The Effects of Stream Slope on Dissolved Gas Concentrations Are Mediated by Scale in an Example Low‐Gradient Stream

Abstract

Channel slope influences the key physical and biogeochemical drivers of dissolved gas dynamics in flowing waters, including turbulence‐driven gas exchange, in situ metabolism, and external inputs. These processes operate at different spatial scales, yet the extent to which channel slope controls longitudinal gas concentration variations—particularly in low‐gradient streams containing pools with high hydrologic retention—remains poorly understood. In this study, we measured dissolved oxygen (DO), carbon dioxide (CO ₂ ), and methane (CH ₄ ) along a ∼8 km stretch of New Hope Creek (NHC), a low‐gradient stream in North Carolina characterized by frequent pools. We paired longitudinally collected gas concentrations with estimates of gas transfer velocity ( k ₆₀₀ ) and channel slopes. We measured substantial spatial variation in the concentrations of all three gases, but this variation was not significantly correlated with k ₆₀₀ , which was low across sites (0.10–15.33 m d ⁻¹ , median = 0.35 m d ⁻¹ ). Instead, channel slope was a better predictor. A flatter slope was associated with greater departure from equilibration (depletion for DO and oversaturation for GHGs), but the relationship occurred at different scales: habitat (∼50 m) for DO and long‐distance scale (>100 m) for GHGs. However, during high‐flow periods enhancing mixing, we measured minimal spatial variation. These results demonstrate that channel slope controls longitudinal gas concentration patterns by influencing biogeochemical processes at varying distances. Our study showed that gas exchange coefficients might not encompass the full reach‐scale variations and underscored the importance of geomorphic context in shaping the longitudinal heterogeneity of dissolved gases in low‐gradient streams.