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.

Surface ocean carbon dioxide variability in South Pacific boundary currents and Subantarctic waters.

To improve estimates of the long-term response of the marine carbon system to climate change a better understanding of the seasonal and interannual variability is needed. We use high-frequency multi-year data at three locations identified as climate change hotspots: two sites located close to South Pacific boundary currents and one in the Subantarctic Zone (SAZ). We investigate and identify the main drivers involved in the seasonal an interannual (2012-2016) variability of the carbon system. The seasonal variability at boundary current sites is temporally different and highly controlled by sea surface temperature. Advection processes also play a significant role on the monthly changes of the carbon system at the western boundary current site. The interannual variability at these sites most likely responds to long-term variability in oceanic circulation ultimately related to climatic indices such as the El Niño Southern Oscillation, the Pacific Decadal Oscillation and the Southern Annular Mode (SAM). In the SAZ, advection and entrainment processes drive most of the seasonality, augmented by the action of biological processes in spring. Given the relevance of advection and entrainment processes at SAZ, the interannual variability is most probably modulated by changes in the regional winds linked to the variability of the SAM.