Scientists are moving closer to understanding the complexity of coral reefs and their response to different environmental impacts with two new studies that might also help initiatives to protect them.
Coral reefs are one of the most diverse ecosystems on the planet and support the livelihoods of more than 500 million people, but three quarters of them are threatened by human activities.
In the first study, a group from the Woods Hole Oceanographic Institute in the US has unravelled the effects of ocean acidification on coral skeletons, as reported in the journal Geophysical Research Letters.
“This is the first unambiguous detection and attribution of ocean acidification’s impact on coral growth,” says lead author Weifu Guo.
“Our study presents strong evidence that 20th century ocean acidification, exacerbated by reef biogeochemical processes, had measurable effects on the growth of a keystone reef building species.”
While it’s long been suspected that acidification, caused by the ocean’s increased absorption of carbon dioxide, weakens corals’ skeletal growth, it’s been difficult to disentangle this from other impacts of warming temperatures and stressors.
The world’s oceans absorb around a third of global carbon dioxide emissions, which has dropped their pH levels by an average 0.1 unit since the preindustrial era and led to a 20% decrease in the concentration of carbonate ions.
Corals rely on carbon carbonate to create their skeletons, so lower levels eat away at their strength, much like osteoporosis weakens human bones.
“The corals aren’t able to tell us what they’re feeling, but we can see it in their skeletons,” says co-author Anne Cohen. “The problem is that corals really need the strength they get from their density, because that’s what keeps reefs from breaking apart.”
Diving into archived skeletal data from 1871, 1901 and 1978 for Porites coral – a long-living, dome-shaped species across the Indo-Pacific – the team applied a numerical model to separate the effects of acidification and rising temperatures on their growth and density.
They found that acidification alone has reduced the corals’ skeletal density by 13% on Australia’s Great Barrier Reef and 7% in the South China Sea since around 1950.
But it had no impact on marine protected reefs in the Phoenix Islands and central Pacific that are not as exposed to pollution, overfishing and effluent from land, suggesting these factors intensify the impacts of acidification.
“This method really opens a new way to determine the impact of ocean acidification on reefs around the world,” says Guo. “Then we can focus on the reef systems where we can potentially mitigate the local impacts and protect the reef.”
To explore what causes some reefs to be more resilient than others, the second group of scientists created a video game-like simulation model that factors in coral reef diversity, which they’ve published in the journal eLife.
“Species diversity is very important for the health of ecosystems in general,” says first author Bruno Carturan from Canada’s University of British Columbia, “and it is now well understood that higher diversity is usually better.”
This creates greater complexity which makes it extremely challenging to predict how they will react to stressors, he adds, and it’s virtually impossible to conduct experiments with coral reefs that embrace the full scope of their diversity.
To get around this, the team built a “monster” of a model, says co-author Corey Bradshaw from Flinders University, Australia. “But when you’re trying to model a complex universe, sometimes you need this degree of complexity to return something ecologically realistic.”
The model draws on the wealth of data scientists have collected on the ecology of coral species, simulating the behaviour of individual colonies in all their complexity. It included information on how different coral species grow, reproduce, compete, die or survive waves, cyclones, thermal stresses and how they influence other processes like herbivory.
Then they used functional traits such as growth rates and fertility to link these factors to colonies of different species.
“For instance, species that have complex branching growth forms are better competitors because they can overtop other species,” says Carturan, “but they are much more fragile and can easily break because of a wave or cyclone.”
They also accounted for colony size, which influences their fertility, resilience to waves or cyclones and ability to compete.
This information was translated into programming language to represent virtually all species and associated processes as realistically as possible, which contrasts with the abstract simplicity of most simulation models, according to Carturan.
What are the key findings?
“Our model works!” he says. After extensive testing that varied species diversity and environmental conditions, they found it realistically simulates coral reef ecosystems and the dynamics of real communities, opening “a new horizon of possibilities”.
The intricacy of the model is particularly suited to testing which aspects of diversity are most important for healthy functioning of coral reef ecosystems, Carturan explains, which would help to manage them and inform active restoration processes.
This includes exploring how problems like bleaching, storm damage and invasive species like crown-of-thorns starfish might change reef ecology and impact species richness and community complexity, says Bradshaw.
Ultimately, they hope that with further development and input by other ecologists, it can predict the future of coral reefs in different climatic and management scenarios.
Natalie Parletta is a freelance science writer based in Adelaide and an adjunct senior research fellow with the University of South Australia.
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