Seasonal variability in carbonate chemistry and air-sea CO2 fluxes in the southern Great Barrier Reef

There is presently little known about temporal variability in CO2 and carbonate chemistry (pH and aragonite saturation state (Omega(arag))) in the Great Barrier Reef (GBR) region. In this study we investigated both the seasonal variability of the carbonate system and the air-sea CO2 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 CO2 (pCO(2)) was found to be the lowest (343 mu atm), rising by 61 mu atm to nearly 404 mu atm during summer. Much of the variance in pCO(2) and pH could be described by sea surface temperature (SST) and its thermodynamic effect on CO2. Despite the relatively large seasonal pCO(2) signal (similar to 60 mu atm), we found little seasonal variability in Omega(arag),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 Omega(arag). These results suggest that within southern GBR waters, future ocean acidification changes can be accurately predicted using various high-CO2 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 CO2, we found these waters to be up to 50 mu atm lower than the atmosphere for nine months of the year, implying an annual CO2 sink. Using the robust relationship between SST and pCO(2), we calculate the region to be a weak sink for CO2 (flux of -665 mmol C m(-2) y(-1)). If we extrapolate our results to the wider southern GBR south of 20 degrees S, it would imply a net CO2 sink of similar to 1 Tg C y(-1). (C) 2013 Elsevier B.V. All rights reserved.