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Coral growth cannot keep pace with rising sea levels

A recent study suggests an uncertain future for coral reefs, as their growth struggles to match the pace of rising sea levels. 

The study, which investigated 9,000 years of coral growth, highlights a discernible decrease in coral growth rates. 

The research was focused on the analysis of 22 drill cores collected from the Belize barrier reef and atolls. The drill cores measured a total of 215 meters.

Dwindling accretion rates

The team used radioisotope dating to estimate the age of the coral fragments. This method allows scientists to infer the age of a material through the decay rates of radioactive samples.

Next, the researchers calculated the distances between the samples in the drill cores to estimate growth rates. 

“Our data show that coral accretion rates in Belize decreased during the Holocene. While at 3.36 millimeters per year, the average accretion rates of reef margins are in the same range as other regions in the western Atlantic, they are somewhat lower than those in the Indo-Pacific,” explained Professor Eberhard Gischler of Goethe University.

This discrepancy has significant implications for tropical island-nations that depend on or are protected by coral reefs and is particularly noteworthy in light of ongoing climate change discussions.

“The growth rates are at the lower end of the United Nations’ Intergovernmental Panel on Climate Change’s (IPCC) predictions of future sea-level rise until 2100,” said Gischler.

Shift in coral composition

The study corroborates the observed decline in live corals in the Caribbean. Today, many reefs in the region are not dominated by corals, but rather by fleshy algae and generalistic weedy taxa.

Older sections of the reef, as noted by Gischler and colleagues, predominantly feature stress-tolerant, reef-building corals. 

“At the base of our cores, directly overlying Pleistocene reef limestone, Pseudodiploria brain corals and Orbicella star corals are most common, illustrating that members of the stress-tolerant taxa are clearly dominating,” said Gischler. 

Centennial-scale growth gaps

The drill cores revealed intriguing centennial-scale gaps in the fossil records of the fast-growing Acropora palmata, or elkhorn coral. 

These gaps, occurring approximately 2,000, 4,000, and between 5,500-6,000 years ago, might be indicative of environmental shifts such as higher temperatures, increased storm activity, or altered nutrient supply.

The 4,000-year-old gap coincides with a potential mass mortality event affecting grazing echinoids in the region. 

The researchers propose that this could have led to increased abundance of fleshy algae or may be related to the 4.2 k-event, a significant climatic shift thought to have caused mid-latitude droughts and warmer sea surface temperatures.

Study implications 

“Studying the drill cores enables us to develop both detailed and systematic reconstructions of the environmental conditions that prevailed during the Holocene, based on which previous ecological and environmental conditions can be reconstructed, allowing us to determine whether the current coral and coral reef declines are in fact unprecedented,” explained Gischler. 

Coral reefs are already under threat due to global warming, ocean acidification, water quality deterioration, and diseases. 

These latest results suggest that tropical coral reefs may become one of the first victims of climate change as they fail to grow fast enough to keep up with rising sea levels.

The study is published in the journal Scientific Reports.

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