Nitrate dynamics within the Pajaro River, a nutrient-rich, losing stream

The major ion chemistry of water from an 11.42-km reach of the Pajaro River, a losing stream in central coastal California, shows a consistent pattern of higher concentrations during the 2 nd (dry) half of the water year. Most solutes are conserved during flow along the reach, but (NO3 � ) decreases by ;30% and is accompanied by net loss of channel discharge and extensive surface-subsurface exchange. The corresponding net NO3 � uptake length is 37 6 13 km (42 6 12 km when normalized to the conservative solute Cl � ), and the areal NO3 � uptake rate is 0.5 lmol m � 2 s � 1 . The observed reduction in (NO3 � ) along the reach results from one or more internal sinks, not dilution by ground water, hill-slope water, or other water inputs. Observed reductions in (NO3 � ) and channel discharge along the experimental reach result in a net loss of 200-400 kg/d of NO3 � -N, ;50% of the input load. High-resolution (temporal and spatial) sampling indicates that most of the NO3 � loss occurs along the lower part of the reach, where there is the greatest seepage loss and surface-subsurface exchange of water. Stable isotopes of NO3 � , total dissolved P concentrations, and streambed chemical profiles suggest that denitrification is the most significant NO3 � sink along the reach. Denitrification efficiency, as expressed through downstream enrichment in 15 N-NO3 � , varies considerably during the water year. When discharge is greater (typically earlier in the water year), denitrification is least efficient and downstream enrichment in 15 N-NO3 � is greatest. When discharge is lower, denitrification in the streambed appears to occur with greater efficiency, resulting in lower downstream enrichment in 15 N-NO3 � .