Alkalinity and CO2 fluxes in a tropical seagrass meadow

Total alkalinity (TA) production in vegetated coastal systems is considered a putative sink for atmospheric CO₂, due to the increase in the seawater buffer capacity when TA is produced in excess of DIC. Much of the TA generated in these habitats is derived from the reduction of NO₃ and Fe, but in oligotrophic tropical waters dominated by carbonate sediments, these sources of TA may be minimal. To address this uncertainty, we measured a suite of sediment-water fluxes (SO₄, N₂, TA, DIC, DOC, etc) in a tropical and calcifying seagrass meadow, allowing us to identify the biogeochemical processes responsible for TA generation and consumption. We placed this information into the context of water-air CO₂ exchange, which was measured by atmospheric eddy covariance. Net N₂ fluxes indicated that denitrification was a negligible TA source in this oligotrophic seagrass meadow, which at times was net N₂-fixing. Instead, sediment-water TA fluxes were dominated by the balance between SO₂ reduction, H₂S oxidation, and carbonate dissolution/precipitation. Air-water CO₂ exchange was small and variable, reflecting the highly-buffered seawater chemistry and oligotrophic nature of this seagrass meadow.