Drifting architects: Plankton, climate, and the race to understand our changing ocean

On a sun-drenched morning off the coast of Villefranche-sur-Mer, the Sagitta III cuts through the cobalt waters of the Mediterranean, past the quiet marinas and pine-fringed terraces of France’s Côte d’Azur. The 40-foot scientific vessel – named after a fearsome zooplankton with hook-lined jaws – rumbles toward a lonely yellow buoy bobbing offshore.

In the distance, the resort town shimmers, a mirage of pastel villas and church towers clinging to the cliffs. But aboard the Sagitta III, the romance ends at the rail. Lionel Guidi, a local scientist at the Villefranche Oceanography Lab — known, with fitting Frenchness, by its acronym LOV — peers into the sea with a practiced intensity.

He is here to fish plankton.

“There’s life!” cries marine technician Anthéa Bourhis

Around him, a veteran crew moves with precision, under the iron fist of Captain Jean-Yves Carval. “Plankton is fragile,” cautions the rugged seaman, who’s spent nearly 50 years navigating freighters, trawlers – and now, scientific boats. “If you go too fast, you make compote.”

The craft slows as it reaches the buoy, a sampling site where Guidi and his LOV colleagues have gathered marine data every day for decades. Below deck, the boat’s bearded chief mechanic, Christophe Kieger, readies a large winch. Its 12,000-foot cable unfurls, sending a fine-meshed net – each pore no wider than a grain of salt – drifting toward the deep. Slowly, it sinks to 250 feet.

Minutes later, the net resurfaced, heavy with a brownish, gelatinous goo.

“There’s life!” cries Anthéa Bourhis, a 28-year-old technician from Brittany, as she carefully transfers the contents into a plastic bucket.

Indeed, that catch holds more than seawater and slime. It is the raw material of the planet’s past – and perhaps its future.

A worrisome trend

Plankton form the beating heart of the ocean’s engine. These tiny organisms absorb carbon dioxide, release oxygen, and underpin the entire marine food web. Without them, life as we know it would not exist.

But what is plankton?

It’s not a single creature, but a vast cast of marine nomads, all bound by one trait: they can’t swim against the current. They drift with tides and eddies, riding invisible flows that govern their lives. Some are no bigger than a speck of dust; others, like jellyfish, can stretch more than a meter wide.

There are two main kinds. Those that harness sunlight: phytoplankton — microscopic marine plants that photosynthesize like greenery on land and, over geological time, have produced more than half the oxygen we breathe. And those that feed: zooplankton — tiny animals that graze on their plant-like cousins, hunt each other, and themselves become prey, sustaining fish, whales, and seabirds alike.

At the Villefranche Oceanography Lab, scientists have been tracking these creatures for decades. Their daily sampling, performed just a few miles offshore, has yielded one of the longest continuous records of plankton in the world.

And that record is now showing signs of stress.

“At our observation site, surface temperatures have risen by about 1.5 degrees Celsius over the last 50 years,” Lionel Guidi tells UN News. “We’ve seen a general drop in phytoplankton primary production.”

The consequences could potentially be far-reaching. Phytoplankton form the foundation of the marine ecosystem, and a decline in their numbers might trigger a cascading effect, disrupting zooplankton, fish stocks, and ocean biodiversity as a whole. It could also weaken their ability to absorb carbon dioxide, drawing it from the atmosphere and carrying it into the deep – what scientists call ‘the biological pump’, one of Earth’s most vital natural climate regulators.

Tiny aliens

Back at the LOV, with the Sagitta III now resting in its berth, Lionel Guidi gestures toward the day’s sample. “Everything starts with plankton,” says the scientist, who, before landing in Villefranche, conducted marine research in Texas and Hawaii.

Meanwhile, Anthéa Bourhis, the young technician, has donned a white lab coat and is bent over the morning’s catch. She fixes the sample in formaldehyde, a step that will store the zooplankton but also kill them. “If they move, it messes with the scan,” she explains.

Once morbidly still, the small animals are fed into a scanner. Slowly, shapes blossom on Bourhis’s screen, as improbably graceful copepods – translucent and shrimp-like, with feathery antennae – float into view.

“You look through the microscope and there’s a whole world,” says plankton specialist Lionel Guidi

“We’ve got some good-looking ones,” she says, grinning.

She begins transferring the digital images into an AI-operated database capable of sorting zooplankton by group, family, and species. 

“They have appendages everywhere,” adds Lionel Guidi. “Arms pointing in all directions.”

One of these deep-sea creatures, called Phronima, even inspired the monster in Ridley Scott’s 1979 film Alien. “You look through the microscope,” Guidi says, “and there’s a whole world.”

From science to policy

A world that is changing – and not fast enough to be understood by satellites or snapshots. That’s why LOV’s long-term series matters: it captures trends that span years and even decades, helping scientists distinguish natural cycles from climate-driven shifts.

“When we explain that if there’s no more plankton, there’s no more life in the ocean. And if there’s no more life in the ocean, life on land won’t last much longer either, then suddenly people become a lot more interested in why protecting plankton matters,” said Jean-Olivier Irisson, another plankton specialist at the LOV.

Next week, just 15 minutes down the coast, the city of Nice is hosting the third UN Ocean Conference (UNOC3) – a five-day summit bringing together scientists, diplomats, activists, and business leaders to chart the course for marine conservation.

Among the gathering’s priorities: advancing the ‘30 by 30’ pledge to protect 30 per cent of the ocean by 2030 and bringing the landmark High Seas Treaty, or ‘BBNJ accord’ to safeguard life in international waters, closer to ratification.

Guidi underscored the urgency of these UN-led efforts, saying: “All of this must be thought through with people who are capable of making laws, but based on scientific reasoning.”

He doesn’t claim to write policy himself. But he knows where science fits. “We convey scientific results; we have proof of a phenomenon. These are not opinions, they’re facts.”

And so, in Villefranche, Lionel Guidi, Anthéa Bourhis and Captain Carval continue their work – hauling life from the sea, capturing it in pixels, counting its limbs, and sharing its data with scientists across the globe. In doing so, they chart not just a threatened ocean, but the unseen threads that bind life itself.