Agricultural headwater streams are critical pathways for phosphorus (P) export from tile-drained landscapes, yet the mechanisms controlling sediment-water P exchange in these systems remain poorly constrained. Shallow, hydrologically modified channels experience strong seasonal and diel variability in dissolved oxygen (DO), temperature, and biological activity, which can alter redox-sensitive P mobilization from sediments. Here we examined spatial and temporal patterns of DO and evaluated their influence on sediment porewater soluble reactive phosphorus (SRP) flux in three agricultural drainage-ditch streams within the Shallow Run sub-watershed of the Maumee River basin, Ohio, during the 2025 growing season.
Monthly reach-scale surveys were conducted from April through September to characterize water chemistry, physical habitat, and biological conditions, while continuous optical sensors quantified high-frequency DO dynamics at the bottom of the reach. In situ diffusive gradients in thin films (DGT) probes were deployed concurrently to provide time-integrated, depth-resolved estimates of labile SRP flux across the sediment-water interface (SWI). Continuous monitoring revealed pronounced seasonal declines in DO and strong diel oscillations, with extended hypoxic and anoxic conditions during mid-summer, particularly at the most nutrient-enriched site.
Sediment porewater SRP concentrations measured by DGT (cDGT) exhibited substantial temporal variability, with the highest porewater SRP concentrations occurring during periods of elevated temperature and reduced DO. Depth-resolved cDGT profiles indicated seasonal shifts in the depth of maximum SRP mobilization within sediments, suggesting dynamic redox controls on P release.
Preliminary data analysis suggests that porewater SRP concentrations are higher under warm conditions, in the presence of vegetation, and when sediment P saturated is high. Porewater SRP concentrations declined with stream water DO concentrations, presumably because this facilitated the release of porewater P into the water column. Stream water SRP concentrations were highest when DO concentrations were low.
These results demonstrate that low-oxygen conditions in agricultural headwater streams can substantially enhance sediment-derived P flux during the growing season. By integrating continuous oxygen measurements with in-situ porewater flux estimates, this study provides mechanistic insight into how redox dynamics regulate internal P loading in intensively managed agricultural stream networks, with important implications for downstream nutrient export and water-quality management.