Progressive seawater acidification on the Great Barrier Reef continental shelf

Coral reefs are highly sensitive to ocean acidification due to rising atmospheric CO₂ concentrations. We present 10 years of data (2009-2019) on the long-term trends and sources of variation in the carbon chemistry from two fixed stations in the Australian Great Barrier Reef. Data from the subtropical mid-shelf GBRWIS comprised 3-h instrument records, and those from the tropical coastal NRSYON were monthly seawater samples. Both stations recorded significant variation in seawater CO₂ fugacity (fCO₂), attributable to seasonal, daytime, temperature and salinity fluctuations. Superimposed over this variation, fCO₂ progressively increased by > 2.0 ± 0.3 µatm year^-1 at both stations. Seawater temperature and salinity also increased throughout the decade, whereas seawater pH and the saturation state of aragonite declined. The decadal upward fCO₂ trend remained significant in temperature- and salinity-normalised data. Indeed, annual fCO₂ minima are now higher than estimated fCO₂ maxima in the early 1960s, with mean fCO₂ now ~ 28% higher than 60 years ago. Our data indicate that carbonate dissolution from the seafloor is currently unable to buffer the Great Barrier Reef against ocean acidification. This is of great concern for the thousands of coral reefs and other diverse marine ecosystems located in this vast continental shelf system.