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

Physical, chemical, hydrologic, and biologic factors affecting nitrate (NO 3 − ) 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 NO 3 − sources, NO 3 − mass balance, in‐stream processing (nitrification, denitrification, and NO 3 − uptake), and NO 3 − 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 NO 3 − input. Streambed processes potentially reduced 45 to 75% of ground water NO 3 − before discharge to surface water. In all streams, transient storage was of little importance for surface water NO 3 − 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 NO 3 − was largely independent of nitrification because both stream and ground water were sources of NO 3 − Unamended denitrification rates extrapolated to the reach‐scale accounted for <5% of NO 3 − exported from the reaches minimally reducing downstream loads. Nitrate retention as a percentage of gross NO 3 − 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 NO 3 − variation. Biotic processing potentially removed 75% of ground water NO 3 − at this site, suggesting an important role for photosynthetic assimilation of ground water NO 3 − relative to subsurface denitrification as water passed directly through benthic diatom beds.