Deep-sea robots are scoping out the secret origins of algae blooms

“They work everywhere, any time.”

A phytoplankton bloom in the Barents Sea.
A phytoplankton bloom in the Barents Sea.NASA image by Jeff Schmaltz and Joshua Stevens

Every spring, the North Atlantic Ocean sees an explosion of tiny, free-floating algae that form the base of the ocean's food chain. While these microorganisms are turning themselves into fish food, they also are gobbling up enormous amounts of carbon dioxide, making them one of nature's key warriors in the fight against climate change.

But scientists have never understood exactly what triggers the annual bloom. It’s a puzzle that has stymied oceanographers for decades. For many years, the traditional way of studying these blooms was via satellites. But that doesn’t work when clouds are overhead, meaning researchers were frequently in the dark.

Now, however, thanks to a new generation of sophisticated sea robots, researchers finally can learn about algae — and other things too.

Satellite image of a bloom in the North Atlantic.
Satellite image of a bloom in the North Atlantic.NASA’s Earth Observatory

The devices, known as "biogeochemical profiling floats," or BGC-Argo floats, can operate more than a mile below the ocean's surface, making it possible to gather data never before collected. These robots are a souped-up version of a previous Argo float.

“The data collected with the floats have been mainly used to improve our understanding of ocean ecosystem and biogeochemical processes,” said Hervé Claustre, a senior scientist with the French National Centre for Scientific Research. “This is particularly important for improving theories and models of the Earth’s carbon system.” The data will used “to monitor and forecast the changing biochemical state of the ocean,” he said.

Understanding the factors that spur the bloom is vital to understanding how algae, which soak up vast sums carbon dioxide, will respond to climate change, explained Alexandre Mignot, a postdoctoral researcher at the Laboratoire d’Océanographie de Villefranche.

Scientists deploy the BGC-Argo float.
Scientists deploy the BGC-Argo float.Léo Lacour

The North Atlantic Ocean punches far above its weight when it comes to scrubbing carbon dioxide. While it accounts for less than 1.5 percent of the total surface area of the world’s oceans, it captures about 20 percent of the CO2 sequestered by the seas. Cold ocean waters help trap planet-warming carbon dioxide lingering in the atmosphere. Then, algae soak up that carbon dioxide during photosynthesis, just like grasses and trees do on land.

From the information collected thus far, the scientists have concluded that the spring bloom is preceded by a “winter simmer,” when algae tend to lay low. The data suggest algae actually can grow — albeit modestly — during the winter during periods of relative calm in the water. While turbulent waters scatter the sun’s rays, tranquil waters allow light to get through, spurring the growth of algae.

"These local blooms lasting a few days could be the starting point for the explosive spring blooms a few months later," Mignot said. Researchers described their work in two papers — one study, published in Nature Communications, and a second study that appeared in Nature Geoscience.

A BGC-Argo float.
A BGC-Argo float.David Lucquet, IMEV.

A typical float spends most of its time 1,000 meters below the ocean’s surface in “sleeping mode,” drifting with the currents. It wakes up every five or ten days to peak its head above water, collecting data on its way up to the surface. Because the floats carry multiple sensors, they can gather information about carbon captured by the ocean, as well as the acidity and oxygen content of ocean waters.

A recently released study, in fact, brought disturbing news about plummeting ocean oxygen levels, saying the amount of water in the open ocean without any oxygen at all has increased more than fourfold during the last 50 years. The problem also has afflicted coastal waters — the same study said that low-oxygen sites have increased more than tenfold in coastal areas since 1950. Moreover, the trend is expected to continue. The paper predicted that oxygen will continue to drop as the Earth warms.

Low and declining oxygen levels in the open ocean and coastal waters affect processes ranging from biogeochemistry to food security.
Low and declining oxygen levels in the open ocean and coastal waters affect processes ranging from biogeochemistry to food security. The global map indicates coastal sites where anthropogenic nutrients have exacerbated or caused O2 declines to <2 mg liter−1 (<63 μmol liter−1) (red dots), as well as ocean oxygen-minimum zones at 300 m of depth (blue shaded regions).Science

“The oxygen concentration is the variable that is the most sampled by the BGC-Argo floats,” Claustre said. The devices are vital to measuring the expansion of areas depleted of oxygen.

Data provided by the robot-powered ocean observation system likely will help commercial fisheries — spring blooms are the foundation for the marine food chain that nourishes the fish wrangled for human consumption. It will also help scientists better understand how climate change impacts marine ecosystems. Even better, “they work everywhere, any time and whatever sea state,” Mignot said.

Marlene Cimons writes for Nexus Media, a syndicated newswire covering climate, energy, policy, art and culture.