Multidecadal trends in CO2 evasion and aquatic metabolism in a large temperate river

Abstract. Rivers play a critical role in the global carbon cycle. However, the environmental and hydro-climatic factors that control the direction and magnitude of river CO2 fluxes across seasons and multidecadal periods are poorly constrained. The origin of excess river CO2 – delivered by soils, wetlands and groundwater or produced by aquatic respiration of organic matter – remains an important unknown in linking terrestrial and aquatic carbon budgets. To address these knowledge gaps, we report on a 32-year high-frequency dataset (1990–2021) from the Loire River, a large, temperate river that underwent a shift from a phytoplankton-dominated regime to a macrophyte-dominated regime around 2005. We estimated daily river-atmosphere CO2 flux (FCO2) and river net ecosystem productivity (NEP) from hourly pH, alkalinity, dissolved oxygen, water temperature and solar radiation. We demonstrate that: (i) annual FCO2 varied an order of magnitude among years (range = 200–2600 g C m2 yr−1) with a long-term decrease trend, mainly linked to decreased groundwater contribution; (ii) the mean annual contribution of internal CO2 production from net ecosystem respiration to total FCO2 was 40 %, increasing from 37±27 % in phytoplankton-dominated regime to 57±10 % in macrophyte-dominated regime; (iii) while the river predominantly acted as a CO2 source, it occasionally functioned as a CO2 sink (FCO2<0) during summer, though this sink behavior constituted a minor component (−0.6 %) of the FCO2 budget; and (iv) FCO2 exhibited strong seasonality linked to discharge, exhibiting hysteresis where FCO2 levels at equivalent discharge were 1.5 to 2 times higher during the rising limb (autumn) compared to the falling limb (spring). The magnitude of this hysteresis diminished in the later macrophyte-dominated regime, indicating a reduced seasonal control by discharge on FCO2. This study demonstrates that river FCO2 and its source are dynamic within and across years, driven by hydro-climatic variations and biological activity. Catchment-scale hydrogeological changes (including groundwater and surface water interactions) can be a more dominant driver of long-term riverine CO2 evasion than in-stream ecological regime shifts (transitions from phytoplankton-dominated to macrophyte-dominated communities), controlling the balance between internal and external CO2 production.