Animals swim by pushing on the water around their bodies. Even small ocean swimmers such as krill — tiny shrimplike animals — make mini currents with each kick. And if there are lots of these tiny creatures? Together they may be powerful enough to stir up water hundreds of meters (yards) deep, a new study suggests.
That water mixing is important because many nutrients are unevenly spread throughout the ocean. Near the surface, microscopic marine plants and bacteria use sunlight to make food. In the process, they use up mineral nutrients in the water. These include nitrogen and phosphorus. So shallow water gets sapped of these nutrients. But deeper water contains lots of such nutrients. Stirring up the ocean helps carry the nutrient-rich deep water up toward the surface where plants can use it.
Wind, tides and large currents all help mix the ocean. But swimming animals move water, too. And researchers wanted to know whether small but very abundant critters might play a role. They were thinking about centimeter (half-inch) sized krill and even tinier crustaceans known as copepods (KOH-puh-podz).
John Dabiri works at Stanford University in California. He studies how fluids move around objects, including animals. Early on, he says, scientists didn’t think creatures as small as krill could have much effect on a big ocean. “The original thinking is that these animals would flap their appendages and create little eddies about the same size as their bodies,” he says.
But then, measurements of krill migrating in the ocean suggested that they might stir up more turbulence than researchers had at first thought possible. So did computer programs used to predict the flow of water around swimming jellyfish and copepods.
In 2014, Dabiri coauthored a study to measure how water moved around swimming brine shrimp in a lab. Brine shrimp are small crustaceans that look similar to krill. Brine shrimp in tanks created jets and eddies in the water much larger than themselves, that study showed. “But there was skepticism about whether those lab results were relevant to the ocean,” Dabiri says.
Why? Ocean water is not uniform. Some spots are warmer, others cooler. Some parts have more salt than others. Such differences can divide the water into layers, each having its own temperature or salt concentration. Such layers don’t easily mix. And the 2014 study hadn’t accounted for such layers. So it wasn’t clear if krill- or brine-shrimp-generated turbulence would be strong enough or travel deeply enough to really mix up a layered ocean.
In the new study, Dabiri’s team asked whether tiny animals had the swimming power to mix ocean layers. To test this, they used two tanks of water in their lab. One was 1.2 meters (4 feet) deep, the other 2 meters (about 6 feet) deep. Each tank contained two layers of water. One layer contained more salt, which made it sink to the bottom. Each tank also held tens of thousands of wiggly brine shrimp. They look like a pink cloud shifting in the water.
Bright lights attract brine shrimp, so the researchers used LED lights to prompt the shrimp to migrate up or down within the tanks. These movements mimicked the massive daily vertical migrations that ocean krill, copepods and other small swimmers make to feed and rest.
The shrimp migrated in a tight bunch. And being so close together helped to magnify their individual efforts.
“As one animal swims upward, it’s kicking backward,” Dabiri says. That same bit of water then gets kicked downward by another nearby animal. Then another, and another. The result is a downward rush that gets stronger as the migration continues. Eventually it extends downward about as deep as the entire group of moving animals. In the ocean, that could be as much as hundreds of meters.
The swimming shrimp moved the water at a speed of about 1 to 2 centimeters (0.5 to 1 inch) per second. The resulting jet was powerful enough to mix the shallow water in each tank with the deeper, saltier water. The researchers reported their findings online April 18 in Nature.
What’s the big deal?
Ocean mixing is an important part of the global climate cycle, Dewar points out. It churns up nutrients that feed will feed blooms of algae at the surface. That mixing also affects how gases, such as carbon dioxide, move between the water and the air above.
Scientists already knew that mixing occurred. But if tiny animals were behind part of that mixing, it might change how scientists studied things moved by ocean currents, he says.
And that question — can tiny animals do any important mixing — was at the heart of the new study, notes William Dewar. He was not part of the study. But as an oceanographer at Florida State University in Tallahassee, he understands the issues here. The new findings “argue quite compellingly” that tiny animals can kick up a good bit of mixing, he says. Those findings, he concludes, “strongly counter the concern that most marine life is simply too small in size to matter.”
The next step will be to try to measure similar effects at sea from ships, Dabiri says. “Previous studies looked for turbulence or eddies on the scale of the animals’ size,” he says. They didn’t look for large downward jets of water. “This paper tells us for the first time what to look for.”
algae Single-celled organisms, once considered plants (they aren’t). As aquatic organisms, they grow in water. Like green plants, they depend on sunlight to make their food.
appendage A finger, leg, ear, antenna or other feature that sticks out from some creature and has some apparent specific function.
atmosphere The envelope of gases surrounding Earth or another planet.
bacteria (singular: bacterium) Single-celled organisms. These dwell nearly everywhere on Earth, from the bottom of the sea to inside other living organisms (such as plants and animals).
bloom (in microbiology) The rapid and largely uncontrolled growth of a species, such as algae in waterways enriched with nutrients.
brine Water that is salty, often far saltier than seawater.
carbon The chemical element having the atomic number 6. It is the physical basis of all life on Earth. Carbon exists freely as graphite and diamond. It is an important part of coal, limestone and petroleum, and is capable of self-bonding, chemically, to form an enormous number of chemically, biologically and commercially important molecules.
carbon dioxide (or CO2) A colorless, odorless gas produced by all animals when the oxygen they inhale reacts with the carbon-rich foods that they’ve eaten. Carbon dioxide also is released when organic matter burns (including fossil fuels like oil or gas). Carbon dioxide acts as a greenhouse gas, trapping heat in Earth’s atmosphere. Plants convert carbon dioxide into oxygen during photosynthesis, the process they use to make their own food.
climate The weather conditions that typically exist in one area, in general, or over a long period.
copepod A type of small crustacean found in salt and fresh water. Some species of them are plankton, floating with the currents. Others spend time on the sea floor. These animals aren’t limited to oceans; copepods also are found in freshwater, from ponds to puddles. They often serve as food for larger species, and most eat phytoplankton — single-celled organisms that get their energy from the sun.
crustaceans Hard-shelled water-dwelling animals including lobsters, crabs and shrimp.
current A fluid — such as of water or air — that moves in a recognizable direction.
environment The sum of all of the things that exist around some organism or the process and the condition those things create. Environment may refer to the weather and ecosystem in which some animal lives, or, perhaps, the temperature and humidity (or even the placement of components in some electronics system or product).
krill Tiny shrimplike crustaceans that live in the ocean and are the main food source of some whales.
LED (short for light emitting diode) Electronic components that, as their name suggests, emit light when electricity flows through them. LEDs are very energy-efficient and often can be very bright. They have lately been replacing conventional lights for home and commercial lamps.
magnify To increase in apparent size or number of something.
marine Having to do with the ocean world or environment.
microscopic An adjective for things too small to be seen by the unaided eye. It takes a microscope to view objects this small, such as bacteria or other one-celled organisms.
migrate To move long distances (often across many countries) in search of a new home. (in biology) To travel from one place to another at regular times to find food or more hospitable conditions (such as better weather).
migration (v. migrate) Movement from one region or habitat to another, especially regularly (and according to the seasons) or to cope with some driving force (such as climate or war). An individual that makes this move is known as a migrant.
mineral Crystal-forming substances that make up rock, such as quartz, apatite or various carbonates. Most rocks contain several different minerals mish-mashed together. A mineral usually is solid and stable at room temperatures and has a specific formula, or recipe (with atoms occurring in certain proportions) and a specific crystalline structure (meaning that its atoms are organized in regular three-dimensional patterns). (in physiology) The same chemicals that are needed by the body to make and feed tissues to maintain health.
molecule An electrically neutral group of atoms that represents the smallest possible amount of a chemical compound. Molecules can be made of single types of atoms or of different types. For example, the oxygen in the air is made of two oxygen atoms (O2), but water is made of two hydrogen atoms and one oxygen atom (H2O).
nitrogen A colorless, odorless and nonreactive gaseous element that forms about 78 percent of Earth's atmosphere. Its scientific symbol is N. Nitrogen is released in the form of nitrogen oxides as fossil fuels burn.
nutrient A vitamin, mineral, fat, carbohydrate or protein that a plant, animal or other organism requires as part of its food in order to survive.
online (n.) On the internet. (adj.) A term for what can be found or accessed on the internet.
phosphorus A highly reactive, nonmetallic element occurring naturally in phosphates. Its scientific symbol is P. It is an important part of many chemicals and structures that are found in cells, such as membranes, and DNA.
sea An ocean (or region that is part of an ocean). Unlike lakes and streams, seawater — or ocean water — is salty.
simulation (v. simulate) An analysis, often made using a computer, of some conditions, functions or appearance of a physical system. A computer program would do this by using mathematical operations that can describe the system and how it might change over time or in response to different anticipated situations.
tides (adj. tidal ) The alternate rising and falling of the sea, usually twice in each lunar day at a particular place, due to the attraction of the moon and sun.
turbulence The chaotic, swirling flow of air. Airplanes that run into turbulence high above ground can give passengers a bumpy ride.
vertical A term for the direction of a line or plane that runs up and down, as the vertical post for a streetlight does. It’s the opposite of horizontal, which would run parallel to the ground.
Journal: I.A. Houghton et al. Vertically migrating swimmers generate aggregation-scale eddies in a stratified column. Nature. Vol. 556, April 18, 2018, p. 497. doi: 10.1038/s41586-018-0044-z.
Journal: M.M. Wilhelmus and J.O. Dabiri. Observations of large-scale fluid transport by laser-guided plankton aggregations. Physics of Fluids. Vol. 26, October 2014. doi: 10.1063/1.4895655.