Disentangling influences on coral health

About one-third of the carbon dioxide spewed into our atmosphere since the start of the Industrial Revolution has been absorbed by the oceans. That absorption has made the oceans more acidic and deprived them of some 20% of their carbonate-ion concentration. Calcifying organisms such as coral, which rely on those ions to build their skeletons, are heavily affected. But despite decades of studies, attributing those effects to acidification remains difficult. Century-long records of the animals’ calcification rates do not show a consistent decline as ocean pH dropped in the 20th century. In some regions, the rates have even increased. That’s because ocean warming, among other factors, also affects coral growth. In particular, skeletal density of coral can increase, even in a lower-pH environment, because calcium carbonate precipitates faster at high temperature.

In a new study, Weifu Guo (Woods Hole Oceanographic Institution) and his colleagues have compiled records of existing and new skeletal growth of a stony, long-lived coral genus known as Porites to disentangle the competing effects. The records came from coral colonies collected from the Great Barrier Reef, the South China Sea, and the central Pacific Ocean and dated from 1871, when significant acidification began, to as recently as 2014. The researchers used the corals’ “extension” (upward growth), skeletal density, historic temperature, and seawater chemistry as inputs to a coral-growth model to isolate the effects of pH and temperature on the animals’ skeletal density in response to constant and varied environmental conditions.

From their simulations, the researchers subtracted model-predicted density changes induced by temperature variations from measured density changes. That allowed them to estimate the consequences of acidification alone. In the Great Barrier Reef, they found that the coral skeletal density induced by ocean acidification fluctuated by ±2% before 1950 and then dropped by 13% over the next 50 years. The decline in coral skeletal density in the South China Sea after 1950 was lower at 7%; and central Pacific reef coral showed no discernable density decline from acidification—likely because those reefs are bathed in open-ocean seawaters and experience only modest decreases in reef-water pH.

Coral’s resistance to mechanical stress drops exponentially as its density decreases. So a 13% drop in skeletal density is equivalent to a 60% drop in compressive strength, which makes the coral increasingly vulnerable to storm damage and bioerosion. That vulnerability, together with other environmental stressors, such as warming waters, sea-level rise, and pollution, will make it all the more difficult for reef ecosystems to survive the 21st century. (W. Guo et al., Geophys. Res. Lett., 2020, doi: 10.1029/2019GL086761 .)