The increasing acidity of the oceans--a consequence of humans' addition of carbon dioxide to the atmosphere--is bad news for many kinds of organisms, particularly those with external skeletons that dissolve (ScienceNOW, 28 September 2005). But for the most part, researchers have studied the possible impacts in the laboratory. Now, a field study heightens scientists' concerns by showing that acidification has widespread effects on an ecosystem. "The whole ecology of the area is radically altered," says lead researcher Jason Hall-Spencer, a benthic ecologist at the University of Plymouth in the U.K.
Hall-Spencer and colleagues studied marine organisms living near a small island off southwestern Italy. The island, called Ischia, is flanked by volcanic vents that release 2 million liters of carbon dioxide per day. The gas raises the acidity of some of the waters to levels predicted in the world's oceans by the end of the century. "It's like swimming through champagne and seeing the future of the ocean," Hall-Spencer says.
When Hall-Spencer first surveyed marine biodiversity off Ischia, he was immediately struck by the absence, in regions of high acidity, of the calcareous algae that usually grows on sea grass. Subsequent research showed that calcareous algae covered more than 60% of the sea floor in areas with normal pH (8.1 to 8.2), but they were absent where the pH was lower than 7.9. To make sure the difference was due to pH, the researchers transplanted calcareous algae to the more acidic waters and found that they completely dissolved. The loss of this algae could be a harbinger of greater devastation: "It's the glue that holds coral reefs together," Hall-Spencer says.
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Other animals with external skeletons also fared poorly. Corals and echinoderms, such as sea urchins, were absent from the more acidic waters. And without the urchins eating them, invasive algae, such as the toxic, fast-spreading Caulerpa, thrived, the team reports online this week in Nature. In other places in the Mediterranean Sea, Caulerpa has crowded out native species, including fish and mollusks. All told, there were 30% fewer species in the acidic waters.
In some ways, Ischia may offer an overly optimistic view of the future. For example, the pH can vary considerably according to the weather; crashing waves temporarily bring the levels back to normal. These reversals make life somewhat easier for some species, such as barnacles, which can close their shells during the periods of higher acidity. And some of the animals near the vents, such as fish and gastropods, may be coming in to feed from nearby water of normal pH. In the future, that might not happen, because the entire ocean will become more acidic.
The impact of acidity at Ischia isn't surprising, other ecologists say, as it matches predictions based on lab experiments. But because it's a large field study, the research might help prompt policymakers to curtail carbon dioxide emissions, says John Guinotte, a marine biogeographer at the Marine Conservation Biology Institute in Bellevue, Washington. "These are not the kind of ecosystem changes we want to see," he says. "If we lose coral and gain algae, that's not a good trade. We are talking about a net loss both biologically and economically."