Longtime readers may remember a cool story about very small ocean creatures mixing the water column with their daily, en masse commutes. A Canadian study had calculated the amount of power exerted by all those millions of tiny, simultaneous wiggles and it was roughly equal to the amount supplied by winds or tides. This was big news – a huge source of power for mixing the ocean,*** hiding in the tiny pink muscles of krill.
Unfortunately, André Visser has gone on record in this week’s Science to puncture that bubble of enthusiasm. Give him credit for letting us down easy, though. His endearing first sentence:
“Every now and then, an idea comes along that is so appealing, it seems bad manners to challenge it.”
Visser points out, using only three Greek letters and two fractional exponents, that although the power all those krill produce is tremendous, it doesn’t mean much mixing gets done. That’s because to mix water you don’t just impart energy on it. You need to get it to form eddies or currents that move far enough to run into some different water. For a krill that’s 1.5 centimeters long, it’s hard to push water that far, no matter how hard they wiggle.
A krill’s “mixing efficiency,” Visser estimates, hovers between 0.01 percent and 1 percent of the mechanical energy of its wiggling. That is to say, between 99 percent and 99.99 percent of the krill’s efforts vanish almost immediately into heat. No word yet as to how good krill are at heating the ocean.
***So who cares? Well, it turns out that mixing the ocean is a lot harder than it sounds. Think about adding half-and-half to your coffee without being allowed to stir it. At first, in a clear mug, it would look pretty cool. But as the cream stalled out in mini-eddies or pooled at the bottom of the mug you would quickly get impatient. Especially if your mug covered 2/3 of the Earth’s surface and was 4,000 meters deep on average. And mixing turns out to be crucial for all sorts of planetary chores: moving heat around, absorbing or releasing carbon dioxide, and stirring nutrients from where they’ve fallen on the sea floor up to the sunlight, where plankton can use them.