Comments on: “Underwater measurements of carbon dioxide evolution in marine plant communities: A new method” by J. Silva and R. Santos [Estuarine, Coastal and Shelf Science 78(2008) 827–830]

Silva et al. propose a new method for quantifying benthic net community production (NCP) of tidal flats under submerged condition, based on the monitoring of water pCO2 in a transparent benthic chamber around high tide. I demonstrate here with theoretical considerations that this method is inappropriate for coastal environments, because it allows only the quantification of the change in the dissolved CO2 which, at classical seawater pH, is only ∼10% of the change of the dissolved inorganic carbon (DIC). Total Alkalinity and/or DIC must be measured at the beginning and end of incubations in order to compute NCP in coastal environments. However, I also demonstrate that when pH is below 7, more than 95% of the DIC change occurs in the CO2 pool. The method proposed by Silva et al. is thus valuable for freshwater environments with acidic, low alkalinity waters, where monitoring the water pCO2 in a vial or chamber provides alone a very close approximation of the planktonic or benthic net community production.