1. We investigated diurnal nitrate (NO3−) concentration variability in the San Joaquin River using an in situ optical NO3− sensor and discrete sampling during a 5-day summer period characterized by high algal productivity. Dual NO3− isotopes (δ15NNO3 and δ18ONO3) and dissolved oxygen isotopes (δ18ODO) were measured over 2 days to assess NO3− sources and biogeochemical controls over diurnal time-scales.
2. Concerted temporal patterns of dissolved oxygen (DO) concentrations and δ18ODO were consistent with photosynthesis, respiration and atmospheric O2 exchange, providing evidence of diurnal biological processes independent of river discharge.
3. Surface water NO3− 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 δ15NNO3 and δ18ONO3 isotopes suggests that algal assimilation and denitrification are not major processes controlling diurnal NO3− variability in the San Joaquin River during the study. The lack of a clear explanation for NO3− variability likely reflects a combination of riverine biological processes and time-varying physical transport of NO3− from upstream agricultural drains to the mainstem San Joaquin River.
4. The application of an in situ optical NO3− 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.