Tiny krill appear to mix up the ocean

If you’ve heard of krill at all, it’s probably in the context of their role as whale food. Nature programs love to point in amazement to the fact that the largest animals on the planet subsist on some of the smallest, namely the krill. But these tiny animals exist independently of their function as food, and a new study suggests that they and their peers may have a significant role in their ecosystems: mixing up the top layers of the ocean.

Krill are crustaceans, as a careful look at them will indicate (although Wikipedia tells us that the cool-sounding name “krill” is simply Norwegian for “small fry of fish”). They don’t tend to grow much larger than a couple centimeters in length, and they feed on even smaller creatures, taking tiny photosynthetic plankton and moving them up the food chain.

But what they lack in size, they make up for with truly astonishing numbers, with some species estimated as having one of the largest total biomasses of anything on Earth. It’s these vast numbers that make them a viable food for the world’s largest creatures and give them the ability to replace the vast numbers gulped down by whales. It’s also at the heart of the new results.

Oceans in the lab

To our untrained tongues, the ocean environment they inhabit may taste generically salty, but its saltiness actually varies. Things like evaporation and the addition of freshwater can influence how salty the surface water is. That surface water, in turn, can form distinct layers as it sits on water that is colder and saltier—and thus more dense. The mixing of the surface water and deeper water plays a key role in the cycling of nutrients and oxygen through the ocean and helps power the conveyor system that drives currents from the tropics to the poles.

Individually, small organisms like krill won’t have any effect on the layers of water near the surface; they’re simply too small and generate too little force. But remember, there are a lot of them. And just as importantly, they undergo mass migrations, moving toward the surface during daytime hours and sinking to greater depths at night. Despite their small size, krill can cover hundreds of meters, easily crossing the layers present in surface waters. So it’s entirely possible that these mass migrations could mix up the oceans.

Finding out whether they do is much harder because lots of other things, including the presence of people or instruments, can also disturb the boundaries between different layers of the ocean. And it’s not always possible to get the organisms to cooperate and migrate when you’re ready to make a measurement. So a team of researchers from Stanford’s mechanical engineering department decided to see what happens under lab conditions.

The researchers used a large, cylindrical tank and carefully added salt solutions at different concentrations to create a two-layer, stratified column of water. Into that, they dumped a whole lot of krill substitute: the brine shrimp Artemia salina, a close cousin of the organisms sold in Sea Monkey kits. And by “whole lot,” we’re talking anywhere from about 20,000 to 140,000 animals for each cubic meter of tank. These concentrations may sound extreme, but those numbers have been seen in the wild.

To get the brine shrimp to migrate, the researchers simply took advantage of the fact that these organisms migrate toward the light. By lighting up a green LED beneath the tank, the researchers were able to get the organisms to settle on the tank bottom. They then lit up a blue LED above the tank, drawing them toward the surface. Small particles in the water reflected the LED light, allowing the team to track the flow of water in the tank. After a half-dozen of these up/down cycles, the researchers left the brine shrimp on the bottom of the tank and read the salinity at a variety of depths.

Collective migration

The results were pretty clear-cut. “In the collective migration,” the researchers write, “a large-scale downward jet developed within the region of upward animal migration owing to the repeated downward fluid displacement by each individual animal propelling itself upward.” Even though each individual animal creates an imperceptible movement of water, they collectively generate a jet of water. And that jet disturbed the sharp boundary between the two layers of water in the tank, smoothing it out into a gradual transition. The effect might have been even stronger, except the brine shrimp generally allowed themselves to passively sink when the blue light at the top of the tank was switched off.

The experiment wasn’t an exact replica of a migration as it would happen in the wild. The blue LED formed a column of light in the tank, which may have concentrated the migration into the tank’s center. In the ocean, the Sun would light up everywhere fairly evenly, which might lead to a broader migration and less of a jet. And the brine shrimp were fairly concentrated in a relatively small tank; while in the ocean, concentrations of krill may be spread across tens of vertical meters.

The authors agree that it would be extremely helpful to observe this phenomenon in the ocean, but they hint that technology has reached the point where it might be possible to do so. And because the mixing of the top layers of the ocean influences so many processes—in their words, it has “numerous potential effects on the physical and biogeochemical structure of the ocean”—figuring out whether organisms like krill influence the process is worth the effort.

Nature, 2018. DOI: 10.1038/s41586-018-0044-z (About DOIs).

https://arstechnica.com/?p=1296587