The God Species_ How the Planet Can Survive the Age of Humans - Mark Lynas [110]
The impacts could be most severe at the base of the food chain, on which all higher life-forms depend. Although so small that they are individually invisible, plankton called foraminifera are present in such huge numbers in the world’s seas that they account for between a quarter and a half of all the carbonate precipitated in the marine ecosystem—a much higher proportion even than coral reefs. Yet there is evidence that the tiny shells precipitated by these ubiquitous plankton are a third smaller than in preindustrial times, perhaps in response to rising oceanic CO2.21 Other micro-size planktonic calcifiers are the coccolithophores, which are so small—and so abundant—that they can be present in the tens or even hundreds of thousands in just a liter of seawater. By scattering sunlight through the upper layers of the ocean, they make it more opaque and help give tropical seas their famed turquoise color.22 If acidification reduces their numbers, the oceans of our planet will subtly change color—to a darker hue, perhaps with a greenish tint where photosynthesizing algae take over from their calcifying brethren, another planetary-scale visual change that might be detected from space.
Scientific studies seem to confirm that coccolithophores are sensitive to ocean acidification: Those organisms exposed to high-CO2 conditions grow degraded or malformed shells and reduce their rate of calcium carbonate production.23 Similar effects have been observed for tiny planktonic floating snails called pteropods, which flourish by the tens of thousands per cubic meter of water in the Arctic and Antarctic seas, and are crucial prey food for pollock, cod, salmon and mackerel.24 In the southern oceans, krill—important there as food for whales, seals, penguins, and fish—are particularly vulnerable because their unique life cycle means they migrate between the higher and lower depths of the ocean and are exposed to very different levels of acidity in the process.25 Acidification also changes the characteristics of seawater in unexpected ways. Acoustically, sound will travel farther, changing the environment for whales and other cetaceans that communicate by hearing.26 Some fish also seem to lose their ability to “smell” properly in more acidic waters, even if they are not directly affected physiologically.27 Some animals seem unaffected as adults, yet are vulnerable in their larval stage.28
But, again as with nitrogen, higher carbon dioxide will also have a fertilizing effect, benefiting some opportunistic, weedy species. Sea grasses are expected to do well in a higher-CO2 ocean. Although acidification will certainly reduce biodiversity overall, it will be a welcome boon for the seaweeds and choking algae that will coat the world’s declining coral reefs as they erode and decay in the carbon-rich water.29 Tiny organisms called diazotrophs, which can fix nitrogen in the sea just as leguminous plants do on land, also seem to proliferate as carbon dioxide levels rise.30 If CO2 emissions continue unchecked, the oceans will see a general shift away from calcifying organisms toward green algae and other photosynthesizers: a change characterized by marine biologist Jeremy Jackson as “the rise of slime.”31 As we saw in the biodiversity chapter, these kinds of shifts in species patterns can be expected to cascade up the food chain, where negative impacts on tiny plankton eventually affect food supplies for the great whales and other top predators. Accordingly, addressing this problem means understanding the interactions between at least four planetary boundaries: those on climate, nitrogen, biodiversity, and, of course, ocean acidification itself.
REEF GAPS
As I showed in the chapter on climate change, scientists have learned a lot about the future by looking at the past. Over many millions of years, the Earth has seen