Over the past twenty years, a fifth of the ocean has seen its photic zone, the thin “illuminated” band that hosts 90% of marine species, thinning.

Over the last twenty years, the'ocean has become darker. The phenomenon affected an area of ​​over 75 million square kilometers, about 20% of the global ocean surface. Researchers from the University of Plymouth found that the photic zone, which is home to about 90% of marine life thanks to the presence of light, is thinning dangerously in some areas of the world.

The ocean, scientists explain, is more alive and dynamic than we are led to believe. And while the implications of certain phenomena are not yet entirely clear, these subtle changes could have repercussions on a huge number of species, including ours.

The ocean is changing color: the study

According to new research published in Global Change Biology, an area of ​​over 75 millions of square kilometers, which is as large as the entire Indian Ocean, has become darker. Using a combination of satellite data and numerical models, researchers from the University of Plymouth and the Plymouth Marine Laboratory found that between 2003 and 2022, the 21% of the global ocean it went dark.

Over the same period, an area similar in size to the African continent has seen its surface shrink by more than 50 metres. depth of the photic zone, the one where there is enough light for photosynthesis, which hosts about 90% of marine life. In 2,6% of the global ocean, this illuminated zone fundamental for marine ecosystems has thinned by over 100 meters.

But as scientists explain, the picture is more complex: while large areas were becoming darker, others followed the opposite trend. Over the last twenty years, approximately 10% of the ocean – more than 37 million square kilometers – has become clearer.

What is the photic zone and why is it getting thinner?

The amount of light present in the ocean's surface waters directly influences ecological interactions in marine ecosystems. A darker ocean, therefore, could have fundamental implications for marine life – and consequently for ours too.

The “illuminated” areas are a very small part of the ocean: beyond a thousand meters of depth there is a completely dark world, which contains over 70% of the ocean volume and that we have just begun to explore. The euphotic zone, the one in which there is enough light to support photosynthesis, is found within 200 meters deep – and despite representing just 2% of the volume of the global oceans, this thin strip of sea (which corresponds to the epipelagic zone) hosts approximately 90% of marine organisms.

Ocean darkening, the researchers explain, occurs when changes in the optical properties of the ocean reduce the depth of its photic zones, narrowing the narrow band of light on which most of the marine species we know depend, from plankton to whales.

As Professor explains Tim Smith, Scientific Officer for Marine Biogeochemistry and Observations at the Plymouth Marine Laboratory,

“If the photic zone shrinks by about 50 meters across large parts of the ocean, animals that need light will be forced closer to the surface, where they will have to compete for food and other resources they need. This could lead to fundamental changes in the entire marine ecosystem.”

Darker Ocean: Research Finds

The researchers used satellite data from theOcean Colour Web NASA's 9km-wide pixel array, which divides the global ocean into a series of XNUMXkm pixels, allowed them to observe changes on the ocean surface with extreme precision. Using an algorithm that measures light in seawater, they then defined the depth of the photic zone at each location.

I more obvious changes are those observed in the North Atlantic, around the Arctic and in Antarctica: these areas, scientists explain, are undergoing the phenomenon in a more pronounced manner due to climate change. The darkening is also widespread in the coastal regions And in the closed seas, such as the Baltic Sea: here, rainfall transports sediment and nutrients from the land to the sea, stimulating plankton growth and reducing the availability of light.

In the United Kingdom, explain the researchers from Plymouth, the picture is contrasting: While areas of the North Sea and Celtic Sea, the eastern coasts of England and Scotland, the coasts of Wales and northern elements of the Irish Sea have become darker over the last two decades, the English Channel and areas north of Orkney and Shetland have become brighter.

The possible impact on ecosystems

According to the researchers, the changes near the coast are likely the result of a increase in nutrients, organic material and sediments caused by factors such as agricultural runoff and increased precipitation. In the open ocean, however, changes are due to phenomena ranging from the dynamics of algal blooms to the variations of the sea ​​surface temperature, which have reduced the penetration of light into surface waters.

While the precise implications of this darkening of the ocean are not entirely clear, researchers say it could have repercussions on a huge number of marine species, and not only. As Dr. Thomas davies, Associate Professor of Marine Conservation,

“Our results demonstrate that these changes are causing widespread darkening that reduces the amount of ocean available to animals that depend on the sun and moon for survival and reproduction. We also depend on the ocean and its photic zones for the air we breathe, the fish we eat, our ability to combat climate change, and the overall health and well-being of the planet. With all this in mind, our results are a serious cause for concern.”

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