Diurnal changes in seawater carbonate chemistry speciation at increasing atmospheric carbon dioxide

Natural variability in seawater pH and associated carbonate chemistry parameters is in part driven by biological activities such as photosynthesis and respiration. The amplitude of these variations is expected to increase with increasing seawater carbon dioxide (C[O.sub.2]) concentrations in the future, because of simultaneously decreasing buffer capacity. Here, we address this experimentally during a diurnal cycle in a mesocosm C[O.sub.2] perturbation study. We show that for about the same amount of dissolved inorganic carbon (DIC) utilized in net community production diel variability in proton ([H.sup.+]) and C[O.sub.2] concentrations was almost three times higher at C[O.sub.2] levels of about 675 ± 65 in comparison with levels of 310 ± 30 µatm. With a simple model, adequately simulating our measurements, we visualize carbonate chemistry variability expected for different oceanic regions with relatively low or high net community production. Since enhanced diurnal variability in C[O.sub.2] and proton concentration may require stronger cellular regulation in phytoplankton to maintain respective gradients, the ability to adjust may differ between communities adapted to low in comparison with high natural variability.
Introduction There is a considerable natural variability in seawater carbonate chemistry speciation, namely carbon dioxide (C[O.sub.2]), bicarbonate (HC[O.sup.-.sub.3]), carbonate ion (C[O.sup.2-.sub.3]) and proton ([H.sup.+]) concentration, as well as pH and [...]