40% of Life in Ocean’s Biodiverse ‘Twilight Zone’ Could Disappear Amid Warming Seas

Just below the surface of the world’s ocean is a layer of deep where sunlight doesn’t reach. Known as the mesopelagic, its depths of 656 to 3,280 feet are cold and dark, but flash with bioluminescent light. In fact, there is more life here than in the rest of the ocean put together. One study found that 95 percent of the world’s fish hide out in this mysterious zone.

Some of the marine species in the midwater, as the layer is also known, are among the biggest on Earth, but most are small and an important part of the ocean’s food web. These creatures ferry enormous amounts of carbon from the surface of the sea into its depths as an essential part of the planet’s climate regulation process.

A new study predicts that the abundance of life hiding in the twilight zone will face considerable declines, even extinctions in some cases, as global waters warm and less food makes its way into the ocean depths.

“The rich variety of twilight zone life evolved in the last few million years, when ocean waters had cooled enough to act rather like a fridge, preserving the food for longer and improving conditions allowing life to thrive,” said Katherine Crichton, lead author of the study and a postdoctoral research fellow at the University of Exeter, as The Guardian reported.

The study, “What the geological past can tell us about the future of the ocean’s twilight zone,” was published in the journal Nature Communications.

The twilight zone, which consists of about one fourth of the volume of the world’s ocean, is home to many species and millions of tons of organic matter, including lanternfish and kite fin sharks, equipped with bioluminescent skin.

The particles of food that float down from the surface of the ocean in the form of marine animal poop and dead phytoplankton are called “marine snow.”

These flecks of sustenance have been degraded faster by bacteria from past warming events so there are less available for marine creatures in the twilight zone, reported CNN.

According to the study, due to the shrinking abundance of food drifting down to the ocean’s midlayer, as much as 40 percent of the marine life living there could be gone by the end of the century.

“According to the studies we have done, 15m years ago there wasn’t all this life [in the twilight zone] and now, because of human activity, we may lose it all. It’s a huge loss of richness,” Crichton told The Guardian. “Unless we rapidly reduce greenhouse gas emissions, this could lead to the disappearance or extinction of much twilight zone life within 150 years, with effects spanning millennia thereafter.”

Lead author of the study Paul Pearson of Cardiff University’s School of Earth and Environmental Sciences said warmer oceans store less carbon due to the fact that the marine snow is being gobbled up by microorganisms closer to the surface, and the less it sinks, the more quickly carbon is released back into Earth’s atmosphere.

Crichton pointed to a bright spot of the study: Although some loss is unavoidable, the most extreme scenario can be avoided if global emissions are curbed.

“We still know relatively little about the ocean twilight zone, but using evidence from the past we can understand what may happen in the future,” Crichton said, as CNN reported.

The study focused on two warm periods — 15 and 50 million years ago — when ocean temperatures were much warmer than they are today.

“We found that the twilight zone was not always a rich habitat full of life,” Pearson said, as reported by CNN. “In these warm periods, far fewer organisms lived in the twilight zone, because much less food arrived from surface waters.”

Three possible outcomes were predicted by the study: a low-carbon possibility with 689 billion tons of total emissions from 2010 going forward; a medium-carbon scenario with 2,756 billion tons of emissions; and a high-carbon scenario with 5,512 billion tons.

“If we get to the medium or high scenario both are very bad news for the twilight zone,” Crichton said, according to The Guardian.