Hydrological and landscape controls on dissolved organic matter dynamics in European wetlands

Dissolved organic matter (DOM) is a key component in aquatic ecosystems, representing the main source of energy for microbial metabolism and playing a crucial role in C sequestration and export. Its optical properties (absorption and fluorescence) provide integrated information on its quality (average molecular weight and aromaticity degree, main sources, presence of protein like and humic-like substances). This paper provides an assessment of the impact of catchment characteristics, seasonality and ecological conditions (altered, restored, well-preserved) on DOM dynamics across six European coastal wetlands of contrasting ecological types. DOM concentration and optical properties show inter-wetland differences, with the highest concentrations of DOC in evaporation-driven Mediterranean systems and the lowest ones in Atlantic daily tidally-flushed wetlands. Water-isotope data and optical properties indicate that hydrological confinement, water-residence time and catchment inputs primarily control DOM accumulation and composition, while microbial and photochemical processing modulate its removal within sites. Wetland restoration effects on DOM are evident where hydrological modification reduced eutrophic stress. In evaporation-driven Mediterranean systems, the recovery of more natural flooding regimes limits connectivity and promotes the accumulation of recalcitrant DOM, potentially contributing to long-term carbon sequestration. In tidally renewed systems, changes in DOM concentration and optical properties, potentially associated with restoration, are less evident, as the strong diel flux overcome possible changes. Because DOM biological lability is directly linked to carbon processing, these patterns suggest that future climate scenarios, characterized by enhanced evaporation, warming and hydrological connectivity, will amplify existing contrasts among wetland types and thereby influence carbon balances. This study provides the first continental-scale baseline of DOM dynamics in European coastal wetlands and highlights the value of optical properties as early-warning proxies to anticipate shifts in carbon cycling under ongoing climate and land-use change.