ratio, photosynthesis-respiration ratio

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