Every night, discreetly, the largest migration on the planet takes place in the oceans. Billions of tiny creatures ranging from zooplankton to lanternfish to krill rise together from the depths to layers closer to the surface. The comings and goings of the Sun and Moon, or at least the little light they can perceive at the bottom of the water, dictate their behavior. Except that in recent years, large areas of the ocean surface have mysteriously darkened. The Atlantic, the Pacific side of South America and the coast of Antarctica would be the most affected.
“We have known for some time that coastal waters are getting darkerexplains Tim Smyth to New Scientist. What is new is that we also observe this trend in the open sea.” With his colleagues, this scientist from the Marine Laboratory in Plymouth (United Kingdom) discovered in 2024 that the pelagic zone of the oceans, i.e. the water column above the bottom, also lets less light filter than before. Analysis of twenty years of satellite photos shows that a fifth of the world’s oceans have darkened, a phenomenon probably linked to the proliferation of phytoplankton, itself due to global warming. Explanation.
Over the years, temperatures rise, marine heat waves become more frequent, and salinity is disturbed in certain regions. These changes influence global ocean circulation. “My hypothesis is that we are looking at a complex interaction between a modification of this circulation on a global scale and some more localized meteorological changes, such as stronger sunshine and more stable surface waters here or there.believes Tim Smyth. All this promotes the growth of phytoplankton and therefore the darkening of the oceans.
This means in concrete terms that in darker regions, the habitable zones which form in the different layers of the ocean shrink vertically by several tens of meters. Imagine the population of Paris crowded into the Buttes-Chaumont park. “If you compress the ability of organisms to grow, move, hunt, communicate, reproduce and photosynthesize into a smaller space, competition for resources becomes tougher.”adds the oceanologist.
The ocean remains extremely resilient
Take zooplankton. Where phytoplankton is at the bottom of the food chain, zooplankton is just above it. Some are the basis of the fish diet. Every day, they migrate vertically, changing layers of the ocean. They descend hundreds of meters from the water column to escape predators that see them in daylight. Then, at night, they return to the surface to feed. It is the largest biomass migration in the world. Several gigatons of zooplankton, or 10 quintillion individuals, make the journey every day.
This migration, and therefore marine ecosystems, will change drastically if light no longer passes through the oceans as deeply. The zooplankton will descend less far since the light will become scarcer more quickly. “In the short term, some predators will be able to hunt more easily, spending less energy tracking down their prey”imagines Tim Smyth. All of this will have cascading effects, both for the marine food chain and for human fishing, which are difficult to envisage today.
The phenomenon is not irreversible, especially near the coasts. The waters there are particularly sensitive to coastal development, particularly agriculture. Fertilizers, soil and organic matter, carried from fields to rivers by runoff, end up in the oceans, where they increase the rate of light-absorbing material in the water. In the open ocean, on the other hand, it is much more difficult to adjust the settings that darken the water. Even if human pollution dropped to zero tomorrow, the oceans would take decades, if not centuries, to clean up.
Still, the oceans have a remarkable capacity for self-healing. “Experience shows that it is enough to leave a marine ecosystem in peace for it to regenerate at a sometimes surprising speed”notes Tim Smyth again. Which is good news for everyone. The oceans cover 70% of the earth’s surface, regulate the climate and absorb enormous quantities of carbon and heat. This is why the scientific community is pushing massively to expand protected maritime zones. Saving wildlife is not the only issue; the oceans are our life insurance.
