If carbon dioxide emissions don't begin to decline soon, the complex fabric of marine ecosystems will begin fraying and eventually unravel completely, two new studies conclude.
The diversity of ocean species thins and any survivors' health declines as the pH of ocean water falls in response to rising carbon dioxide levels, scientists from England and Florida reported February 18 at the annual meeting of the American Association for the Advancement of Science. What's more, affected species aren't restricted to those with shells and calcified support structures features particularly vulnerable to erosion by corrosive seawater.
Jason Hall-Spencer of the University of Plymouth, England, and his colleagues have been collecting data from marine sites off Italy, Baja California and Papua New Guinea, where high concentrations of carbon dioxide percolate out of the seabed from volcanic activity below. Directly above these CO2 seeps, pH plummets to at least 7.8, a value that is expected to occur widely by 2100 and which is substantially lower than the normal level for the area, 8.1. These sites offer a preview of what may happen to seafloor ecosystems as CO2 levels continue to rise, causing ocean water pH to drop.
Compared with nearby normal-pH sites, species richness in low-pH zones was diminished by 30 percent, Hall-Spencer reported. “Coral and some algae are gone. And the sea urchins are gone,” he said. Fish may be present, but unlike in areas with a normal pH, they won't deposit their eggs there.
Although seagrasses appear to survive just fine in the low-pH seawater, close inspection showed that fish had nibbled the fronds, Hall-Spencer found. He identified one likely explanation: At low pH, these grasses no longer produced the phenolic defense compounds that typically deter munching by grazing animals.
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