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Since 2014, researchers have deployed more than 150 biogeochemical Argo floats in the Southern Ocean.

GRETA SHUM/SOCCOM

Fleet of robotic probes will monitor global warming’s impact on microscopic ocean life

A single drop of seawater holds millions of phytoplankton, a mix of algae, bacteria, and protocellular creatures. Across the world’s oceans these photosynthesizing microbes pump out more than half of the planet’s oxygen, while slowing climate change by capturing an estimated 25% of the carbon dioxide (CO2) released from humanity’s burning of fossil fuels. But the scale of this vital chemistry is mostly a guess, and there’s little sense of how it will change as temperatures rise. “What’s happening out there? We have no idea really,” says Susan Wijffels, a physical oceanographer at the Woods Hole Oceanographic Institution.

Soon, 500 drifting ocean floats studded with biogeochemical sensors will deliver answers. Today, the National Science Foundation (NSF) announced it will spend $53 million to fund the new floats, marking the first major expansion of the Argo array, a set of 4000 floats that for 15 years has tracked rising ocean temperatures. “This is going to be revolutionary,” says Wijffels, a leader of the original Argo program.

The biogeochemical (BGC) Argo floats, in development for nearly as long as Argo itself, will operate much like their forerunners. After being tossed off a ship, each of the skinny, 1-meter-tall floats drifts with deep ocean currents 1000 meters down. Every 10 days or so, it uses an oil-filled bladder to change its density, dropping to 2000 meters and then slowly rising to the surface, where it beams the resulting profile home. Although instruments lowered from ships can make deeper and more precise measurements, ship campaigns are expensive, and often limited to common ocean trade routes, says Ken Johnson, a chemical oceanographer at the Monterey Bay Aquarium Research Institute and a leader of the new program. The floats, he says, are “not as good as a ship, but they’re there all the time.”

In addition to standard Argo measurements of temperature and salinity, the new floats will have sensors measuring oxygen, sunlight, particles, chlorophyll (a gauge of phytoplankton abundance), nitrate (a key nutrient), and pH (acidity). Researchers will be watching that last reading closely, because acidity reflects both the ocean’s uptake of CO2 and its pernicious effect. When the gas dissolves in seawater, it forms carbonic acid that eventually splits into bicarbonate and hydrogen ions, the latter increasing the water’s acidity. Ecologists are concerned that acidification, already 30% worse in surface waters than preindustrial times, will make it more difficult for some phytoplankton, corals, bivalves, and many other species to assemble their shells of calcium carbonate.

Researchers have been testing more than 150 prototype BGC floats in the Southern Ocean since 2014, and the findings are tantalizing. This ocean, encircling Antarctica, is home to two seasonally contrasting CO2 fluxes. In the summer, algae draw down CO2. But in the winter, ancient carbon, stashed away for centuries in deep currents traveling south from the Atlantic and Pacific oceans, wells up, some escaping into the atmosphere. Scientists have long assumed that the carbon absorption dominates, with the Southern Ocean accounting for a significant share of the oceans’ global CO2 storage. But because almost no ships ventured to the stormy, cold Southern Ocean, they couldn’t check whether the winter release really was smaller.

In the past few years, Alison Gray, an oceanographer at the University of Washington, Seattle, and her colleagues have used the trial BGC floats to show that the winter exhalation of CO2 comes close to canceling out the summer’s gains. She is now seeking to understand why the outgassing seems to peak on the Pacific side of the ocean. Surface winds or currents smashing into undersea ridges might sweep up more of the deep waters there. Another explanation is biological: The region’s phytoplankton might be less capable than their Atlantic counterparts at staunching the CO2 outflow.

The BGC floats could also shed light on a less-known ocean trend: a slow drop in oxygen. Since the mid–20th century, it has declined by some 2%. Much of the loss takes place in anoxic dead zones like one in the Gulf of Mexico, where nutrient runoffs from overfertilized lands lead to algal blooms, and, eventually, bacterial surges that use up oxygen in a frenzy of aerobic decomposition, suffocating fish. But researchers have also found that vast tracts of open ocean are losing oxygen because of warming, which limits water’s ability to hold oxygen, and decreased downward mixing. BGC-Argo could reveal the true extent of these minimum zones. “If you eat seafood or care about sea life, you should care about deoxygenation,” Gray says.

The floats aren’t only useful for large-scale trends. For instance, the trial floats discovered massive phytoplankton blooms in the Southern Ocean, far from typical nutrient sources like melting sea ice, suggesting a seafloor hydrothermal vent was providing the nutrients. The floats should also be able to detect short-lived phenomena such as the oxygen loss after a hurricane’s upheaval, or the injection of nutrients for species in the North Atlantic twilight zone, up to 1000 meters down, that come from late-winter plunges of surface water. “The ability to observe these biogeochemical properties in three dimensions is going to be huge,” says Katja Fennel, an oceanographer at Dalhousie University, who is pushing for Canada to add 40 floats to BGC-Argo.

Researchers plan to begin deploying the new floats next year in the equatorial Pacific, where El Niño and La Niña drive large temperature swings every year or two. The floats could show how the swings affect the ability of phytoplankton to soak up carbon, offering clues to how a warming climate will change the ocean’s overall carbon uptake.

After the first 5 years of NSF financing, Johnson and his colleagues would likely need support from the National Oceanic and Atmospheric Administration, which pays for much of the main U.S. Argo fleet. Expanding to 1000 floats, as the team hopes, would require funding from France, Australia, Canada, China, and other countries. At some point, they hope the Argo fleet will be bolstered by 1200 deep Argo floats, which can sink to 6000 meters without crushing. But money for upgrading the fleet, which would remain at 4000 total floats, has been elusive—even though Argo is a relative bargain compared with ships.

If anything, the pandemic has underscored that point, Johnson says. “We’re all sitting at home now,” he says. Yet out in the ocean, the robotic fleet calls home week after week, capturing otherwise invisible changes. “Ten years ago, the ship was the observing platform,” Johnson says. “Now, it’s the tender.”