The world’s oceans are losing light – and it could change marine life as we know it, according to a new study.
The research found that over the past two decades, more than 21% of the global ocean has become darker.
This change affects the photic zone – the upper layer of the ocean that sunlight penetrates – where around 90% of marine life lives.
The loss of light in these waters may have far-reaching impacts on marine ecosystems, as well as the humans who rely on them.
The study, recently published in the journal Global Change Biology, was led by scientists from the University of Plymouth and Plymouth Marine Laboratory.
Is the ocean getting darker?
By analysing nearly two decades of satellite data alongside advanced ocean modelling, researchers found that the depth of sunlit ocean waters has decreased in many parts of the world. In some areas, the depth of the photic zone has reduced by more than 100 metres.
According to the study, 21% of the global ocean – over 75 million square kilometres – has seen a decline in light penetration between 2003 and 2022.
Around 9% of the ocean has lost over 50 metres of its photic depth, and 2.6% has lost more than 100 metres.
However, the pattern isn't entirely one of loss. Around 10% of the ocean – equivalent to more than 37 million square kilometres – has become lighter over the same period.
This mixed picture reflects the complexity of ocean systems and the many factors that influence water clarity, say the authors of the study.
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Why is the ocean getting darker?
The reasons for ocean darkening vary by location, explain the researchers. In coastal areas, increased sediment and nutrient runoff from agriculture and heavier rainfall due to climate change are likely culprits. These pollutants can feed blooms of plankton that reduce the amount of light penetrating the water.
In the open ocean, darkening may be linked to changes in plankton communities, driven by shifting sea surface temperatures and nutrient levels.
Where is ocean darkening happening the most?
Geographically, some of the most significant changes were observed in the Arctic and Antarctic regions, and in areas influenced by the Gulf Stream. These are all regions under strong pressure from climate change, further linking ocean darkening to global environmental shifts.
Darkening was also widespread in enclosed seas such as the Baltic, where river runoff carries nutrients and sediments from land, further limiting light in the water.
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What does this mean for marine life?
The photic zone is vital to ocean life. It's where sunlight powers photosynthesis in microscopic plants known as phytoplankton, which form the base of the marine food chain. It also supports the behaviours of countless species that rely on light for survival and reproduction.
A shrinking photic zone could force animals to move closer to the surface, where competition for resources is greater – and may even alter entire ecosystems.
“We also 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,” says co-author Dr Thomas Davies, associate professor of marine conservation at the University of Plymouth. “Taking all of that into account, our findings represent genuine cause for concern.”
Professor Tim Smyth, who also co-authored the study, adds: “The ocean is far more dynamic than it is often given credit for,” says Smyth. “For example, we know the light levels within the water column vary massively over any 24-hour period, and animals whose behaviour is directly influenced by light are far more sensitive to its processes and change.
“If the photic zone is reducing by around 50m 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. That could bring about fundamental changes in the entire marine ecosystem.”
What about light levels at night?
Interestingly, while night-time changes in light levels were generally smaller than those during the day, they were still found to be ecologically significant.
Some marine species, especially those active at night, are particularly sensitive to even subtle changes in light levels.
How exactly was the study carried out?
To track changes in light levels, the researchers used NASA’s Ocean Colour Web data, which divides the ocean into 9km-wide pixels, offering a detailed global view of surface conditions.
An algorithm was then used to estimate the depth of the photic zone across each pixel, combining satellite data with solar and lunar light models.
Find out more about the study: Darkening of the Global Ocean
Main image: dark ocean. Credit: Getty
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