In the quest to learn what the oceans will look like if we don’t cut back on carbon dioxide emissions, scientists have turned to volcanic vents, which pump carbon dioxide into the waters around them. The news is bad, according to two papers published in the same edition of Nature Climate Change. Few species adapt to the relatively acidic conditions the CO₂ produces.

In addition to the deep sea hydrothermal vents, around which life is thought to have evolved, shallow water carbon dioxide seeps exist. Some may be a product of climate change (as well as a cause), but a few longstanding natural vents have been found in waters less than 10 meters (32 feet) deep.

Dr. Ivan Nagelkerken of Adelaide University and first author of one of the papers told IFLScience, “Now that ocean acidification is being recognized as so important, people are looking for these vents because they are natural laboratories.” Research under artificial conditions not only fails to capture the complexity of real-world environments with multiple interacting species, but it also doesn’t usually give time for species to adapt or evolve.

“Using these CO₂ seeps, we’ve been able to get a unique preview of what the future ocean will look like under current projections for the end of the century – and it’s not good,” Nagelkerken said.

Even when the vents are ancient, most fish species continue to avoid the carbon dioxide-rich waters. Generalist species appear more adaptable to the acidic environment, with gobies and triplefin fishes doubling or tripling in number thanks to reduced competition and an absence of predators. However, Nagelkerken told IFLScience that the total biomass, as well as biodiversity, is greatly reduced.

Laboratory fish exposed to acidic conditions display increased risky behavior, no longer avoiding predators and sometimes even swimming towards them. Nagelkerken says this was seen around the vents, but only when fish were in open habitat. “If they were near a hiding place they would hide, just like fish at control sites.”

Nagelkerken acknowledged that the acidified area around the vents is too small to reveal much about the effect on larger species such as sharks that operate over wider ranges, but the differences at this small scale are stark.

Co-author Professor Sean Connell said, “One minute you’re in a kelp forest with one metre high kelp and lots of different fish. Then you move into the vent area where everything is barren with short turf algae, just a few centimetres high and devoid of the life and colour of the other areas.”

^{What the kelp beds look like further from the volcanic vent in waters with less carbon dioxide. Credit: Ivan Nagelkerken}

A second paper explores the effects of vents releasing carbon dioxide near coral reef systems in the Northern Mariana Islands. Unsurprisingly, given the known effects of low pH on calcium carbonate formation, the study observed results that were even more devastating than for the kelp habitats Nagelkerken investigated.

“We show that acidification-related stress significantly influences the abundance and diversity of coral reef taxa, leading to the often-predicted shift from a coral to an algae-dominated state,” the paper reports. “This study provides field evidence that acidification can lead to macroalgae dominance on reefs.”

"While we’ve done lab simulations of how increased carbon dioxide influences coral growth, this is the first field evidence that increasing ocean acidification results in such a dramatic ecosystem change from coral to algae,” said first author Dr. Ian Enochs, a scientist with NOAA.

Enochs noted, “A shift from coral to algae-covered rocks is typically accompanied by a loss of species diversity and the benefits that reefs provide.”