Ocean Acidification

A new study of coral reefs in Papua New Guinea shows ocean acidification simplifies coral structure, making crucial habitat less appealing to certain fish species.

While much media attention has focused on heat stress-induced coral bleaching, this finding, by a University of Adelaide research team led by Professor Ivan Nagelkerken, adds nuance to concerns about how global warming affects coral reefs.

Ocean acidification is caused by an increase in the level of carbon dioxide in oceanwater, leading to a reduction in pH. This makes calcium carbonate less available in the ocean, which corals use to build and repair their skeleton.

Professor Nagelkerken and his team show that, while ocean acidification in some instances does not reduce overall coral cover on a reef, the structures are less branched and therefore less appealing as habitat to some fish species.

Researchers observed two reefs in Upa-Upasina, Papua New Guinea: one located next to a volcanic seep releasing a steady stream of carbon dioxide, causing natural acidification, and another located 500 metres away unaffected by the volcanic gases.

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“Ocean acidification has the potential to reshuffle ecological communities globally, lead to the loss of key habitats and biodiversity, reduce fisheries’ productivity, and have negative physiological impacts on many marine animals and plants,” says Professor Nagelkerken, from the University of Adelaide’s School of Biological Sciences.

“It might also lead to a reduction in populations of various fish species, which could create novel species community structures that might have lower biodiversity and not be as resilient as present-day communities. It could also clearly distinguish winner species from loser species. And if this ocean acidification affects fisheries species, some species that recreational and commercial fishers target might become less abundant.”

The acidification conditions observed in the research, which was published in the Journal of Animal Ecology, at the reef beside the volcanic seep are expected to occur in the ocean more broadly as the increasing level of human-caused carbon emissions in Earth’s atmosphere are absorbed by the ocean.

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Credit: NEOM/ Unsplash

A new study of coral reefs in Papua New Guinea shows ocean acidification simplifies coral structure, making crucial habitat less appealing to certain fish species.

While much media attention has focused on heat stress-induced coral bleaching, this finding, by a University of Adelaide research team led by Professor Ivan Nagelkerken, adds nuance to concerns about how global warming affects coral reefs.

Ocean acidification is caused by an increase in the level of carbon dioxide in oceanwater, leading to a reduction in pH. This makes calcium carbonate less available in the ocean, which corals use to build and repair their skeleton.

Lemon Damselfish and Dascyllus at a shallow coral reef. Credit: Placebo365/iStock.

A new study of coral reefs in Papua New Guinea shows ocean acidification simplifies coral structure, making crucial habitat less appealing to certain fish species.

While much media attention has focused on heat stress-induced coral bleaching, this finding, by a University of Adelaide research team led by Professor Ivan Nagelkerken, adds nuance to concerns about how global warming affects coral reefs.

Ocean acidification is caused by an increase in the level of carbon dioxide in oceanwater, leading to a reduction in pH. This makes calcium carbonate less available in the ocean, which corals use to build and repair their skeleton.

The French Polynesian island Moorea is the most beautiful isle in the world, some say. Its lagoons are surrounded by reefs dominated by Porites corals.  

These corals and other calcifying marine species are the world’s primary reef-builders. Therein lies the trouble. The seas in which they dwell are turning acidic, a result of rising atmospheric carbon dioxide (CO2). Marine life that depends on calcium carbonate struggles to form shells or, in the case of coral reefs, skeletons.   

Porites reefs, say scientists Peter Edmunds and Robert Carpenter of California State University at Northridge, are among the most sensitive of all corals. Edmunds, Carpenter and Steve Doo of the University of Hawaii published results in the journal Limnology and Oceanography revealing the consequences of ocean acidification. The reefs may not be able to grow and reproduce.

In the actual context of global changes, ocean warming and acidification are both closely related to increased atmospheric carbon dioxide concentration and are expected to deeply impact biological communities through generalized effects on the entire oceanic system. Indeed, environmental changes may challenge the growth and stability of various marine productions due to the emerging consequences associated with ocean warming and acidification. Among commercial marine species, sea urchins undergo a variety of physiological, transcriptomic and immunological changes as a result of the conditions imposed by ocean warming and acidification, raising questions about the future management of several populations.

In addition to their importance in terms of ecological services, sea urchins are widely exploited for commercial purposes, supporting a growing market of considerable value. Although recent studies emphasize the growing importance of echinoid farming within integrated multitrophic aquaculture (IMTA) systems, sea urchin fisheries account for more than 99.9 percent of total sold per year, with aquaculture providing the remainder.

A school of yellowmouth barracudas in Scandola Marine Reserve, Corsica, France on June 17, 2017

A new study has found that the planet’s oceans are experiencing a “triple threat” of oxygen loss, extreme heat and acidification.

The researchers discovered that, as global heating has worsened, increasing stress has been placed on marine species, with as much as 20 percent of the world’s oceans affected by these threats.

“The global ocean is becoming warmer, more acidic, and losing oxygen due to climate change. On top of this trend, sudden increases in temperature, or drops in pH or oxygen adversely affect marine organisms when they cannot quickly adapt to these extreme conditions,” the study said.

Paris, 3 Juin 2024 – With contributions from more than 100 scientists from nearly 30 countries, UNESCO’s State of the Ocean Report 2024, published with the support of Iceland, reveals alarming new data on threats facing the ocean. This comprehensive assessment provides an evidence-based review of challenges including ocean warming, rising sea levels, pollution, acidification, de-oxygenation, blue carbon and biodiversity loss.

Click here to download the report

“This UNESCO report shows that climate disruption is having an increasingly strong impact on the state of the ocean. Temperature, acidification, sea level: all the alarm bells are ringing. In addition to implementing the Paris Climate Agreement, we call on our Member States to invest in the restoration of marine forests and to better regulate marine protected areas which are important reservoirs of biodiversity,” said Audrey Azoulay, Director-General of UNESCO.

The rate of ocean warming has doubled in 20 years

While atmospheric temperatures tend to fluctuate, the ocean is steadily and constantly heating up. The State of the Ocean Report indicates that the ocean is now warming at twice the rate it was twenty years ago, with 2023 seeing one of the highest increases since the 1950s. While the Paris Agreements pledged to keep global warming below 2°C above pre-industrial levels, ocean temperatures have already increased by an average of 1.45°C, with clear hotspots above 2°C in the Mediterranean, Tropical Atlantic Ocean and Southern Oceans.

One dramatic consequence of this warming is an increase in sea levels across the globe. The ocean absorbs 90% of the excess heat released into the atmosphere, and as water heats up it expands. Warming ocean temperatures now account for 40% of the global rise in sea levels, and the rate of rising has doubled over the past 30 years totalling 9cm.

Computer models forecast a worrying future for the Ascension Island Marine Protected Area.

Above: Georgetown beach, situated on the west coast of Ascension Island 
The seas around the remote Ascension Island, situated in the equatorial Atlantic Ocean, have been protected from fishing and deep-sea mining since 2019 and it is the 8th largest marine protected area (MPA) in the ocean. Despite efforts to safeguard this marine area, computer models forecast a worrying future for the area if the rate of climate change continues, even in low emission scenarios. 

Read the paper Impacts of Climate Change on the Ascension Island Marine Protected Area and its ecosystem services >>Unsustainable fishing and climate change rank as some of the most widespread drivers of marine biodiversity loss worldwide, threatening both the health of our ocean and human well-being. Conservation efforts aimed at curbing these losses are often centre around the establishment of MPAs, which – when appropriately managed and enforced – have been proven to be highly effective in reducing and reversing the impact of fisheries. But beyond those benefits to marine ecosystems, MPAs also offer multiple socioeconomic benefits: increasing the abundance of local fishing stock (and biomass of fish) and improving social capital. 

Butterfly fish are an example of an herbivorous species often mapped in the lower trophic levels. Scientists have been observing how this trophic level might be the one most heavily affected by climate change and ocean warming (Image Source: “Butterfly fishes” by Consuelo Puchades, licensed under CC BY-NC-ND 2.0 DEED).

Everything gets energy from somewhere. Plants make their own energy from sunlight, but all other life on earth needs to eat to get energy. In the ocean, for example, tiny photosynthetic phytoplankton are food for krill, which are then food for whales. These all constitute different trophic levels. Trophic levels are essentially different tiers or levels within a food chain or food web. Think of them as steps in a ladder, each representing a stage of energy transfer within an ecosystem.

At the very bottom are the primary producers, which are usually plants or algae that convert sunlight into energy through photosynthesis. They’re the foundation of every ecosystem, transforming sunlight and nutrients into organic matter. Next up are the primary consumers, also known as herbivores, which eat the primary producers. They’re like the first level of consumers in the food chain, munching on plants to get their energy. Moving up, we have secondary consumers, which are typically carnivores or omnivores. They feed on the primary consumers, gaining energy from the plants indirectly through the herbivores they eat. Then there are tertiary consumers, which are predators that eat other predators. They’re often at the top of the food chain in many ecosystems, feasting on secondary consumers. It is important to note, though, that not every ecosystem will have organisms for every single trophic level described, but trophic levels are an important tool to better understand how much energy goes into maintaining an ecosystem.

Each trophic level represents a transfer of energy from one organism to another, and as you move up the levels, there’s typically less energy available because some is lost on each step. Trophic levels help us understand the flow of energy and nutrients within ecosystems and how different species depend on each other for survival. But our ecosystems are ever-changing. Global climate change and ocean acidification are real and tangible pressures that marine ecosystems are facing—and that’s why Dr. Nan Hu and their collaborators embarked in a meta-analysis to define how the effects of ocean acidification and warming would affect different marine trophic levels. My question to you would be, would you rather be predator or prey in a warming and acidic ocean?

Newly hatched Pacific cod larva. (Image credit: NOAA Fisheries/Emily Slesinger)

As the climate changes, the ocean is simultaneously warming and acidifying. Both have been shown to adversely affect the vulnerable early life stages of Pacific cod. But until now, the interplay of these two environmental stressors was unknown.

A new NOAA Fisheries study takes a look into the future by replicating predicted Alaska ocean conditions in the laboratory. The study is the first to look at the interactive effects of ocean warming and acidification on Pacific cod. Researchers evaluated the growth and survival of eggs and larvae at combinations of temperature and acidity representing current and future ocean environments. The findings can help improve predictions to ensure climate-ready Alaska fisheries and communities in the future.

A selection of historic and modern mussel shells from the American Museum of Natural History’s collection used in the study. Credit: Daniel Kim/ AMNH

Researchers at the American Museum of Natural History have found that over the last 120 years, the porosity—or small-scale holes—in mussel shells along the East Coast of the United States has increased, potentially due to warming waters. The study, which analyzed modern mussel shells in comparison to specimens in the Museum’s historic collection, was published in the journal PLoS ONE.

“Mussels are important on so many levels: They provide habitats on reefs, they filter water, they protect coasts during storms, and they are important commercially as well—I love mussels and I know many other people do, too,” said Leanne Melbourne, a Kathryn W. Davis postdoctoral fellow in the Museum’s Master of Arts in Teaching program and the lead author on the study. “Human-caused environmental changes are threatening the ability of mussels and other mollusks to form their shells, and we need to better understand what risks will come from this in the future.”

Imagine a vibrant underwater metropolis, teeming with life and color. This is the coral reef, a complex ecosystem that supports a quarter of all marine life.

But beneath the dazzling surface lies a hidden world, a symphony of microbial life that plays a vital role in the reef’s health. Recent studies published shed light on this unseen orchestra, revealing how these tiny organisms conduct the health of the reef.

Microbial Maestros: Keeping the Coral in Tune

(Photo : DAVID GRAY/AFP via Getty Images)

Corals themselves are a marvel of nature – a partnership between a polyp, a soft-bodied animal, and microscopic algae called zooxanthellae.

These algae provide the coral with energy through photosynthesis, but the relationship is delicate. When stressed by environmental changes like rising water temperatures, corals can expel the algae, leading to coral bleaching – a sickly white appearance that signifies the breakdown of the coral-algae partnership. This is where the microbial community steps in.

Researchers have discovered that a diverse and healthy microbiome, the collection of microbes living within the coral, is essential for coral resilience.

Hairtail landed in Fujian. The impact of climate change on China’s fisheries is alarming scientists, and research gaps need addressing. Professor Tian Yongjun says: “We’re still not certain where the spawning grounds of the ‘big four’ families of fish are,” referring to hairtail, large and small yellow croakers, and cuttlefish. (Image: Zhang Guojun / Alamy)

As climate change brings warmer, more acidic waters to China, its offshore fish stocks are coming under pressure.

A number of fish populations – including the large yellow croaker, sea bream and sandlance – are at risk, according to a study by researchers based in the US and China, and published in the Proceedings of the National Academy of Sciences (PNAS).

Ocean warming has been more pronounced in China’s offshore waters than almost anywhere else. Winter surface temperatures in the Bohai, Yellow and East China seas, off the country’s eastern seaboard, rose by nearly 2C from 1958 to 2014 – well above the global average. And with concentrations of atmospheric CO2 rising, more of the gas is absorbed into the ocean, resulting in acidification of surface water. The trend is particularly evident in the coastal waters of southern Jiangsu, the Yangtze estuary and Hangzhou Bay.

Redfin needlefish hiding under the sea surface near Curaçao. Study co-author Juliette Jacquemont / University of Washington Seattle

The impacts of climate change on marine life, from rising sea surface temperatures to ocean acidification, have long been studied, but new research is shedding light on the extent of these effects both currently and in the future.

Scientists developed a method that fully considers the consequences of warming oceans and acidification on fish and invertebrate animals, without canceling out certain other impacts, such as when one species begins eating more and another eats less.

“To gain a better understanding of the overall worldwide impact of climate change, marine biologists calculate its effects on all fish or all invertebrate species lumped together,” Katharina Alter, of the Royal Netherlands Institute for Sea Research (NIOZ) and lead author of the study, explained in a statement. “Yet, effects determined in different individual studies can cancel each other out: for example if invertebrate animals such as snails profit from a certain environmental change and other invertebrates, such as sea urchins, suffer from it, the overall effect for invertebrates is concluded to be zero, although both animal groups are affected.”

Large anthropogenic emissions of carbon dioxide don’t only cause global warming, but also acidification of the oceans. In the Baltic Sea pH is expected to decrease between 0.1 and 0.4 units during this century depending on future emissions. The effects of ocean acidification on organisms have already been visible and are expected to increase, but not for the reason previously thought.

Sam Dupont, University of Gothenburg. Photo: Lisa Bergqvist

The oceans absorb about 25 percent of the global carbon dioxide emissions. One effect of this is that pH in the water is decreasing – the oceans are being acidified.

“The global trend is currently a pH decrease of 0.02 units per decade”, explains Erik Gustafsson, oceanographer and researcher at Stockholm University Baltic Sea Centre.

Future acidification is directly related to the development of carbon dioxide emissions. If the goals of Paris agreement (to limit global warming to well below 2 degrees) are met, the pH decrease will level out and pH begin to increase again during this century. A more moderate mitigation of emissions would lead to a continuous decrease and in the worst-case scenario, where emissions continue to increase significantly, pH would decrease by up to 0.4 units until the end of this century.

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Complex situation in coastal seas

In coastal seas, like the Baltic Sea, however, the development of pH is much more complex and variable compared to in the open oceans.

“In coastal seas we have strong interactions between land, and sea and there are other processes that can counteract and enhance ocean acidification, such as properties of the freshwater input, changes in run-off and salinity and changes in production and respiration patterns over time”, says Erik Gustafsson.

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Future development in the Baltic Sea

Modelling of the future development in the Baltic Sea shows a mean pH decrease similar to the one in the open oceans – about 0.1 units 2100 with moderate CO2 mitigation, and 0.3-0.4 in the worst-case scenario. But the large seasonal variations also means that the exposure time for harmfully low pH levels increases. 

In a scenario where the Baltic Sea has recovered from eutrophication, the mean pH decrease is larger, but the seasonal variations are smaller, which altogether means less exposure time for harmful pH levels. 

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Ecological effects of acidification

Sam Dupont, senior lecturer at the Department of Biological and Environmental Sciences at University of Gothenburg, has been studying the consequences of ocean acidification for species and ecosystems for many years.

He highlights that extensive ocean acidification has occurred before. At the end of the Permian period (250 million years ago), there was a change in volcanic activity and a lot of carbon dioxide was released into the atmosphere, which led to climate change and ocean acidification.

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A new University of Alaska Fairbanks–led study has found that razor clams accelerate their shell development when exposed to more acidic ocean conditions, but that those young shells are built with more fragile substances.

How shellfish react to changing marine conditions is a growing concern as climate change gradually shifts ocean chemistry. The threat of ocean acidification is particularly significant in Alaska, in part because cold waters absorb more carbon dioxide, leading to a more rapid rise in acidity.

The study, published in the journal Frontiers of Marine Science, tracked newly hatched razor clam larvae through their first 28 days of shell development in various levels of acidic water.

One group of razor clams was raised in water that mirrors the current average pH levels in Alaska’s Cook Inlet, while another was raised in a more acidic environment that simulates conditions that are projected in the region in 2100.

A third experiment simulated variable conditions under the 2100 projections, which were meant to mimic the constantly shifting pH levels in lower Cook Inlet. Real-world levels of ocean acidity change on a 12-hour cycle driven by tidal activity in the region, and the study is the first to simulate how those variable conditions could affect a concretion-forming bivalve.