Enhancing alkalinity in Ria Formosa by deployment of alkaline substrates: variability in nutrients concentration and fluxes.

Reducing atmospheric CO2 levels is essential to mitigate global warming. Natural weathering of alkaline minerals removes CO2 over geological time scales. Supplying alkaline substrates to coastal environments has been proposed to accelerate this process. However, the efficiency and ecological impacts of such interventions remain insufficiently studied. Mineral dissolution rates depend on substrate type, grain size, temperature, pH, and salinity. The concomitant release of nutrients and trace elements during weathering may affect marine biota. During the first in-situ experiment of its kind in Europe, we investigated the effects of adding fine and coarse-grained olivine and basalt on alkalinity and nutrient dynamics in an undisturbed saltmarsh dominated by Sporobolus maritimus at Ria Formosa lagoon (southern Portugal), a productive coastal system. Initiated in September 2022, the experiment involved monthly measurements of temperature, salinity, oxygen, and pH, as well as total alkalinity and nutrient concentrations in overlying and porewaters. The overlying water was collected while the tide was falling, 15-30 min after emergence and before the substrates dried out. Porewater samples were obtained within few hours after emergence, using rhizon samplers. Nutrients were analyzed spectrophotometrically using the OSIL Marine Nutrient Kit. Results showed that sediments acted as nutrient sources in both control and treatment plots due to organic matter remineralization, bioturbation, and geochemical processes linked to mineral dissolution. Nutrient concentrations, particularly in porewater, increased by up to an order of magnitude compared to the overlying water and up to fourfold relative to the control during the first three months. Thereafter, they gradually decreased over time, except for nitrate and, to a lesser extent, silicate. Substrate and grain-size effects became evident after six months, with fine-grained olivine showing the highest silicate and nitrate levels. Seasonal variability, biological activity, and sediment heterogeneity were considered to exert a governing influence on the observed fluctuations. Overall, alkaline substrates enhanced alkalinity during the first six months, with minor long-term nutrient impacts, which likely were moderated by Ria Formosa's high water renewal rate.