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

• We measured seasonal carbonate chemistry variability at a site in southern GBR.
• Seasonal pH and pCO2 variability were temperature-driven.
• There was no seasonal variation in aragonite saturation state.
• We calculate the region to be a weak sink for CO2 (− 665 mmol C m− 2 y− 1).

There is presently little known about temporal variability in CO2 and carbonate chemistry (pH and aragonite saturation state (Ωarag)) 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 (pCO2) was found to be the lowest (343 μatm), rising by 61 μatm to nearly 404 μatm during summer. Much of the variance in pCO2 and pH could be described by sea surface temperature (SST) and its thermodynamic effect on CO2. Despite the relatively large seasonal pCO2 signal (~ 60 μatm), we found little seasonal variability in Ωarag, 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 Ωarag. 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 μatm lower than the atmosphere for nine months of the year, implying an annual CO2 sink. Using the robust relationship between SST and pCO2, 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°S, it would imply a net CO2 sink of ~ 1 Tg C y− 1.