Seasonal Variation in Flow and Metabolic Activity Drive Nitrate and Carbon Supply and Demand in a Temperate Agricultural Stream

In‐stream biogeochemical processing, typically associated with base flow conditions, has recently been assessed at higher discharges, aided by high frequency monitoring. However, the potential for nutrient and carbon processing is still largely unknown in streams impacted by agriculture, representing major pathways for eutrophication and diffuse pollution. In this study, we measured solute concentrations and gross primary productivity (GPP) and ecosystem respiration (ER) to infer nitrate (NO₃⁻) and dissolved organic carbon (DOC) supply and demand across contrasting hydrological conditions. As expected, solute supply greatly surpassed in‐stream biological demand for both NO₃⁻ and DOC for intermediate to large discharges. However, during four consecutive weeks in summer, lowered NO₃⁻ supply and high metabolic activity led to a 60% and 31% reduction in stream NO₃⁻ and DOC export. We also compared metabolism‐discharge versus solute concentration‐discharge patterns during storm events to better understand biogeochemical responses to high flows. Metabolic rates showed a contrasting response to storm events: ER increased while GPP decreased following declines in NO₃⁻ concentrations. The positive correlation between GPP and NO₃⁻ concentrations suggests that GPP suppression can be partially attributed to decreased NO₃⁻ availability during storm events. This study supports the idea that agricultural streams have a limited capacity to biologically process DOC and NO₃⁻. However, it also emphasizes that the balance between supply and demand can vary from severe saturation to limitation, depending on seasonal fluctuations in discharge and metabolic activity, highlighting the crucial role of mitigating pollution at its source during hydrologically active periods to improve water quality.