Acidic oceans of the future show extinction

A glimpse of future ocean chemistry finds that acidification transforms entire ecosystems.

Daily Climate staff report

A sea urchin off the coast of Italy. Researchers studying sea life near
acidic ocean vents found that increasing ocean acidification
can lead to wholesale ecosystem changes that mimic
extinction. Photo by Kristy Kroeker/University of California, Davis.

Ocean acidification may create an impact similar to extinction on marine ecosystems, according to a study published recently.

The study, exploring naturally acidic waters near volcanic vents in the Mediterranean Ocean off Italy, suggests that ocean acidification as a result of human emissions can degrade entire ecosystems – not just individual species, as past studies have shown.

The result, scientists say, is a homogenized marine community dominated by fewer plants and animals.

  

"The background, low-grade stress caused by ocean acidification can cause a whole shift in the ecosystem so that everything is dominated by the same plants, which tend to be turf algae," said lead author Kristy Kroeker, a postdoctoral researcher at the Bodega Marine Laboratory at the University of California, Davis.

The study was published in the journal Proceedings of the National Academy of Sciences.

Colorful patches

The oceans have absorbed roughly 30 percent of the carbon humans have pumped into the atmosphere by burning fossil fuels, buffering the globe from the harm posed by greenhouse gases. But it comes with a price: seawater has become more acidic as it absorbed all that carbon.

Today the ocean's pH is lower than anything seen in the historical record in the past 800,000 years, scientists say. As the acidity increases, organisms such as corals, oysters, snails and urchins have trouble pulling minerals from the seawater to create protective shells. The study released Monday buttresses ecologists' fears that such changes could ripple through entire ecosystems – and that ocean acidification could prove as consequential and catastrophic for the globe as any changes in air temperature associated with climate change.

Castello Aragonese d'Ischia in the Mediterranean Sea off
the coast of Italy

Most ecosystems have numerous, colorful patches of different plants and animals – algae, sponges, anemones, among others, Kroeker said in a statement. "With ocean acidification, you lose that patchiness.... Everything looks the same."

Kroeker and colleagues studied waters surrounding Castello Aragonese, a 14th century castle off the coast of Italy where volcanic vents naturally release bubbles of carbon dioxide gas. The vents create different levels of acidity on the reef. These gradients gave the scientists a glimpse of what a future marked by increasingly acidic ocean waters could look like – and how the creatures and plants living in those environments may react to a disturbance.

The researchers selected three reef zones: low, high and extremely high acidity, representing world ocean conditions for the present day, 2100 and 2500, respectively. Then they removed animals and vegetation from the rocks there. Every few months for three years, Kroeker dived to the study plots to photograph them and watch how plots in each zone recovered.

Variety through time

Coral reef in ambient carbon dioxide levels shows a
"patchy" ecosystem with a variety of species,
while coral in a highly acidic environment (right)
shows algae as the dominant species.
All photos by Kristy Kroeker/UC Davis

Kroeker found that acidic water reduced the number and variety of species. In the non-acidic plots, many different plants and animals, including turf algae, would colonize and grow. Sea urchins, snails and other so-called "calcareous species" would then eat them, allowing for variety through time.

But in both the high and extremely high acidic plots, urchins and other grazers either never reappeared or did not graze, allowing fleshy turf algae to steadily increase and ultimately overtake the zones. 

Calcareous grazers play key roles in maintaining the balance within marine ecosystems. They are also considered among the most vulnerable species to ocean acidification, previous studies have found.

"If the role of these grazers changes with ocean acidification, you might expect to see cascading effects of the whole ecosystem," Kroeker said. "If the pattern holds for other calcareous grazers, this has implications for other ecosystems, as well."