Stable but Transient—Episodic Sensitivity in a Buffered German Karst Headwater Stream

Abstract

Karst headwaters often exhibit stable carbonate-system behaviour because carbonate buffering maintains consistent relationships among pH, alkalinity, dissolved inorganic carbon, calcite saturation and partial CO₂ pressure (pCO₂). However, it remains unclear whether this chemical stability translates into predictable CO₂ evasion patterns ($F_{{\mathrm{CO}}_2}$), and how hydroclimatic shifts affect the resilience of buffered spring-fed systems. This study examines a karst headwater stream in northern Bavaria over 15 months, complemented by a 32 h diel survey during warm low-flow conditions. System sensitivity was evaluated as deviations from baseline carbonate-system behaviour and as spatial decoupling between pCO₂ and $F_{{\mathrm{CO}}_2}$. Under baseline conditions, the stream showed a consistent carbonate-system evolution over the first 2000 m. Alkalinity remained stable, while pCO₂ declined from ~22 910 to ~3420 μatm, dissolved inorganic carbon decreased by ~0.7 mmol L⁻¹, and δ¹³C_DIC became enriched by ~1.4‰. However, this hydrochemical consistency did not translate into predictable $F_{{\mathrm{CO}}_2}$ patterns. Although pCO₂ was highest at the spring, $F_{{\mathrm{CO}}_2}$ peaked ~443 m downstream at ~9000 mmol m⁻² day⁻¹, indicating that reach-scale hydromorphology and gas-transfer conditions modulated CO₂ evasion. Hydroclimatic extremes and transitions caused short-lived deviations from baseline behaviour, especially during warm low-flow, rewetting (after drought) and high-flow conditions, pointing to transient sensitivity rather than persistent system destabilization. Diel observations supported this interpretation by showing stronger short-term oxygen and flux variability downstream, while the spring remained comparatively buffered. Overall, this study illustrates that carbonate buffering can maintain hydrochemical stability while CO₂ evasion remains locally controlled, spatially heterogeneous and transiently sensitive to hydroclimatic variability.