A more acidic ocean under climate change threatens to reconfigure entire ecosystems by advantaging some fish species to the detriment of others, a new study has found. The research is one of only a few that go beyond the lab to study how species interactions are changing in nature under more extreme conditions.
Researchers from the University of Adelaide and the University of Hong Kong showed that a higher concentration of carbon dioxide in the oceans, which reacts to turn seawater more acidic, favors common fish species, allowing them to double their populations. But that might also mean the downfall of rarer, subordinate competitors, leading to biodiversity loss and a total restructuring of fish communities, with numerous ecological impacts.
“For 10 to 20 years now, scientists have conducted lab experiments to understand how changes in seawater chemistry will affect individual marine organisms. These have been really useful, but they are inaccurate representations of nature,” says David Koweek, a postdoctoral research scientist at the Carnegie Institution for Science, who was not involved in the research. “We have very little understanding about how ocean acidification causes shifts in organisms’ interactions and how these scale to create different outcomes for ecosystems and the resources people rely upon.”
The study, published in Current Biology Journal, looked at how ocean acidification might influence species interaction and how that might determine who thrives and who doesn’t. As species sharing the same waters interact, the disappearance of some could reverberate far along the food chain and destabilize ecosystems.
Oceans absorb about 30 percent of CO2 from the atmosphere. But the oceans’ chemistry changes as the level of CO2 in the air increases, with seawater dropping in pH and turning more acidic.
Understanding how ocean acidification affects biodiversity is important, as a rich variety of species, each with their specific roles in the environment, keeps ecosystems functioning and helps buffer against environmental changes.
“If there are more species, the network of their interactions is stronger and more resources are being used, making the community more robust to disturbances such as climate change,” said Ivan Nagelkerken, a professor of marine ecology at the University of Adelaide and lead author of the study.
Nagelkerken and his team spent three years analyzing fish communities around natural CO2 vents in shallow temperate reefs. Venting of carbon dioxide in the ocean happens naturally along faults and fractures in geologically active areas, usually volcanic ones. These vents release bubbles of CO2 that dissolve in the water in amounts similar to predicted CO2 concentration levels for the year 2100 (around 1,000 parts per million, according to some models), Nagelkerken said.
Researchers conducted the study on Whakaari (White Island), a volcanic outcrop in the Bay of Plenty off New Zealand’s North Island, where they compared two vents to two control sites.
The study found that the number of species was lower around CO2 vents than in areas less rich in carbon (nine versus 14 species). Perhaps more importantly, the density of predator species – which play a key role in maintaining population levels among more common fish species – was 50 percent lower near CO2 vents, while the total density of fish was 1.5 times higher.
Community structures were significantly altered, as populations of dominant species such as the common triplefin (Forsterygion lapillum) and the crested blenny (Parablennius laticlavius) doubled in density, while those of subordinate species, such as the Yaldwin’s triplefin (Notoclinops yaldwyni) and blue-eyed triplefin (Notoclinops segmentatus), almost halved.
In carbon-richer waters, dominant species also benefited from a higher abundance of invertebrates to feed on. Previous studies have shown that a drop in seawater pH affects fish neurological functions and triggers unusual behaviors; in this case, ocean acidification appeared to reward riskier behavior. Combined with a plummeting population of predators and more available resources, such an environment gives common species the upper hand.
Reducing overfishing of predators is one of the keys to preventing biodiversity loss in increasingly acidic waters, according to the study’s authors. Healthy predator populations would help control common species and give their rarer competitors better survival odds, even as marine ecosystems may become further destabilized as ocean acidification worsens.
While CO2 vents don’t provide a perfect picture of future acidic oceans, they allow scientists to study real-life conditions that are otherwise difficult to replicate in the lab.
These results discount a scenario in which fish species would adapt to higher carbon concentrations to counter the effects of ocean acidification.
Another area of interest, according to Koweek, is looking at how ocean acidification and ocean warming may interact to change our oceans’ biodiversity and ecosystems.
Since the dawn of the Industrial Revolution some 150 years ago, surface waters have become 30 percent more acidic; but by 2100 they could be 150 percent more so if greenhouse gas emissions remain at current levels, according to the National Oceanic and Atmospheric Administration.
While research on ocean acidification has mostly looked at its effects on coral reefs, the impact is not limited to tropical waters.
“Colder waters are capable of absorbing more carbon dioxide,” Koweek explained. “So ocean acidification is generally thought to be progressing much faster in colder temperate and polar waters. The cold waters that reach the west coast of North America every spring and summer are thought to be some of the world’s most rapidly acidifying.”
So what should be done? The same measures that will slow climate change, such as phasing out fossil fuels for clean energy sources, will also combat ocean acidification.
“The only effective way to prevent ocean acidification from getting worse at a global scale is to stop using the air and sea as waste dumps for our carbon dioxide pollution,” said Ken Caldeira, a climate scientist at the Carnegie Institution for Science.
I. Nagelkerken et al (2017). Species interactions drive fish biodiversity loss in a high-CO2 world. Current Biology, 27, 2177-2184.
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