Sea butterflies are in the news, stressed by ocean acidification.
What do we now know about the decline in pH of ocean waters?
Well, we know that the pH has dropped from 8.2, where it was at the start of the Industrial Revolution, to its current level of about 8.1, and that the rate of change has increased in the past several decades. This may not sound like much, but in fact it indicates a 30% increase in the concentration of H+ ions in sea water. That is plenty to stress species that depend on carbonate ions in the water to build the calcium carbonate shells and skeletons that they depend on.
At current rates of global atmospheric CO2 emissions, ocean pH will drop further to 7.8 by the end of the century.
Ocean acidification has occurred before on the planet, but this event is different: it is happening 100 times more rapidly than any previous events we know of. Geochemists are looking 300 million years into the past, and there is nothing like it.
And it matters. Anything with an exposed calcium carbonate shell or skeleton will be affected – think mollusks, corals, and shellfish like crabs, shrimp and lobsters. With more CO2 dissolved in the water, there are more more bicarbonate ions along with the greater levels of H+ ions, and as a result less carbonate is available to make calcium carbonate. The shells are vulnerable to dissolution unless the surrounding water is saturated with carbonate ions, for they then lose calcium back into the water. As the shells erode and weaken, the animals become stressed, misshapen and potentially dead.
We’ve known about the increasing threat of ocean acidification for some years, but perhaps it has seemed a more distant threat than others associated with our increased CO2 emissions. But we know there already has been an impact on shell growth of oysters and mussels, and we know that coral reefs are particularly vulnerable as pH continues to decline. We have certainly been warned.
Now we are warned once again, this time by sea butterflies. Also known as pteropods, these actually are pea-sized snails that live in the plankton where they are predators of other plankton and the common prey of fish. They are very beautiful to us – translucent, graceful, with the snail foot modified into what look like flapping wings. The shell is much reduced, though still very present, and the shells of species living in the California Current along the west coast of the US are showing signs of unusual erosion from exposure to the lower pH.
There are several key issues here. The rate of ocean acidification is unprecedented, and we don’t really know what lies ahead. We also know that vulnerable organisms will have insufficent time to adapt even if adaptation were possible. Eliminating vulnerable species like pteropods – or brittle stars, corals, mollusks or crustacesns – from ecosystems where they play a critical role as prey or predator will change the communities in ways that may also effect the top predators we want to catch. We are not short of discouraging examples of such community restructuring.
Do poster species help? Though the looming loss of coral reefs has not galvanized us to action, effective conservation campaigns have been built on the images of a variety of mammals, from whales and polar bears to koalas and pandas.
But sea butterflies as poster species? Because of their beauty, perhaps that isn’t impossible. It can’t hurt.