Hydrothermal CO2 venting, pH anomalies, and biogeochemical consequences in a shallow hydrothermal system (Calent Mound, Western Mediterranean)

Shallow marine hydrothermal systems with natural CO₂ emissions generate localized chemical gradients and perturbations in seawater carbonate chemistry, offering opportunities to investigate coastal biogeochemical processes under elevated CO₂ conditions. We present a multidisciplinary characterization of the Calent Mound hydrothermal field (Columbretes Islands, Western Mediterranean) based on two oceanographic surveys conducted in 2020 and 2021. Hydrographic measurements revealed pronounced pH anomalies, with reductions of up to 1.12 units relative to reference conditions localized above active venting areas. Water-column temperature and salinity anomalies were minimal, whereas subsurface sediments exhibited thermal anomalies up to +5.7 °C below the seafloor. CO₂ emission-frequency analysis revealed heterogeneous degassing patterns, from sporadic to continuous, producing an estimated flux of ~3.3 kt yr^-1 over an active area of 17,000 m². Dissolved inorganic nutrient concentrations in vent fluids were markedly enriched relative to open-water reference values, particularly for phosphate, nitrate + nitrite, and silicate. Sequencing of microbial mats revealed distinct prokaryotic and eukaryotic communities associated with hydrothermal influence, including sulfur-, iron-, and ammonia-oxidizing taxa. Interannual variability was evident, although several key microbial taxa were consistently detected across both surveys. These observations characterize the chemical processes governing natural CO₂ venting effects on local and regional biogeochemistry and highlight the influence of hydrothermal inputs on carbonate chemistry, nutrient dynamics, and microbial community structure in a shallow marine environment.