A Fast-Forwarded Look at How Our Oceans Might Be in a Few More Decades

This is scary stuff, when you get right down to it: With atmospheric CO₂ concentrations rising, where does it all go? Well, lots of it goes into the ocean. There, it's estimated to have caused a 30% increase in hydrogen ion concentration since the beginning of the last century. An increase in H⁺ levels means a drop in pH as the ocean becomes more and more acidic.

Our oceans are turning into vats of acid.

Does that scare you?

It should scare calcareous organisms more, since they're the ones who will be bearing the immediate brunt of ocean acidification: as acidity increases, their calcium carbonate shells disintegrate. Many, if not most of, the ocean's coral reefs, clams, snails, etc are threatened by this. And of course, not only them, but the ecosystems--and economies--based around them. Detailed discussion of the mechanisms and effects of ocean acidification has been made before, you can find one at wikipedia.

We might be able to visualize some of the effects that ocean acidification will have in our children's lifetime right now, according to a new letter in the journal Nature. [454:96-99] Hall-Spencer et al. use marine CO₂ vents in the Mediterranean as a natural experiment to see the correlation between high carbon dioxide levels and marine life community structure. The vents they monitor flux in CO₂ output normally throughout their existence, and they chose vents that emit only CO₂, without poisonous sulphur compounds or heat. Thus, they were able to limit confounding effects other than CO₂, and come to stronger conclusions about the association of CO₂ and reduced numbers of calcifers in the ocean.

They found a 30% reduction in species numbers (mostly calcifiers) near the vents versus the control sites. Coral reef cover (from Family Corallinaceae, the red algae corals) was 60% outside of the vent areas but only 0% within it. (*poof* Gone.) As if to take these Corallinaceae's place, a number of algal genera grew fantastically within the CO₂-rich regions near the vents. These algae include some of the most invasive algae species in the world, like sargassum, which forms thick, floating mats, and Caulerpa, which is already threatening the Mediterranean with its ability to crowd out all other species.

Sea urchin numbers were also significantly lower around the vents, juvenile gastropods were nowhere to be found, and adult gastropods' shells were flimsy. Barnacles seemingly were able to close themselves off to protect themselves from the acidic water, except for the lowest-pH sites, where they, too, were decreased in number.

The results of this study by Hall-Spencer and his team corroborate theory as well as earlier experimental studies of a more limited nature that couldn't take into account ecosystem-scale effects as well as this study could. The authors acknowledged that their work with vents could not perfectly model world-wide ocean acidification due to temporal variation in pH at the vents, the patchiness of the high CO₂ regions in the ocean landscape due to the nature of the vents, and other abiotic factors that may have had some effect. And of course, it's correlational and not causational. Nevertheless, this study provides valuable insight into the drastic effect that anthropogenic sources of CO₂ may have on ocean ecosystems if greenhouse gases continue to be emitted at the present rate.

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note: 15 minutes after posting this, I realized with a big, forehead-slapping "duh" that in this post I was committing the very same error I was deriding in my earlier post about the incorrect inference of causation from observational studies. I had to go back and edit myself. How embarrassing...