Oceans around the planet are darkening – what’s going on?

At dusk, Earth’s largest biomass migration across the oceans takes place almost silently, mostly unnoticed. Trillions of often tiny creatures – zooplankton, krill, lamprey – rise synchronously from the depths, drawn by blooms of phytoplankton in the uppermost layers of water. They feast overnight, safe from predators who hunt by sight, and then retreat at sunrise.

The ebb and flow of the sun and moon dictate the behavior of many sea creatures. But in recent decades, large areas of the ocean’s surface have mysteriously darkened. Tim Smyth, a marine scientist at Plymouth Marine Laboratory in the United Kingdom, and his colleagues were the first researchers to notice this pattern in the open ocean last year. Since then, he has continued to study how the oceans are shifting in response to global warming along with changes in land use—and the key role that light plays in these habitats.

Smyth says The new scientist about the origins of ocean darkening, the implications for marine ecosystems, and what can be done to allow more light to penetrate the surface layers of the oceans.

Thomas Lewton: How did you find out that large swathes of the ocean are darkening?

Tim Smith: We first approached this problem from an unexpected direction. I have been working together for the last decade Tom Daviesmarine conservation scientist at the University of Plymouth to understand the effects of artificial light pollution at night. In this work, we analyzed 20 years of global satellite data to track changes in the optical properties of the ocean. To our surprise, we found consistent patterns of darkening, indicating that surface waters are more opaque to incoming light. Rather than random patches scattered across the global ocean, these changes form large, connected areas. Overall, we found that roughly a fifth of the world’s oceans have darkened in some way.

Why is the ocean darkening?

In coastal areas, the darkening of the ocean is closely linked to changes in the rivers that flow into the sea. Along the coast, changes in land use affect what is dissolved or suspended in the water, which in turn changes the optical quality of the water entering the ocean. For example, when a landscape changes from forested to agricultural, it affects how materials are washed into rivers. During floods, rivers carry much more suspended particles and much higher levels of colored dissolved organic matter, the stuff that gives rivers their “soaked tea” color.

Another key factor in coastal ocean darkening is nutrient loading. Fertilizers used in industrial agriculture are washed into rivers and stimulate the growth of phytoplankton. When phytoplankton blooms, it reduces how deep light can penetrate into the water column. We’ve known for some time that coastal waters are darkening, but what we’ve now discovered is that these changes extend beyond the coast: there’s also a wider darkening of the open ocean.

What is causing the changes in the open ocean?

They may be linked to shifts in phytoplankton blooms caused by climate change. Globally, we are witnessing rising ocean temperatures, more frequent sea heat waves and changes in salinity in some regions. Together, these changes affect large-scale ocean circulation patterns.

Phytoplankton blooms themselves depend on a combination of light, nutrients, temperature and the vertical structure of the water column. In winter, the open ocean is usually well-mixed by storms that sweep over the surface. With the arrival of spring, however, more stable surface layers begin to form. These stratified surface layers limit vertical mixing, concentrating light and nutrients in the upper ocean where phytoplankton can grow most efficiently.

I suspect we are seeing a complex interplay of altered global circulation patterns and more localized weather changes, such as sunny conditions and increasingly stable surface waters, all of which promote phytoplankton growth and contribute to a broader darkening of the open ocean.

How does ocean darkening affect marine ecosystems?

It helps to think about the different levels in the ocean of the food web. At the lowest level are the primary producers, phytoplankton, which can be one of the causes of darkening. Then the next level up are zooplankton, such as Calanus copepods that fish feed on. Calanus copepods are really important because they are at the center of the first part of the food web. They perform what is known as part of a vertical migration, moving hundreds of meters up and down in the water column each day.

During the day, they descend to depths of 200 to 300 meters, where light levels are much lower, making it difficult for visual predators of their prey, and then return to the surface at night to feed.

It is the largest migration of biomass on the planet. When you think of the seasonal migration of species on the planet, you immediately think of David Attenborough’s wonderful narrative about the wildebeest in the Serengeti. About several million wildebeest migrate in the Serengeti. But what we see in the oceans is a far, far larger—but largely invisible—migration of zooplankton on a scale that dwarfs that of wildebeest. Several gigatons of zooplankton, about 10 quintillion individuals, do this every day.

So what happens to these creatures if the light doesn’t penetrate so deep into the water?

The overall effect, where you have areas of darkening, is that we are vertically compressing the usable surface ocean habitat by tens if not hundreds of meters – a bit like squeezing the population of London into the size of Hyde Park. If you compress the ability of organisms to grow, move, hunt, communicate, reproduce, and photosynthesize into a smaller area, then competition for resources becomes acute. In the short term, some species may be easier prey because they can expend less energy hunting the quarry. All of this has a knock-on effect on things like food webs and global fisheries – although we don’t yet know what the wider consequences will be.

Fish that rely on vision to hunt, from small schooling species to larger predators such as tuna, may also find their hunting grounds compressed closer to the surface. Meanwhile, phytoplankton—microscopic plant-like organisms that are at the base of the marine food chain and produce about half the oxygen we breathe—can find depths at which photosynthesis can shift as the ocean darkens.

Is ocean darkening still a problem at night?

Yes. Daylight isn’t the whole story. We also looked at what happens under the moonlight. To the human eye, the sea appears almost completely black at night, but for many marine animals, the faint glow of the moon is surprisingly important. It helps guide nocturnal migration, signaling when it’s safe to climb to the surface to feed and when it’s safe to slip back into the darkness below.

Our lunar modeling suggests that as the ocean becomes darker, this faint light struggles to penetrate the water. The result could be a subtle but meaningful shift in the undersea nightscape: the thin layer of ocean illuminated by moonlight may become shallower. For creatures that depend on these subtle light cues that could push their nocturnal world closer to the surface, potentially reshaping who meets whom in the dark.

What are the global implications of all these changes?

Ocean darkening also depends on things like carbon cycling. If zooplankton don’t go as deep as they used to to avoid predation because light levels are limited to this highest level, it means they aren’t as efficient at removing carbon from the atmosphere. When zooplankton die, they sink to the bottom of the ocean and lock the carbon stored in their bodies away, but if they don’t go to such great depths, then they are less able to transport carbon to the deep ocean. Instead, it is likely to stay more in the upper layers, where it can be breathed back into the atmosphere rather than being locked away for decades or centuries.

But managing the export of carbon from the illuminated upper layers of the ocean to the seafloor is challenging. Satellites give us this fantastic global scale, but really only on the surface. We have perhaps only a handful of sufficiently long-term field observations that measure carbon precipitation from the top of the water column to the seafloor.

Can anything be done to reverse the darkening of the ocean?

In some places yes. Coastal waters are particularly sensitive to what happens on land, especially agriculture. Fertilizers, soil and organic matter washed off fields can end up in rivers and eventually the sea, where they increase the amount of light-absorbing material in the water. This means that improving the way we manage the land could help restore some of the cleanliness of the coast. One effort to tackle this is the AgZero+ programme, led by the UK’s Center for Ecology and Hydrology, which brings together scientists and farmers to develop low-pollution, climate-neutral farming systems that reduce runoff while protecting soil, biodiversity and water quality. The project is testing approaches such as smarter use of fertiliser, nature-based solutions such as agroforestry and better river basin management to move water – and the nutrients it carries – more slowly from land to sea. These kinds of changes could help limit darkening in coastal waters.

On the open ocean, however, the fight with drivers is much more difficult. Even if global emissions dropped to net zero tomorrow, it would take decades, if not centuries, for the ocean to respond.

Is there still hope for the oceans?

Absolutely. One of the most encouraging discoveries of recent years is how resilient the ocean can be when given the chance. Marine ecosystems can recover surprisingly quickly when key species and habitats are protected. Take the kelp forests along the California coast. After intense marine heat waves between 2014 and 2016, scientists discovered that the algae was growing in well-managed marine protected areas [regions of the ocean established to safeguard habitats and species] bounced back faster than the forests beyond. Where predators, grazing and other ecological relationships remained intact, underwater forests were better able to bounce back.

This is one of the reasons why there is now a global push to expand marine protected areas. When properly enforced, they act as ecological breathing spaces that allow marine life to recover and ecosystems to restore their natural balance. In a warming world, they can also help ecosystems withstand climate shocks such as heat waves.

So yes – there is reason for optimism. The ocean still has a remarkable ability to heal itself. Give marine ecosystems some room to recover and they often respond surprisingly quickly. And that is important for all of us. Oceans cover about 70 percent of the Earth’s surface, regulate our climate, and absorb vast amounts of carbon and heat. Protecting them is not just about saving wildlife, it’s about protecting the planet’s life support system.

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