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 (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 . 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 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 N-NO3 is greatest. When discharge is lower, denitrification in the streambed appears to occur with greater efficiency, resulting in lower downstream enrichment in N-NO3 .