Combining isotope measurements, water quality sensors and computational methods to unravel in-stream carbon dynamics of a complex stream network in the Italian Alps

Freshwaters are key players in global carbon (C) cycle as they collect C leaked from the terrestrial ecosystem and host in-stream production and respiration processes. C transported by streams and rivers can be out-gassed to the atmosphere in the form of carbon dioxide (CO2), buried in the sediments or reach the coastal oceans. In the last decades, the relevance of these exchange fluxes to global CO₂ emissions has been recognised, as well as the importance of describing the C dynamics at the stream sediment-water-atmosphere interfaces. Describing the functioning of fluvial C cycling under varying hydrodynamic and morphological traits is even more critical in mountain catchments due to the rapid change they are facing under global warming. However, estimation of these fluxes is largely uncertain and requires the integration of multidisciplinary theoretical and observational studies.

This work illustrates the planned activities and the preliminary results of two synergistic research projects aimed at resolving C cycling and stream metabolism in an alpine catchment: project CONSTRAIN (CarbON exchange processes across STReAm INterfaces) funded by the Italian Ministry of Research, and project iNEST (Interconnected North-Est Innovation Ecosystem) funded by the European Union Next-Generation EU. 

The projects focus on the Rio Valfredda, a pristine mountain stream network draining a 5.3 km² catchment in the Italian Alps. The planned activities include the continuous, high frequency measurement of dissolved oxygen and carbon dioxide, along with environmental ancillary variables like photosynthetic active radiation, stream temperature, barometric pressure, pH and electrical conductivity, in four reaches within the stream network. Through a newly proposed model that couples the diel fluctuations of O₂ and CO₂, we aim to jointly estimate stream metabolism (i.e. gross primary production and ecosystem respiration), lateral input of dissolved inorganic carbon (DIC) and CO₂ out-gassing to the atmosphere. 

We aim at linking C cycling patterns with hydrologic traits of the selected reaches. To that end, water stable isotopes (δ¹⁸O and δ²H) are being monitored in several tributaries of the stream network (grab samples with monthly frequency), at the catchment outlet (at daily frequency) and in the precipitation collected by rain gauges placed at different altitudes. From the analysis of the isotopic signature of streamflow and precipitation we reconstruct summary statistics of the travel time distribution of water within the hillslope with the goal of relating it with the lateral flow of DIC.

The comprehensive set of information collected, together with the previous knowledge about the hydrological dynamics of the Valfredda catchment, which has been closely monitored for the past 5 years in the framework of the DyNET project funded by the European Research Council, will allow upscaling C cycling at the level of the whole network rather than focusing on individual reaches. These projects will enhance our understanding of the role played by hydrology on the metabolism of complex river networks, unraveling the multifaceted dynamical relations that link rivers with the surrounding environment and allowing a robust assessment of the contribution of freshwaters to CO₂ emissions.