Ocean Acidification Has Impacted Coral Growth on the Great Barrier Reef

Abstract

Ocean acidification (OA) reduces the concentration of seawater carbonate ions that stony corals need to produce their calcium carbonate skeletons and is considered a significant threat to the functional integrity of coral reef ecosystems. However, detection and attribution of OA impact on corals in nature are confounded by concurrent environmental changes, including ocean warming. Here we use a numerical model to isolate the effects of OA and temperature and show that OA alone has caused 13 ± 3% decline in the skeletal density of massive Porites corals on the Great Barrier Reef since 1950. This OA-induced thinning of coral skeletons, also evident in Porites from the South China Sea but not in the central Pacific, reflects enhanced acidification of reef water relative to the surrounding open ocean. Our finding reinforces concerns that even corals that might survive multiple heatwaves are structurally weakened and increasingly vulnerable to the compounding effects of climate change Plain Language Summary Measurable anthropogenic-induced acidification of the oceans (OA) has occurred over the last four decades. But its impact on coral reef ecosystems, such as coral calcification, has yet to be unambiguously demonstrated. This problem with detection and attribution of OA impacts is due, in large part, to the fact that multiple co-varying environmental and biological factors influence coral growth at the same time, and our inability to deconvolve them. Here, we use a numerical model of coral growth to isolate the contributions of ocean acidification to long coral growth timeseries generated on multiple Indo-Pacific reefs over the 20th century. We show that ocean acidification has had a significant negative impact on skeletal growth of a keystone reef-building genus across the Great Barrier Reef and in the South China Sea, where the rate of reef acidification outpaces that of the surrounding open ocean. Conversely, the OA-induced thinning of coral skeletons observed on these reefs has not yet affected corals in the central Pacific, where the rates of reef acidification have been lower. Nevertheless, as ocean acidification accelerates over the next few decades, even these reefs will be affected, resulting in a measurable weakening of coral reef structures across the global tropics.

Porites Calcifying Fluid pH on Seasonal to Diurnal Scales

“…Besides pH cf , other calcifying fluid carbonate system parameters calculated from bulk derived δ 11 B and B/ Ca also exhibit a seawater temperature dependency, such as DIC cf (r 2 = 0.72, p < 0.01, n = 10, Figure 6c) and TA cf (r 2 = 0.63, p < 0.01, n = 10, Figure 6d). Similarly, sub-annual variations in skeletal density suggest some positive relation with temperature, with lower density values in autumn and winter when seawater temperatures are low, and higher density in spring and summer under higher seawater temperatures (Figures 3a and 3b), consistent with faster aragonite precipitation and thus skeletal densification at higher temperatures (Guo et al, 2020;Mollica et al, 2018). However, these sub-annual skeletal densities exhibited an apparent negative correlation with aragonite Ω cf (r 2 = 0.44, p = 0.04, n = 5, Figure 6e), different from the positive correlation between inter-annual skeletal density and Ω cf observed in Porites collected from several Pacific reefs in Mollica et al (2018).…”

“…(2018). This difference likely reflects the interplay of multiple factors influencing coral skeletal density (e.g., extension rates, temperature, and Ω cf ) on different time scales (Guo et al., 2020). Overall, results support that at our study site coral calcification is affected by physiological modification of the calcifying fluid carbonate chemistry and that these modifications primarily follow variations in ambient seawater temperatures.…”

Increasing coral calcification in Orbicella faveolata and Pseudodiploria strigosa at Flower Garden Banks, Gulf of Mexico

“…There was a significant decline in density, however, during the last seven years for P. strigosa that could not be attributed to river runoff or bleaching (Table 2). This decline in density within the past decade could be the start of a longer-term decline like that found in massive Porites on the GBR and in three different species of coral on the Florida Reef Tract, including the two studied herein (Helmle et al 2011;Rippe et al 2018;Guo et al 2020). Indeed, Hu et al (2018) showed that the rate of OA at FGB was higher than the rate in the open ocean from 2007 to 2017, which overlaps with the significant decline in skeletal density of P. strigosa (Table 2).…”

“…The impact of runoff from the Mississippi and Atchafalaya Rivers can effectively erase any OA signal because runoff impacts surface productivity, as well as directly modifies seawater chemistry, both of which introduce variability that makes a long-term OA signal difficult to detect. OA is expected to reduce coral calcification and/or cause a decline in skeletal density (Chan and Connolly 2013;Mollica et al 2018;Guo et al 2020). There were no declines in long-term calcification rates or skeletal density of the corals analyzed from FGB; calcification rates have increased in both coral species, whereas skeletal density did not exhibit any trends (Fig.…”

“…As Cooper et al (2012) discussed, this is contrary to the idea that ocean acidification (OA) impacts will occur first, and most severely at high latitudes due to lower aragonite saturation states. They suggested that the rate of change of the thermal environment appears to be the overall dominant driver of recent changes in coral growth rates, although newer evidence suggests that corals may be experiencing the impacts of OA as a decline in skeletal density (Mollica et al 2018;Guo et al 2020).…”