In a new research, scientists have warned that as oceans become more acidic by the year 2100, coral ecosystems may start to vanish because of an invasion of opportunistic marine species like algae and mollusks, who are better able to survive in the new environment.
According to a report in New Scientist, an exploration of natural "bubble streams" of carbon dioxide (CO2) in shallow Mediterranean waters off the coast of Italy is the first to document the effects of ocean acidification in a real ocean setting.
It is well understood that oceans will become more acidic as CO2 concentrations rise is well understood. By the year 2100, ocean acidity is predicted to be 7.8 pH, compared to 8.2 pH in 1900.
But all the studies of how this will affect marine ecosystems have been carried out in laboratories, many involving organisms with shells being placed in low-pH seawater and watching them slowly dissolve.
The experiments give little indication of the degree to which this would happen in the open oceans, affected by currents and the population dynamics which regulate ecosystems.
Now, Jason Hall-Spencer and colleagues at the University of Plymouth, UK, have looked at just this process. They found a site off the island of Ischia in southern Italy where geologic CO2 naturally seeps through the seafloor.
The Ischia site offers an usual opportunity to study cool, acidified ecosystems that are not modified by the toxic effects of sulphur.
It also allowed the researchers to study how the Mediterranean ecosystem changed across a pH gradient. On its outer edges, the pH was a "normal" 8.2; in water immediately above the seep, it dropped to 7.4.
At a pH of 7.8, the team noticed a marked change.
Populations of coralline algae, which hold reefs together, suddenly crashed. Sea-urchins also disappeared.
Also, there was a transition from a coral ecosystem to one dominated by lush sea grasses. These, along with invasive algae originally from Asia, thrived in the acidic waters.
The researchers warn that as waters become more acidic, opportunistic invasive species that are better able to survive at low pH could move in.
Another marked effect of the acidified water was something that has been described in laboratory settings - the hard shells of animals such as limpets softened.
"They were paper-thin. You could push your thumb through them," said Hall-Spencer.
According to Hall-Spencer, the ecosystem changes he and his colleagues documented in Ischia are relevant to other ocean ecosystems because the types of organisms they studied - sea urchins, corals and sea-grasses - are found around the world.