Linking catchment transit time heterogeneity with fluvial metabolism to unravel carbon cycling in an alpine stream network

Freshwater systems such as rivers, streams, and groundwater are critical for the ecosystem functioning, as they shape the fate of solutes in terrestrial environments. Runoff and leaching waters drain superficial and deep soil layers, vegetation, and rock weathering zones, transferring nutrients and other compounds to downstream ecosystems and eventually to oceans. Freshwaters also facilitate chemical reactions, such as ion exchange, mineral precipitation, and biological uptake, which dictate solute concentrations and forms during transport. The transport of nutrients and other solutes is crucial to the productivity and health of aquatic and riparian ecosystems. For example, the delivery of organic carbon supports the dynamics of the microbial and food web, while transport of metals and pollutants impacts water quality, affecting drinking water sources, aquatic life, and ecosystem services. Catchment hydrologic response and connectivity must be carefully untangled in order to characterize fluvial and aquatic metabolism and the fluxes of exchange between soil, water, and the atmosphere. 

Our research focuses on the Rio Valfredda, a pristine mountain stream network draining a 5.3 km² catchment in the Italian Alps, with the ultimate goal of linking carbon (C) cycling patterns with hydrologic traits. To that end, extensive data acquisition and field campaigns have been carried out since November 2023. Activities include the measurement of dissolved oxygen (DO) and C dioxide (CO₂), along with environmental ancillary variables such as photosynthetic active radiation, temperature, barometric pressure, pH, total alkalinity, dissolved inorganic C (DIC) and electrical conductivity, in different reaches. Water stable isotopes  (δ¹⁸O and δ²H) are also being monitored in several springs and tributaries of the stream network, at the catchment outlet, and in the precipitation at three different altitude rain gauges.

By comparing the isotopic signatures of water in precipitation and streamflow, we develop a modeling framework to reconstruct the spatial variation in water transit time distribution (TTD) across multiple Valfredda sub-catchments. Variations in TTD across catchment springs, tributaries, and the outlet reveal the spatial heterogeneity of hydrologic connectivity and act as indicators of lateral inputs to the stream. TTD results are thus compared  with the available environmental data collected within the Valfredda network to unravel sub-catchment transport dynamics and their effect on the C exchange fluxes in the critical zone. We focus specifically on the spatial variation of DIC, connecting its behavior to the proportion of young water mobilized within the system, proving the age of mobilized water serves as a proxy for the transfer of DIC from green to blue ecosystems.

We believe our approach marks a significant advancement in understanding freshwater solute transport and the coupling dynamics of water and C cycling at the catchment scale and can ultimately support resource management and pollution mitigation efforts, contributing to the long-term sustainability of aquatic ecosystems.