| Summary: | There is presently little known about temporal variability in CO and carbonate chemistry (pH and aragonite saturation state (Ω)) in the Great Barrier Reef (GBR) region. In this study we investigated both the seasonal variability of the carbonate system and the air-sea CO fluxes in waters offshore of Lady Elliot Island, southern GBR, between the austral spring of 2009 and winter 2010. During winter, the partial pressure of CO (pCO) was found to be the lowest (343μatm), rising by 61μatm to nearly 404μatm during summer. Much of the variance in pCO and pH could be described by sea surface temperature (SST) and its thermodynamic effect on CO. Despite the relatively large seasonal pCO signal (~60 μatm), we found little seasonal variability in Ω, which maintained a level of 3.6 throughout the seasons. Seasonal changes in dissolved inorganic carbon (DIC) and total alkalinity (TA), were found to offset each other during the seasons, thereby resulting in little seasonal variability to Ω. These results suggest that within southern GBR waters, future ocean acidification changes can be accurately predicted using various high-CO future scenarios without the need to account for seasonal variability that has been found to modulate the timing or onset of future oceanic acidification elsewhere in the ocean. For CO, we found these waters to be up to 50μatm lower than the atmosphere for nine months of the year, implying an annual CO sink. Using the robust relationship between SST and pCO, we calculate the region to be a weak sink for CO (flux of -665mmolCmy). If we extrapolate our results to the wider southern GBR south of 20°S, it would imply a net CO sink of ~1TgCy.
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