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 sign and magnitude of river CO₂ fluxes across seasons and multi-decadal periods are less 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 eutrophic, phytoplankton-dominated regime to an oligotrophic, macrophyte-dominated regime in ca. 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); ii) the mean annual contribution of aquatic metabolism to total FCO₂ was 40 %, but this also varied according to year and trophic regime, ranging from negative to 100 % contribution; iii) the river occasionally acted as a CO₂ sink (FCO₂ < 0) during summer, especially during the eutrophic period of 1990–2000, but this flux was negligible (-0.6 % of the FCO₂ budget); and iv) FCO₂ exhibited hysteresis with discharge, with FCO₂ levels ranging from 1.5 to 2 times higher in autumn compared to spring at equivalent discharge rates, and the degree of which was depended on trophic regime. This study makes clear that river FCO₂—and the source of this CO₂—is dynamic within and across years and that global changes affecting the river trophic regime control the balance between internal and external CO₂ production.