Progressive seawater acidification on the Great Barrier Reef continental shelf

Abstract

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.

Environmental memory gained from exposure to extreme pCO 2 variability promotes coral cellular acid–base homeostasis

“…Long-term studies show remarkable consistency in pCO 2 measurements recorded at the same location between years [2], suggesting pCO 2 variability measured within these identical reef habitats over the same time period may be similar across years.…”

“…In-field measurements (temperature, photosynthetically active radiation (PAR) and nutrients) were recorded concurrently with the manipulative experiment at the same locations where corals were collected (8 January to 18 March 2021), whereas pCO 2 was recorded over the same season, but in 2016 (8 January to 18 March 2016; electronic supplementary material, Methods). Long-term studies show remarkable consistency in pCO 2 measurements recorded at the same location between years [ 2 ], suggesting pCO 2 variability measured within these identical reef habitats over the same time period may be similar across years.…”

“…Some studies have shown neutral or positive effects of pH variability on corals and other calcifiers, whereas other studies have shown negative effects, with observed differences probably a result of the magnitude of pH amplitude, the duration of the experiment and/or the response variables examined in addition to regional- and species-specific responses [ 6 , 9 , 10 , 16 , 17 ]. Further, most experiments did not characterize the long-term pH or pCO 2 conditions from which the study specimens were collected, which can fluctuate dramatically on diurnal, seasonal and spatial scales [ 2 , 6 , 18 ]. As a result, the question remains on whether lifelong exposure to pCO 2 variability promotes increased tolerance to acidification stress.…”

“…The increasing frequency and intensity of climate-driven marine heatwaves has provided powerful visual evidence of our changing climate, manifested by mass coral bleaching events and subsequent coral mortality [1]. The impacts of ocean acidification on reef-building corals and other reef fauna and flora are not as visually striking, though they are occurring in response to progressive increases in seawater pCO 2 and declines in pH observed across coastal reef systems [2,3]. It is difficult to disentangle multiple co-occurring stressors and individually quantify acidification-specific effects, which often require microscopic or physico-chemical techniques to accurately examine [4][5][6].…”

A tale of two reef systems: Local conditions, disturbances, coral life histories, and the climate catastrophe

“…The spread of severe heat stress (Eakin et al, 2019; Skirving et al, 2019) is impacting all coral reefs and leaving too little time for recovery (Thompson & Dolman, 2010; Osborne et al, 2017; Hughes et al, 2018). Climate change is also expected to bring more severe cyclones (Kossin et al, 2020), which are the most frequent disturbance to reefs in northwest Australia, disease outbreaks (Heron et al, 2010; Maynard et al, 2011), as well as sea level rise, and ocean acidification (Anthony, 2016; Fabricius et al, 2020). The long‐term consequences of these cumulative pressures are perhaps greater for isolated reefs, such as those at SR and RS, because of their reliance on survivors to repopulate communities and to provide the genetic diversity underlying future adaptation (Gilmour et al, 2013; Thomas et al, 2020; Underwood et al, 2009).…”