Plymouth (United Kingdom) (AFP)

The craft brought back from the water on the deck of the ship, off Plymouth in Great Britain, looks like a small spaceship drawn by a child: it is a device that continuously collects plankton, a Continuous Plankton Recorder ( CPR).

For 90 years, merchant ships and fishing boats have been trailing behind them on their maritime routes across the world to bring back to researchers samples of the smallest inhabitants of the oceans.

Plankton is at the base of the marine food chain, but it also produces a large part of the oxygen we breathe and plays an essential role in the carbon cycle.

As ocean currents change and the distribution areas of marine life change, samples collected by the CPR have shown a clear change in plankton in recent decades.

Planktons living in warmer waters replace those in colder waters, often with different seasonal cycles, forcing the species that feed on them to adapt or leave.

"The big worry is when change happens so quickly that the ecosystem cannot recover," Clare Oster told AFP.

Rising ocean temperatures can lead to "the collapse of entire fisheries," she says as nearly half of humanity depends on fish for their protein intake.

Ecosystem under strain

Plankton brings together aquatic species carried by the current, animals such as jellyfish for zooplankton and plants such as algae for phytoplankton, as well as bacteria and viruses.

A crew member retrieves a device which collects continuous plankton off Plymouth, UK, August 26, 2021. DANIEL LEAL-OLIVAS AFP

Through the process of photosynthesis, which uses the sun's rays to transform carbon dioxide (CO2), phytoplankton allows the oceans to produce about half of the oxygen on Earth, scientists estimate.

It also makes it possible to store at least a quarter of the CO2 emitted by fossil fuels burned by humans.

Phytoplankton are consumed by zooplankton, which are themselves swallowed by predators, from birds to whales.

When phytoplanction and its predators die and sink to the bottom of the sea, they take the carbon they have stored with them.

It is the "biological carbon pump".

But the researchers warn that climate change is putting stress on the ecosystem: as the temperature of the oceans increases, less nutrients from the bottom rise to the surface and the level of acidification of the waters increases.

A researcher analyzes a sample of plankton under a microscope at the Marine Biological Association in Plymouth, UK, August 26, 2021. DANIEL LEAL-OLIVAS AFP

Global warming "exposes ocean and coastal ecosystems to conditions unprecedented for centuries and millennia, with consequences for plants and animals living in oceans around the world," write UN climate experts , the IPCC, in a draft report to be published in 2022.

They warn of "growing impacts on marine life" and believe that deteriorating ocean conditions will lead to a decline in phytoplankton over the next century.

The total average phytoplankton biomass - which is measured by weight or quantity - is expected to drop 1.8-6% depending on the level of emissions.

Map of the different locations of devices continuously collecting plankton samples, at the headquarters of the Marine Biology Association, Plymouth, UK, August 26, 2021 DANIEL LEAL-OLIVAS AFP

Even a modest reduction can affect the food chain and potentially lead to a 5-17% reduction in marine life.

This could alter the carbon sequestration cycle, says Plymouth University scientist Abigail McQuatters-Gollop.

"Blooms"

Sustainable fishing, a reduction in pollutants, especially agricultural pollutants, and a reduction in CO2 emissions are part of the solution.

So far, environmental protection has focused on "creatures that are large, cute or of immediate monetary value", such as whales, turtles or cod, notes McQuatters-Gollop.

But everything depends on the plankton, tiny and yet visible from space during these efflorescences, also called "blooms", which give the sea this emerald or electric blue color.

Satellite image taken by NASA of a "bloom", an efflorescence of plankton, giving the sea an electric blue color, in the Chukchi Sea, between Russia and Alaska, on June 1, 2018. HO NASA / AFP

Like terrestrial plants, phytoplankton need nitrates, phosphates and iron to grow.

But an excess of nutrients can lead to environmental catastrophes, like last summer in Turkey when the coasts were invaded by "sea snot".

A phenomenon that can poison or block sunlight, depriving underwater flora and fauna of oxygen.

At the same time, a study published by the scientific journal Nature concluded that the iron carried by the smoke following the fires in Australia in 2019 and 2020 had triggered a giant plankton bloom from miles away, which may have sucked up a substantial part of it. of CO2 ...

The "blooms" can also be nourished by nutrients conveyed by sandstorms or volcanic eruptions.

Natural phenomenon

This natural phenomenon inspired David King, founder of the Center for Climate Repair in Cambridge, who wants to fertilize planktonic blooms by dispersing iron on the surface of the water.

The beach of Kocaeli, in Turkey, on the Sea of ​​Marmara, invaded by "sea snot", June 12, 2021. Yasin Akgul AFP / Archives

In theory, this should allow more CO2 to be stored and also increase underwater life, even whale populations decimated by hunting.

More whales means more of their faeces, which are rich in nutrients useful for plankton.

David King, who is to test this technique in the Arabian Sea in a fenced area, hopes to restore "a virtuous circular economy" even if the process is debated.

It was not until the 1980s that researchers named the planktonic bacterium prochlorococcus, the main source of photosynthesis in the world.

They could see that some wander, that others live in community.

When planktons leave corals due to warming, the corals become discolored.

When others associate with algae, they transform them into energy generators, notes Johan Decelle, a researcher at CNRS and the University of Grenoble.

Plankton samples recovered from the Atlantic Ocean, exhibited at the Plymouth Marine Biology Association, UK, August 26, 2021. DANIEL LEAL-OLIVAS AFP

Scientists have used them to track climate change or recognize the presence of plastic in the seas.

Much remains to be learned.

"There's an entire galaxy down there," Clare Ostle notes from the boat off Plymouth, where the waters seem calm in the sunlight but every drop teems with life.

© 2021 AFP