Baby corals will have more trouble replenishing the world’s largest reef in anticipated hotter climates unless water-quality guidelines are adjusted to manage deadly human-generated sediment, researchers recently found.

Their study of the viability of inshore coral recruits, larvae that have settled on the ground to become a tiny baby coral that will slowly form a colony, is the first to quantify how realistic future climate conditions could alter their ability to withstand sediment deposition. It is slated to be published in the forthcoming May issue of Science of the Total Environment.

Its findings: Under conditions expected to prevail in 2100, recruit survival could be slashed in half.

These numbers are striking because coral recruitment is critically important for sustaining the animals’ populations, which the study noted are under pressure from ocean acidification, rising water temperatures and mass bleaching events that could become more challenging as the climate changes. Heat stress drove an 89% reduction in larval recruitment in 2018 compared to historical levels, according to a 2019 study from Nature.

To test the resilience of Acropora millepora recruits, the researchers behind the new study replicated the 14-week period after corals attach to a reef using the Australian Institute of Marine Science’s advanced National Sea Simulator facilities. 

Focusing on this branching coral found in shallow habitats along the Great Barrier Reef, researchers raised the recruits under scenarios representing the current as well as “realistic ‘medium’ and ‘high’ climate scenarios” that also incorporated ocean warming and acidification. At either five and 10 weeks after settling or at only 10 weeks after, the recruits were exposed to different sediment loads representing typical river runoff and dredging operations near those habitats.

“Recruits raised under future climate scenarios were more vulnerable to sediment deposition stress,” the study concluded. “The lethal sediment deposition loads (LC50) showed consistent decreases by about 50% under high future climate conditions for both five- and ten-week-old recruits, and after either one or two deposition events.”

The highest climate scenario is comparable with the “business-as-usual” path along which the world is currently tracking.

But the researchers were surprised to find that a coral’s resilience appeared to depend largely on its age. While corals under 10 weeks old suffered high mortality rates under the largest realistic sediment load in current and future climate scenarios, older ones were better able to survive elevated deposition levels, thanks to their longer tentacles that could push the material away.

“They clearly need that time to grow, to become bigger,” said first author Christopher Brunner, a Ph.D. candidate at AIMS@JCU, a partnership between AIMS and James Cook University, and the Australian Research Council Centre of Excellence for Coral Reef Studies.

The only recruits to survive the highest sediment load were grown under current climate conditions and didn’t face exposure until 10 weeks, “highlighting the influence of both climate and recruit age,” the study said.

The team is sending the information to Australian government officials, who could incorporate the findings in their planning.

“We want to support the management now and in the future of the Great Barrier Reef,” Brunner told The Academic Times. “We hope to highlight with our study that it’s necessary to account for the changes in the future.”

Water-quality guidelines are important to preservation in the face of economic development along Australia’s eastern and western coasts, the authors say, as some of the more than 2,000 individual reefs comprising the Great Barrier Reef sit in “relatively close proximity” to shipping facilities and coastal development.

As harbors are built and maintained, they say, associated dredging that results in the release of sediments could be timed to avoid over-exposing baby corals to sedimentation. The northeast coast of the continent in particular experiences a large amount of dredging activity because of oil and other natural resources found there.

The role of age in coral survival is particularly important, Brunner said. As a result of previous research, Australia has already established some regulations to minimize the amount of sediment that reaches the reefs around the time corals are known to spawn. 

The study suggests policymakers should regulate the timing of activities such as dredging that increase sediment levels in reef waters “to prevent periods of high sediment levels coinciding with recruitment.”

Land use poses another set of problems, according to Brunner. Agriculture operations are found around most of the continent’s northeastern coast along the reef, and during the wet season of December through April, sediment runoff can inundate rivers. However, Australia has strongly supported best farming land-management practices to protect river beds and minimize runoff into the reef lagoon, he said.

“You always have the interests of economy and protection,” Brunner continued, “and finding that balance is really difficult.”

While at least half of corals vanished from the Great Barrier Reef in the last few decades, with disease outbreaks and habitat degradation also contributing, restoration efforts such as outplanting are on the rise, according to scientists.

In a subsequent stage of their project, the AIMS team completed a similar study taking the climate scenarios but incorporating light limitations to simulate the effect of turbidity, which can have a more subtle effect than straight deposition. Sediment coming into a reef area can reduce light availability for up to a month as it slowly settles and smothers everything on the ground.

“It’s clearly a quite big picture where you have to account for a lot of different pressures which are pressing on these coral recruits,” Brunner said. “It is not just the sediment and not just the climate; there are many different aspects to that story.”

The study “Climate change doubles sedimentation-induced coral recruit mortality,” published in the May 10, 2021 issue of Science of the Total Environment, was authored by Christopher A. Brunner, James Cook University, Australian Research Council Centre of Excellence for Coral Reef Studies, Australian Institute of Marine Science and AIMS@JCU; Sven Uthicke, Australian Institute of Marine Science and AIMS@JCU; Gerard F. Ricardo, Australian Institute of Marine Science; Mia O. Hoogenboom, James Cook University and Australian Research Council Centre of Excellence for Coral Reef Studies; and Andrew P. Negri, Australian Institute of Marine Science and AIMS@JCU.