Researchers say they have identified “extreme corals” for the first time in mangrove lagoons around Australia’s Great Barrier Reef—a discovery that has implications for reef conservation and challenges our understanding of the conditions corals can tolerate.
A team of scientists, led by Emma Camp from the University of Technology Sydney in Australia, found 34 species of coral that were regularly exposed to high levels of environmental stress—including extremely low pH, low oxygen levels and large swings in temperature—in two mangrove lagoons, known as the Woody Isles and Howick Island.
The team says the corals in these two “hot spots” of resilience were surviving unexpectedly—although coral cover was low and patchy. That’s according to a study published in the journal Marine Ecology Progress Series.
“Through our research we have identified that mangrove lagoons often have extreme environmental conditions—specifically warmer, more acidic and lower oxygen seawater conditions than adjacent coral reefs,” Camp told Newsweek. “The mangrove lagoon conditions are comparable to, or even exceed what is predicted for, the open-ocean under climate change in the year 2100.”
“The presence of corals in these extreme mangrove habitats challenges our understanding of the conditions some corals are capable of living in,” she said. “The extreme mangrove lagoons act as a natural laboratory to allow us to study the mechanisms that support coral resilience to sub-optimal environmental conditions—predicted to become the norm for coral reefs in the future. We can also assess the trade-offs associated with living in harsh environmental conditions. For example, we found that the corals calcify less, meaning they are building less of their skeleton.”
Coral reefs around the world are facing significant threats, in most part, because of human impacts on the planet. These threats consist of localized stressors, such as pollution, resource over-exploitation, and unsustainable fishery practices, as well as climate stressors including ocean warming, reduced levels of oxygen in seawater and ocean acidification.
For example, events linked to global warming have damaged the Great Barrier Reef to such an extent that the ability of its corals to recover from shocks has been severely disrupted.
According to a study published in the journal Nature, the replenishment of new coral fell by a staggering 89 percent following two severe mass bleaching events in 2016 and 2017, which were caused by significant spikes in water temperature.
The two bleaching events were the worst in recorded history, leading to a catastrophic die-off in many regions of the 3,800 individual reefs that comprise the world’s largest reef system.
Bleaching happens when corals are placed under stressful conditions—such as high temperatures—to which they respond by expelling tiny plant-like organisms called zooxanthellae that live inside their tissue. This causes them to turn white.
This is not immediately fatal to the corals, because they can reabsorb the zooxanthellae. But if the stressful conditions persist for too long, they die.
In the past 20 years, the Great Barrier Reef has experienced four mass bleaching events (1998, 2002, 2016 and 2017). Scientists predict that these could occur twice a decade from 2035 and every year after 2044, under projections based on current greenhouse gas emission trends. This would mean the gap between each shock would shorten, giving the coral less and less time to recover.
Camp says that the latest findings could have significant implications on our attempts to save stricken coral reefs.
“The mangrove lagoons hold a potential stock of stress-hardened corals that could be considered for reef intervention activities, such as local-scale coral transplantation to degraded reef areas,” she said. “Further work is required to assess how this could be scaled-up.”
The importance of coral reefs to the world’s oceans is hard to overstate. While they cover less than 1 percent of the Earth’s surface, they are home to around 25 percent of known marine life and host the highest biodiversity of any ecosystem globally. Furthermore, they also play a hugely influential role in the world’s economy.
“It has been estimated that 500 million people directly rely on coral reefs for food, resources and livelihoods and they have an estimated economic impact of $375 billion dollars a year,” Camp said. “Important ecosystem services of coral reefs include, but are not limited to: acting as natural barriers providing coastal protection, supporting fisheries, habitat for many marine species, important source of pharmaceutical compounds, cultural significance and value, part of nutrient cycling in the marine environment, and important for tourism.”