Largest Migration on Earth Changes Ocean Chemistry with Animal Pee

The largest migration on the planet isn't completed by monarch butterflies or peregrine falcons. Instead, it's the movement of small animals from the surface of the open ocean to the sunless depths, where they hide from predators during the day. Now, scientists have found that this massive migration actually shapes the chemistry of our oceans-by the animals' pee.

"I'm very fascinated by these massive migrations," said Daniele Bianchi, lead author of the new study, in a news release. "To me, it's exciting to think about the effects of animal behavior on a large scale in the ocean."

These migrating animals include tiny zooplankton, crustaceans such as krill, and fish such as the creepy-looking lanternfish. Even though these animals are small in size, though, their abundance throughout the ocean allows them to drastically impact their surroundings.

The researchers mined data from underwater sonar surveys in order to calculate how many animals are migrating to which depths, and where. Then, they gauged the combined effect of the animals' daytime digestion.

In the end, the researchers found that in certain parts of the ocean, ammonia released from the animals drives a big part of the oxygen-free conversion of ammonium and other molecules to nitrogen gas, a key chemical transition.

"We still think bacteria do most of the job, but the effect of animals is enough to alter the rates of these reactions and maybe help explain some of the measurements," said Bianchi.

Understanding these low-oxygen zones is important for future studies. Not only that, but it's predicted that under the effects of climate change, these low-oxygen zones will expand. This means that understand these zones today is crucial for better understanding what may happen in the future as temperatures continue to warm.

The findings are published in the journal Proceedings of the National Academy of Sciences.