Stirring Up the Oceans

Biology plays a much larger role in climate systems than we sometimes realize.

Researchers at Caltech just published the results of a very interesting study in the journal Nature.

The researchers have found that many small organisms, from tiny krill, copepods and amphipods to jellyfish and a number of other organisms, move large amounts of deep ocean water with them as they migrate from the ocean’s depths each day.

As these organisms move, they create vacuumlike conditions, similar to drafting, behind them, and because they move in very large masses, they play a role in ocean circulation.

The researchers call this “Darwinian mixing” after Charles Darwin (not THE Charles Darwin, but rather, his grandson, Charles Galton Darwin, a physicist and son of George Howard Darwin) who first discovered and described his idea about the motion of bodies in water creating drift behind them, known as Darwin drift.

Now, with the help of modern technology and science, researchers are much closer to validating the idea that Darwin’s theory has real consequences for understanding ocean water circulation.

The effects of this mixing aren’t estimated to be small, either. One of the researchers calculated that, in total, biogenic or Darwinian mixing could have an energetic effect on ocean currents equivalent to wind forcing or tidal forcing.

This is interesting for a number of reasons. First, to think that such small organisms could play a big role in ocean currents and water exchange, and by extension play a role in global climate, is really cool.

Second, this discovery might shed light on how interrelated ocean systems function and how they can be influenced. I’ve mentioned geoengineering schemes to mitigate anthropogenic climate change in the past (see my blog titled “Coral Reef Sunburn” for a discussion of plans to add iron to the oceans in an attempt to sequester carbon).

One such plan calls for adding large amounts of iron to parts of the ocean that are otherwise rich in nutrients phytoplankton need to grow. The idea is that as the phytoplankton consume the iron and other nutrients along with carbon from the atmosphere, they will eventually die and sink to the bottom, taking the carbon with them. But researchers testing this idea have found that their simple idea doesn’t work as they planned. They found that carbon sequestration does not occur at a high enough level of efficiency to make the scheme worthwhile.

Part of the reason it doesn’t work is due to the fact that, though the phytoplankton do consume carbon, die and sink, most of the carbon they were carrying ends up circulating back to the upper reaches of the ocean.

Some of the researchers studying this geoengineering idea suggest that this might be due to organisms consuming the phytoplankton early, before they’re able to sink all the way to the bottom of the ocean where the carbon would be locked up for long periods of time.

This is still just supposition, but it’s possible that this biological circulation mechanism may be playing a role in keeping the unsequestered carbon “in the system,” so to speak.

Researchers will have to take Darwinian mixing into account when they study such geoengineering plans in the future. The effects of such geoengineering schemes are poorly understood, and just as playing with tidal forces could cause massive and unforeseen impacts on the environment, so too may plans to alter or influence the forces of Darwinian mixing.

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