A school of anchovies swims in shallow water in the Bahamas.
Anchovies may contribute to ocean mixing in shallow coastal waters. Credit: Paul Harrison/Wikimedia Commons, CC BY-SA 4.0

Beneath the calm surface of the water off the Galician coast of the Iberian Peninsula, anchovies gathered to spawn. While these fish were getting lucky, seafaring scientists got lucky too—for the first time, researchers observed tiny creatures agitating the water enough to contribute to ocean mixing.

It may not be intuitive that little fish swimming in a big ocean can produce mixing on a scale larger than themselves, said John Dabiri, a fluid dynamicist at the California Institute of Technology in Pasadena who wasn’t part of the work. The idea wouldn’t seem so far-fetched, however, to those who study fluid dynamics.

The concept of biomixing dates back to the 1960s and an oceanographer named Walter Munk, who raised this idea of animal mixing in the ocean as a joke, Dabiri said. More recently, he continued, researchers have tried to declare the idea dead on multiple occasions, publishing papers saying it wasn’t possible. “I guess somebody forgot to tell the animals.”

In the lab, Dabiri’s team has documented turbulent mixing from swimming brine shrimp.

Though those critters don’t dwell in the ocean, their motions mimic the nightly vertical migrations of ocean-abundant krill, showing that mixing due to vertical migrations is physically possible. Other researchers have spotted creature-caused turbulence in the ocean but haven’t been able to tie it to mixing of layers of water with different properties, such as temperature or salinity. Still others have tried to measure mixing by shoals of fish and have come up empty. The lack of evidence prompted some researchers to maintain that biomixing doesn’t occur, Dabiri said.

A Fishy Surprise

Then in the summer of 2018, scientists who knew a thing or two about ocean turbulence happened to be in the right place at the right time. “It was a fortuitous observation,” said Bieito Fernández Castro, an oceanographer at the University of Southampton in England and lead author of a study published in Nature Geoscience. Fernández Castro and his colleagues, on a project led by University of Vigo researchers from Spain, were planning to study a different topic—how turbulence affects the growth of algae that produce toxins. Blooms of this algae typically don’t occur when there’s turbulence, so the team had picked a spot where they thought there would be little ocean mixing, Fernández Castro said.

For 15 days, the researchers waited in the same spot for the bloom to appear. To detect turbulence, they used a tool that senses small-scale temperature and velocity fluctuations. Repeatedly, they let the instrument fall in the water and brought it back up, like a yo-yo. During the day, the water’s turbulence was weak. But each night, they measured strong turbulence at 10 meters and deeper. “The values of turbulence we were getting were just huge,” Fernández Castro said. “We were thinking this should feel like a big storm.”

At first, that puzzled the scientists. “It was very funny to be on this cruise,” Fernández Castro noted. It took a few days to rule out that wind and tides didn’t cause the turbulence.

The ship happened to have an instrument that provided a vital clue. The vessel, which is also used for fisheries research, was equipped with a device that uses sound waves to spot the presence of fish. A nightly uptick in the acoustic signals corresponded to the ocean commotion.

The team didn’t have quite everything needed to study fish, so they improvised. Each night, they cast their plankton nets into the water and hauled in a load of anchovy eggs, suggesting the mingling and mixing fish were spawning.

“When we realized what it was, we got very excited because we know about all this controversy [about biomixing],” Fernández Castro said. This controversy has continued, in part, because it’s been tricky to find turbulent mixing in the wild.

Mixing to Fuel Life

It’s not clear how widespread biomixing is. Fernández Castro and his colleagues caught this ocean agitation in a relatively shallow embayment where properties such as temperature have a sharp gradient. This may be one of the requirements for small creatures to stir up the ocean, the authors wrote. In the deep ocean, where such gradients aren’t as strong, small fish may not be able to impart enough energy to mix layers of water together.

“Anything that promotes vertical exchange in stratified waters is generally good for the ecosystem because it provides what is lacking.”

The team isn’t sure whether such mixing occurs in other parts of this bay, let alone in other ecosystems. But where it does occur, such mixing could fuel life. Vertical mixing brings oxygen that’s plentiful in the upper waters down to the depths, where it helps sustain life on the seafloor. And mixing can carry nutrients, which come from the sunken remains of algal die-offs, back to the surface. “Anything that promotes vertical exchange in stratified waters is generally good for the ecosystem because it provides what is lacking,” Fernández Castro said.

As a connected network, the ocean’s transport of carbon, oxygen, and nutrients and its density structure could be affected by biomixing, Dabiri said. And if this mixing is found to occur on a wider scale, it will present a computational challenge for researchers who model the ocean’s climate and circulation. Although we can’t yet rule out that biomixing isn’t happening on a large scale, Dabiri said, it’s hard to overstate the impact it could have.

—Carolyn Wilke (@CarolynMWilke), Science Writer

Citation: Wilke, C. (2022), Tiny creatures may play a difficult-to-detect role in ocean mixing, Eos, 103, https://doi.org/10.1029/2022EO220247. Published on 20 May 2022.
Text © 2022. The authors. CC BY-NC-ND 3.0
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