Ocean acidification increases the cost of calcification by reducing the
number of carbonate ions available to begin with. To extend the
construction metaphor, imagine trying to build a house while someone
keeps stealing your bricks. The more acidified the water, the greater the
energy that’s required to complete the necessary steps. At a certain point,
the water becomes positively corrosive and solid calcium carbonate
begins to dissolve. This is why the limpets that wander too close to the
vents at Castello Aragonese end up with holes in their shells.
Lab experiments have indicated that calcifiers will be particularly
hard-hit by falling ocean pH, and the list of the disappeared at Castello
Aragonese confirms this. In the pH 7.8 zone, three-quarters of the missing
species are calcifiers. These include the nearly ubiquitous barnacle
Balanus perforatus, the hardy mussel Mytilus galloprovincialis, and the keel
worm Pomatoceros triqueter. Other absent calcifiers are Lima lima, a
common bivalve; Jujubinus striatus, a chocolate-colored sea snail; and
Serpulorbis arenarius, a mollusk known as a worm snail. Calcifying seaweed,
meanwhile, is completely absent.
According to geologists who work in the area, the vents at Castello
Aragonese have been spewing carbon dioxide for at least several hundred
years, maybe longer. Any mussel or barnacle or keel worm that can adapt
to lower pH in a time frame of centuries presumably already would have
done so. “You give them generations on generations to survive in these
conditions, and yet they’re not there,” Hall-Spencer observed.
And the lower the pH drops, the worse it goes for calcifiers. Right up
near the vents, where the bubbles of CO 2 stream up in thick ribbons, Hall-
Spencer found that they are entirely absent. In fact, all that remains in
this area—the underwater equivalent of a vacant lot—are a few hardy
species of native algae, some species of invasive algae, one kind of shrimp,
a sponge, and two kinds of sea slugs.
“You won’t see any calcifying organisms, full stop, in the area where
the bubbles are coming up,” he told me. “You know how normally in a
polluted harbor you’ve got just a few species that are weedlike and able to