Comparison of CO2 Dynamics and Air-Sea Exchange in Contrasting Tropical Reef Environments

Multiyear high temporal resolution CO₂ records in three differing coral reef settings were obtained using buoys deployed in coastal waters of Oahu since June 2008. The buoys are located on the barrier reef of Kaneohe Bay and offshore of Honolulu, on the south shore of Oahu. Annualized CO₂ air-sea fluxes at the three buoys ranged from +0.05 mol C/m²/yr offshore Honolulu on a fringing reef well mixed with the open ocean to -1.12 mol C/m²/yr on a barrier reef flat in Kaneohe Bay (positive values represent CO₂ sinks from the atmosphere and negative values represent sources). These fluxes compare well to those estimated from previous studies in Kaneohe Bay as well as in other tropical reef environments. pCO₂ measurements, made every 3 hours, at each location show strong temporal cycles on multiple time scales ranging from diel to seasonal at each buoy and an anticorrelation with pO₂. These records, when combined with those of a prior buoy deployment in southern Kaneohe Bay and several synoptic studies, allow us to examine how the principal biological cycles of productivity/respiration and calcification/carbonate dissolution are influenced by changing water column properties, physical processes (e.g. residence time) and atmospheric conditions and how these processes ultimately impact the exchange of CO₂ between the ocean and atmosphere on hourly to interannual cycles. The data clearly demonstrate the need for high frequency pCO₂ data to characterize completely and accurately short-term local changes in the CO₂-carbonic acid system parameters and how these changes overprint the longer scale process of ocean acidification as a result of invasion of CO₂ into the ocean due to emissions of anthropogenic CO₂ to the atmosphere. Since many coral reef ecosystems are still sources of CO₂ to the atmosphere because of positive net ecosystem calcification, and in some instances net heterotrophy, such data are even more critical in terms of assessing future changes in the direction and magnitude of the CO₂ exchange flux in these systems.