Measurement of community metabolism and significance in the coral reef CO2source-sink debate

Two methods are commonly used to measure the community metabolism (primary production, respiration, and calcification) of shallow-water marine communi ties and infer air-sea CO2 fluxes: the pH-total alkalinity and pH-O-2 techn iques. The underlying assumptions of each technique are examined to assess the recent claim that the most widely used technique in coral reefs (pH-tot al alkalinity), may have provided spurious results in the past because of h igh rates of nitrification and release of phosphoric acid in the water colu mn [Chisholm, J. R. M. & Barnes, D. J. (1998) Proc. Natl. Acad. Sci. USA 95 , 6566-6569], At least three lines of evidence suggest that this claim is n ot founded. First, the rate of nitrification required to explain the discre pancy between the two methods recently reported is not realistic as it is m uch higher than the rates measured in another reef system and greater than the highest rate measured in a marine environment. second, fluxes of ammoni um, nitrate, and phosphorus are not consistent with high rates of nitrifica tion and release of phosphoric acid. Third, the consistency of the metaboli c parameters obtained by using the two techniques is in good agreement in t wo sites recently investigated. The pH-total alkalinity technique therefore appears to be applicable in most coral reef systems. Consequently, the con clusion that most coral reef flats are sources of CO2 to the atmosphere doe s not need revision. Furthermore, we provide geochemical evidence that calc ification in coral reefs, as well as in other calcifying ecosystems, is a l ong-term source of CO2 for the atmosphere.