Stream macronutrient cycling is often thought to occur within fixed boundaries because of the unidirectional flow in running waters. However, recent studies showed that stream connectivity, which is the exchange and pathways of energy and matter throughout the stream, influences water chemistry by integrating biogeochemical processes within the stream's different compartments, such as the surface channel (SW), the hyporheic zone (HZ), and the adjacent riparian zone (RZ). Yet, these macronutrient cycling dynamics become more complex in intermittent streams because the flow intermittence disrupts the connectivity. It is still unclear how the timing and the spatial variation in flow connectivity affect stream nitrate dynamics. Here, we examined the influence of hydrological connectivity across the SW, the HZ, and the RZ on nitrate concentrations. Our four study sites were located along a 5-km reach of an intermittent stream in the Midwest Ozarks, USA. During the rewetting period (October to June), we collected biweekly water samples for chloride (CL-) (used as a conservative tracer) and nitrate (NO3-) from four wells in the RZ and two in the HZ. We also collected water samples from the surface channel. We observed strong hydrological connectivity at the HZ, RZ, and SW interface, likely influenced by precipitation. The HZ NO3- concentrations ranged from 0.011 to 1.631 ppm, and the RZ NO3- concentrations ranged from 0.018 to 1.092 ppm. The NO3- in the stream ranged from 0.001 to 3.731 ppm, increasing as the rewetting period progressed and showing the highest concentrations in the summer before the stream dried up, while the HZ and RZ concentrations lowered. Differences in NO3- between compartments were observed in the fall, coinciding with lower hydrological connectivity between the stream compartments. Overall, we will present opportunities to advance our understanding of biogeochemistry in intermittent streams in the central region of the US.

Seasonal variations of nitrate and chloride in an intermittent stream in the Midwest Ozarks

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