Benthic Ecosystem Calcification Measured with Coupled pH and O₂ Aquatic Eddy Covariance

We present a method to quantify benthic ecosystem calcification from simultaneous pH (proton) and O₂ eddy covariance flux measurements. In benthic ecosystems, photosynthesis is a proton sink, while calcification is a proton source. Where calcification is the dominant nonmetabolic proton source, it can be isolated as the residual between the measured proton flux and the flux predicted from O₂-derived metabolism. We demonstrate this technique in Posidonia oceanica seagrass meadows near Ischia, Italy, where coralline algae epiphytes are the primary calcifiers. The method resolved a diurnal calcification signal consistent in magnitude with previous estimates for seagrass epiphytes. However, our pH measurements and proton fluxes also revealed widespread, diffusive CO₂ vent influence at both the vent-adjacent site and the control site (670 m away), demonstrating that control sites near natural CO₂ vents may not provide the stable baseline often assumed. Excluding the vent-affected data removed substantial portions of the data set, resulting in high uncertainty, while also illustrating the insights that high-speed multiparameter sensing provides. Our error analysis identifies accuracy in pH, alkalinity, and the ecosystem photosynthetic quotient as critical constraints on this and other pH-O₂ based calcification measurements, particularly in environments where calcification rates are small relative to metabolic fluxes.