Swarms of tiny ocean organisms known collectively as zooplankton may have an outsize influence on their environment.
These clusters of centimeter-long individuals, each beating tiny feathered legs, can, in aggregate, create powerful currents that may mix water over hundreds of meters in depth.
Although the work was carried out in the lab, the research is the first to show that migrating zooplankton—or any organism—can create turbulence at a scale large enough to mix the ocean’s waters. The findings could alter the way scientists think about global nutrient cycles like carbon, phosphate, and oxygen, or even currents themselves.
“Ocean dynamics are directly connected to global climate through interactions with the atmosphere,” says John Dabiri, professor of civil and environmental engineering and of mechanical engineering at Stanford University and senior author of the paper in Nature.
“The fact that swimming animals could play a significant role in ocean mixing—an idea that has been almost heretical in oceanography—could therefore have consequences far beyond the immediate waters where the animals reside.”
The findings could also help scientists understand how the ocean sequesters carbon dioxide from the atmosphere and lead to updates in ocean climate models.
“Right now a lot of our ocean climate models don’t include the effect of animals or if they do, it’s as passive participants in the process,” Dabiri says.
One of the most common zooplankton, krill are among the most abundant marine organisms and migrate daily in giant swarms, heading hundreds of meters deep by day and up to the ocean’s surface by night to feed.
Dabiri knew that in terms of forces that drive the mixing of oceans, wind, and tidal currents are thought to play the largest role. But he wondered if giant zooplankton migrations could also be involved—an idea first proposed by oceanographer Walter Munk in 1966, and since then debated but never systematically explored.
Dabiri and graduate student Isabel Houghton tried to answer that question not in the ocean but in the relatively controlled environment of large water tanks in the lab. The pair worked with Jeffrey Koseff and Stephen Monismith, professors of civil and environmental engineering, to create flow environments that mimic the ocean with saltier water on the bottom of the tank and less salty water on the top. The resulting gradient mirrors ocean conditions that any organism would need to disrupt in order to cycle nutrients between the ocean’s surface and water deep below.
“There’s no appreciable deep mixing of oxygen or carbon dioxide in the ocean if you can’t overcome the stabilizing influence of salinity and temperature gradients,” Koseff says.
In the lab, the group was looking to see whether the tiny organisms they studied—mostly brine shrimp (also known as sea monkeys) as a stand-in for less lab-hardy krill—are simply churning water locally, leaving the gradient intact, or redistributing salt into a more uniform mixture. If they can mix layers in the lab, chances are they can do the same in the ocean.
To carry out the study, Houghton placed brine shrimp in the tank and activated laser or LED lights from either above or below. Because brine shrimp are attracted to light, they migrated toward the source. When she reversed the lights the tiny creatures scurried to the other end in a migration that lasted about 10 minutes.
With cameras closely recording the animals’ movements, researchers were able to measure the individual water eddies surrounding each brine shrimp and the larger currents in the tank. From these, they showed that turbulence from individual organisms aggregates into a much larger turbulent jet in the wake of the migration. What’s more, those flows were powerful enough to mix the tank’s salt gradient.
“They weren’t just displacing fluid that then returned to its original location,” Houghton says. “Everything mixed irreversibly.”
Before the current study, scientists thought that krill and other zooplankton could only create turbulence in their own size range—on the order of centimeters. That’s hardly enough to move nutrients on a meaningful scale.
Now it appears that zooplankton have the capacity to mix ocean waters, at least regionally. Furthermore, the findings might not just apply to organisms like krill in the upper kilometer of the ocean, but also to jellyfish, squid, fish, and mammals that swim even deeper, potentially churning the entire water column.
The findings need to be verified in the ocean, Dabiri says, which will involve finding and following swarms of krill in locations as diverse as the California coast and frigid Antarctic waters.
But if they continue to see mixing at the scales the lab work suggests, the findings could change the way ocean scientists think about the role of animals in influencing their watery environment—and potentially our climate on land.
Source: Stanford University