The Sea Butterfly Effect

Gareth here, following up on Aleck’s post yesterday about the differences in seawater chemistry his group has been seeing in the measurements they’re making on our current cruise vs. what they measured last year on our cruise to similar latitudes in the Atlantic.

Like Aleck said, our Ocean Acidification Pteropod Study capitalizes on the fact that the chemistry of these two oceans is naturally very different. As Aleck’s data show, the 'compensation depth' at which the water becomes corrosive to aragonite, the particular form of calcium carbonate that is found in the pteropod shell, is much shallower in the Pacific than the Atlantic. At the northernmost point of our surveys, Aleck’s graphs show that the aragonite compensation depth in the Atlantic was 2500m and in the Pacific was 135m! And as we move southwards along our Pacific transect, the compensation depth is becoming gradually deeper.

Schematic of pteropod diel vertical migrations: shallow at night to eat and deep during the day to avoid being eaten!

 

So the question for us biologists onboard is how these differences in chemistry between and within oceans affects pteropods and their aragonitic shells. Many species of pteropod undergo vertical migrations of hundreds of meters between day and night, migrating up into shallow waters by night to feed and back down to depth by day to avoid predators that use vision to hunt. By sampling with our MOCNESS net system at different depth intervals during day and night we can infer whether the pteropods are migrating vertically and over what depth range.

A key question is whether the pteropods' vertical migratory behavior changes when the compensation depth is shallow vs. deep, and if not, how they are able to cope with pH levels that should be corrosive to their shell. In addition to capitalizing on differences between the oceans in seawater chemistry, we can capitalize on the fact that many of the same species of pteropod occur in both oceans. We found Clio pyramidata, for instance, along much of our Atlantic survey line, and so far along most of our Pacific line too. It’s not entirely clear whether these Atlantic and Pacific versions are sub-species or perhaps genetically distinct species that just morphologically appear very similar (that’s something we’ll be testing with our genetic collaborators), but being able to compare essentially the same species in such very different chemical environments is very powerful.

Clio pyramidata (Photo: N. Copley)

Last year in the Atlantic we caught a variety of species and life stages of pteropods. Many of these exhibited a diel vertical migration and we caught pteropods all the way down to the maximum depth we sampled of 1000m. As is the case for many types of organisms, the diversity of pteropod species was highest but at low abundance in the southern, sub-tropical, portion of the survey area in the low productivity waters of the Sargasso, while diversity was lower but abundance high in the colder and more productive temperate waters north of the Gulf Stream and offshore of the Grand Banks.  It’s too early to say how our results from the current cruise compare to what we saw in the Atlantic, but we hope to report back soon!

Survey line for our 2011 cruise to the northwest Atlantic

In coming decades, the aragonite compensation depth is predicted to shoal substantially. Our hope is that by comparing how pteropods respond to these natural levels of variation in compensation depth that we're quantifying in the modern ocean we can gain insight into how they might respond to the changing chemistry of the future ocean.