Assessing the sources and magnitude of diurnal nitrate variability in the San Joaquin River (California) with an in situ optical nitrate sensor and dual nitrate isotopes

Summary 1. We investigated diurnal nitrate (NO 3 − ) concentration variability in the San Joaquin River using an in situ optical NO 3 − sensor and discrete sampling during a 5‐day summer period characterized by high algal productivity. Dual NO 3 − isotopes (δ 15 N NO3 and δ 18 O NO3 ) and dissolved oxygen isotopes (δ 18 O DO ) were measured over 2 days to assess NO 3 − sources and biogeochemical controls over diurnal time‐scales. 2. Concerted temporal patterns of dissolved oxygen (DO) concentrations and δ 18 O DO were consistent with photosynthesis, respiration and atmospheric O 2 exchange, providing evidence of diurnal biological processes independent of river discharge. 3. Surface water NO 3 − concentrations varied by up to 22% over a single diurnal cycle and up to 31% over the 5‐day study, but did not reveal concerted diurnal patterns at a frequency comparable to DO concentrations. The decoupling of δ 15 N NO3 and δ 18 O NO3 isotopes suggests that algal assimilation and denitrification are not major processes controlling diurnal NO 3 − variability in the San Joaquin River during the study. The lack of a clear explanation for NO 3 − variability likely reflects a combination of riverine biological processes and time‐varying physical transport of NO 3 − from upstream agricultural drains to the mainstem San Joaquin River. 4. The application of an in situ optical NO 3 − sensor along with discrete samples provides a view into the fine temporal structure of hydrochemical data and may allow for greater accuracy in pollution assessment.