Estimating Gross Primary Production in Mediterranean catchments using isotopic mass balance: A multi-source ecohydrological approach applied to Corsica (France)

Mediterranean catchments are strongly constrained by water availability, with pronounced seasonal contrasts that tightly couple hydrological processes and ecosystem functioning. Understanding how water fluxes regulate carbon uptake through Gross Primary Production (GPP) remains a key challenge in ecohydrology, particularly in Mediterranean regions where direct carbon flux measurements are scarce.

The aim of this study is to assess GPP at the catchment scale using a water-balance approach constrained by stable water isotopes (δ¹⁸O, δ²H). The method relies on the ecohydrological coupling between plant transpiration and carbon assimilation, quantified through water-use efficiency (WUE). Isotope-based partitioning of evapotranspiration is combined with WUE to derive basin-scale GPP.

The approach is applied to 11 Mediterranean catchments in Corsica (France), covering a wide range of spatial scales (15–950 km²), elevation gradients (sea level up to 2700 m), climatic conditions, and geological contexts (dominated by fractured granitic and metamorphic schists bedrock in the mountains and detrital sedimentary formations in downstream lowland areas). These catchments offer contrasted hydrological regimes, from perennial mountain rivers to strongly water-limited lowland coastal systems, making them a natural laboratory to investigate water–carbon interactions.

The methodology framework is based on one year of monitoring of rainfall and river water stable isotopes (δ¹⁸O and δ²H). Rainfall isotope data collected at 20 stations were used to generate isotope precipitation isoscapes, while river isotopes were monitored at 11 sites (one per catchment). These isotope datasets were combined with ERA5-Land climate data and CORINE Land Cover (CLC) vegetation information.

Monthly GPP calculated results reveal strong seasonal and spatial contrasts across the 11 Corsican catchments. GPP shows a pronounced Mediterranean pattern, with very low values during summer drought (often <20 gC.m⁻²) followed by a sharp recovery in autumn and peak values exceeding 200 gC.m⁻² during late autumn and early winter. The seasonal amplitude exceeds one order of magnitude, highlighting the dominant control of water availability on carbon uptake. Low-elevation and coastal catchments exhibit stronger summer GPP reductions than higher-altitude mountain and/or larger catchments, suggesting a buffering effect of elevation, groundwater storage capacity and geological context on ecohydrological functioning. However, some limitations remain regarding the difficulty to assess evaporation and thus transpiration values.

To better constrain the temporal dynamics of water fluxes underlying these patterns, an isotope-enabled hydrological model based on the J2000-iso framework is being developed. By improving the partitioning of hydrological fluxes, particularly evaporation and transpiration, this approach is expected to reduce uncertainties in GPP estimates and help to better link hydrological processes with ecosystem productivity.

Overall, this study provides one of the first basin-scale applications of isotope-based GPP estimation in Mediterranean environments at the regional scale and illustrates the potential of ecohydrological approaches to better quantify water–carbon coupling under ongoing climate change.