Explainer: Why Are the World’s Oceans Getting Darker?

The darkening of the ocean is a manifestation of global warming. Research reveals that coastal regions and open seas have become significantly darker. The darkening means sunlight and moonlight penetrate less deeply, reducing the depth of the photic zone, which eventually will intensify competition for resources, threaten marine ecosystems, oxygen production, and ocean’s health. 

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New research from the University of Plymouth found that between 2003 and 2022, 21% of the global ocean, including massive areas of coastal regions and open ocean, have become darker. 

What Is Ocean Darkening?

According to the paper, published in May, the drivers of darkening change depending on the area. 

In coastal areas such as the Baltic Sea, rainfall on land brushes sediment – including rocks, minerals, and the remains of plants and animals – as well as nutrients into the sea. This increased biological activity can reduce light penetration into the water, leading to a darker environment. Meanwhile, in the open ocean, more frequent and intense algal blooms, driven by rising ocean temperatures, can deplete oxygen in the water, reducing light penetration.

Over the past two decades, seawater near coastal regions has changed. According to the study, elevated concentrations of plankton, suspended particulate matter and other optically active constituents that can change the track of light under the ocean can reduce the penetration of natural light, thereby negatively affecting the amount of light available for photobiological processes in the oceans.

In other words, the oceans darken when light finds it harder to penetrate the water. Not only has such a phenomenon impacted plankton’s ability to photosynthesize and produce energy, but the habitats for light-dependent marine life are also shrinking.

Photic Zone

Researchers found that more than 9% of the ocean – 32 million square kilometres, roughly the size of Africa – have seen photic zone depths reduced by over 50 metres (about 165 feet), while 2.6% of the ocean saw a reduction of over 100 metres. 

Photic zone reductions typically occur during daytime, while nighttime reductions were smaller due to lower moonlight irradiances. 

Photic zones are the uppermost layers of a body of water that receive enough sunlight for photosynthesis to occur. 90% of all marine life, including plankton, fish, and other light-dependent organisms, live in the photic zone of the oceans, where sufficient light penetrates to stimulate photobiological processes such as photosynthesis and hydrogen production. The zone is one of the most productive habitats on the planet and fundamental to the maintenance of healthy global biogeochemical cycles. 

“There has been research showing how the surface of the ocean has changed colour over the last 20 years, potentially as a result of changes in plankton communities. But our results provide evidence that such changes cause widespread darkening that reduces the amount of ocean available for animals that rely on the sun and the moon for their survival and reproduction,” said Thomas Davies, Associate Professor of Marine Conservation at the University of Plymouth.

Scientists estimate that about half of the Earth’s oxygen is produced in the oceans. Most of it comes from marine plankton, such as floating plants, algae, and some bacteria that photosynthesize.

More creatures are competing for a limited amount of food in an increasingly smaller space. “If the photic zone is reduced by around 50 metres in large swathes of the ocean, animals that need light will be forced closer to the surface, where they will have to compete for food and the other resources they need,” said Tim Smyth, a professor at Plymouth Marine Laboratory. “That could bring about fundamental changes in the entire marine ecosystem,” Smyth added.

“We rely on the ocean and its photic zones for the air we breathe, the fish we eat, our ability to fight climate change, and for the general health and wellbeing of the planet,” Davies said. “Our findings represent genuine cause for concern.” 

Potential Impact

According to the study, the most significant decreases in photic zone depth were observed at the top of the Gulf Stream and around both the Arctic and Antarctic, where the planet is experiencing the most obvious effects of climate change. The team also found that the North Sea and Celtic Sea, the eastern coasts of England and Scotland, the coastlines of Wales, and the northern elements of the Irish Sea have all become darker over the past two decades.

The scale of loss in photic zone depth caused by ocean darkening over the last 20 years is so profound that the research team said they represent one of the largest losses of habitat on Earth. Ocean darkening will eventually impact carbon storage, which would lead to worse climate change, and affect the livelihood of trillions of people who depend on fish and the sea. 

Phytoplankton play a crucial role in absorbing atmospheric carbon dioxide (CO2) through photosynthesis. When they die, some of this carbon sinks to the seafloor, effectively removing it from the atmosphere and helping to reduce greenhouse gases. However, without favorable conditions for their growth, less CO2 is absorbed and less oxygen is produced, further accelerating global warming.

On the other hand, without enough light to grow, move, hunt, communicate, reproduce, and photosynthesize, marine organisms will continue to migrate into an increasingly smaller area of the ocean. This will expose them to tighter competition for resources and decrease their chances of survival. 
“[Ocean darkening] can disrupt marine food webs, alter species distributions, and weaken the ocean’s capacity to support biodiversity and regulate climate,” said Oliver Zielinski, Director of the Leibniz Institute for Baltic Sea Research in Germany. “Coastal seas, being closest to human activity, are particularly vulnerable, and their resilience is crucial for both ecological health and human wellbeing.”