Journal Article

Total alkalinity flux in coral reefs estimated from eddy covariance and sediment pore-water profiles

Kayanne, Hajime, Kuwae, Tomohiro, Tokoro, Tatsuki, Yamamoto, Shoji, Watanabe, Atsushi

2015

Overview

Ocean acidification decreases the pH of seawater and the seawater saturation state with respect to CaCO₃ minerals. In the event of ocean acidification, Mg-calcite is considered to be the first mineral to dissolve. Dissolution of Mg-calcite is more prevalent at depth in the sediment than at the sediment interface because of production of CO₂ resulting from microbial decomposition of organic matter. Rates of CaCO₃ dissolution can be estimated from total alkalinity (AT) fluxes calculated from concentration gradients and diffusion coefficients. We estimated AT flux in a sandy area of the Shiraho coral reef under natural hydrodynamic conditions using eddy covariance and sedimentary AT profiles. The calculated nighttime AT flux at the sediment–water interface was 0.4–2.6 mmol m−2 h−1. Analysis of the sedimentary profile at a depth of 0–20 mm indicated that respiration by organisms consumed oxygen and produced CO₂ during night and that photosynthesis enhanced O₂ concentrations during the day. However, dissolved oxygen was depleted at all times in sediments deeper than 20 mm. The pore-water aragonite saturation state (=Ωₐ) was constant at ∼ 3.0, which is equivalent to a value of 1.0 for the saturation state with respect to foraminifera (=Ωfora), as determined in a previous study. Both organic reactions (e.g., respiration) and inorganic Mg-calcite dissolution occur in the sediment, leading to a constant Ωfora value in the sediment. These data confirm the metastable equilibrium of pore water with respect to Mg-calcite from foraminifera, which is the most soluble phase in the sediment.

• Publisher: John Wiley and Sons, Inc