Understanding the effects of hydrological connectivity on the concentrations and quality of dissolved organic carbon in an intermittent stream located in the Ozarks

Streams are connected to terrestrial and groundwater environments by longitudinal, lateral, and vertical water fluxes. This connectivity influences stream water quality and biogeochemical processes by transporting sediments, solutes, and organisms. Studies have shown that hydrological connectivity is the main driver of dissolved organic carbon (DOC) and dissolved organic matter (DOM) dynamics in streams. In intermittent streams, however, these dynamics become more complex because the flow intermittence disrupts connectivity. To date, it is still unclear how the timing and the spatial variation in flow connectivity affect the dynamics of DOC and DOM. We examined the influence of hydrological connectivity across the surface channel (SW), hyporheic (HZ), and riparian (RZ) compartments on DOC quantity and quality. Our four study sites were located along a 5-km reach of an intermittent stream running through karst geology in the Midwest Ozarks, USA. We collected data through the rewetting period that started in Oct until the stream dried up in Jun 2022. At each sampling site, biweekly, we collected water samples for chloride, DOC, and DOM analysis from SW and 25 wells in the RZ and HZ. Results show that hydrological connectivity between SW and RZ varied across time by being disconnected from the main channel until Jan, briefly losing connectivity in March, and reconnecting until the dry season. This was likely influenced by precipitation events. DOC in the stream ranged between 1.3 and 10 ppm, increasing as the rewetting period progressed and decreasing until the stream dried up. The HZ DOC concentrations were higher and more similar (1.9 to 10.9 ppm) to the SW than in the RZ, but in the RZ, DOC varied (1.5 to 7.9 ppm) dependent on precipitation. Differences in DOC between compartments were observed in the fall, coinciding with lower connectivity between the stream compartments (p-value < 0.05). During the dry season, the compartments lost connectivity showing a decrease in the DOC in the HZ and RZ. DOC quality is currently being assessed via optical metrics of E2:E3, SUVA254, and spectral slopes for evidence of biogeochemical processing. Overall, we will present opportunities to advance our understanding of biogeochemistry and hydrological connectivity in intermittent streams in the central region of the USA.