VANCOUVER — 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 that 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.
His team also transplanted a host of healthy marine species to sites along a gradient of pH values leading up to an Italian seep, then monitored the immigrants’ health for a year.
Even shelled animals initially survived from fall to spring, in some cases bumping up their calcification in an attempt to cope with the corrosive waters. The surprise, Hall-Spencer notes: When peak temperatures arrived in August, many transplanted corals and mollusks died “due to the double whammy effect of high CO2 and high temperature.”
In lab experiments, Chris Langdon of the University of Miami Rosenstiel School of Marine and Atmospheric Science and colleagues raised coral larvae at pH values representing the South Pacific today and at levels expected in 50 and 100 years. Compared with newly spawned larvae at normal pH, the metabolism of those raised in the lowest pH environment dropped 65 percent, Langdon reported.
“You can think of this” — the lowered metabolism — “as a ball and chain attached to the leg of every coral larva,” Langdon says. “It’s not killing it outright, but each will have to go through life dragging this ball and chain behind.” Langdon also found that the larvae’s ability to make energy from nutrients in the water also suffered in the reduced pH. “So it’s like they’re starving at the same time,” he said.
Finally, there was a 60 percent decline in the number of larvae that could settle out onto a simulated reef surface, Langdon reported. One reason may have to do with the effects of acidification on turf algae in their environment. These algae made less of two key pigments. Ordinarily, the pigments “call out to the larvae, saying this is a nice place to settle,” he explains.
In Papua New Guinea, Langdon found evidence that the same thing appears to be happening in the wild at CO2 seeps with comparable pH values.
Other scientists reported at the meeting that at some sites, such as along the West Coast of the United States, seawater regionally — and regularly — falls to a pH of 7.5 or lower owing to natural factors other than CO2 seeps. Such new data may explain the occasional catastrophic wipeouts of young farmed shellfish in recent years, notes Gretchen Hofmann of the University of California, Santa Barbara.