Enter the twilight zone: scientists dive into the oceans’ mysterious middle

Enter the twilight zone: scientists dive into the oceans’ mysterious middle 28.02.2020

Enter the twilight zone: scientists dive into the oceans’ mysterious middle

The vast, wild depths between light and shadow face increasing threats from climate change and overfishing.

It is home to a majority of the marine fish biomass and helps to remove an estimated 4 billion tonnes of carbon dioxide from the atmosphere each year. Now, scientists are gearing up to dive into the twilight zone, the largely unexplored ocean layer 200 to 1,000 metres deep that some worry is threatened by a changing climate and increased pressure from fishing.

As part of a US$25-million mission, NASA will travel to the north Atlantic in April to study the movement of carbon between the atmosphere and the deep ocean. Others will join in the expedition thanks to a collaborative venture unveiled at the American Geophysical Union’s ocean-science meeting in San Diego, California, last week.

“This is literally the biggest investment ever made in the twilight zone,” says Dave Siegel, an oceanographer at the University of California, Santa Barbara. He is heading the NASA mission, dubbed Export Processes in the Ocean from Remote Sensing, or EXPORTS. The addition of a network of collaborators promises to bolster data-sharing and coordination with other research efforts around the world. “If we can federate, we can help each other.”

Plumbing the depths

The twilight zone begins where photosynthesis fails, and reaches down to where light tapers off entirely. Here, countless creatures depend on faecal pellets and dead organisms that fall from above — typically referred to as marine snow. Tiny grazers also rise into the upper ocean to hunt each night, marking the largest animal migration on Earth. Bigger predators, such as whales and sharks, frequently forage in this band of water, and humans are increasingly eyeing its bounty as well. Commercial fishing operations in Norway and other countries have already begun to harvest krill that typically inhabit the twilight zone.

Some scientists fear that exploitation of this largely untapped store of protein will increase in the future owing to rising demand for food. That could affect the marine food web and ultimately the climate, says Philip Boyd, a marine ecologist at the University of Tasmania in Hobart, Australia, who is heading up a project to investigate how much carbon drops into the abyss in the Southern Ocean.

The ocean provides a crucial service to humanity by pulling carbon out of the atmosphere, and that depends on what happens in the twilight zone, says Ken Buesseler, a marine radiochemist at the Woods Hole Oceanographic Institution (WHOI) in Massachusetts. “It’s that simple,” he says, “but it’s not easy to measure.”

NASA gave twilight-zone research a major boost with its investment in the EXPORTS project, which kicked off in 2018 with an expedition to the north Pacific Ocean. Low iron concentrations in that region of the ocean restrict blooms of photosynthetic phytoplankton, and preliminary results confirm that less carbon moves deep into the ocean there.

Read more on: www.nature.com

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