One-fifth of global ocean darkened, shrinking vital sunlit zones, study finds

Over the past two decades, more than one-fifth of the global ocean—covering over 75 million square kilometres—has undergone significant darkening, says a new study published by Global Change Biology.

The phenomenon, known as ocean darkening, occurs when the ocean's optical clarity decreases, reducing the depth of its photic zones—the sunlit layers where 90 per cent of marine life thrives, Caliber.Az reports.

The study, conducted by researchers from the University of Plymouth and Plymouth Marine Laboratory, used satellite data from NASA’s Ocean Color Web and numerical models to analyze changes in photic zone depth between 2003 and 2022. They discovered that in addition to the 21 per cent of the ocean becoming darker, more than 9 per cent saw a photic zone depth reduction of over 50 meters, while 2.6 per cent experienced reductions exceeding 100 meters.

Dr. Thomas Davies, associate professor of marine conservation at the University of Plymouth, emphasized the ecological implications:

"There has been research showing how the surface of the ocean has changed color 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."

Researchers believe that coastal darkening is primarily driven by increased runoff from agriculture, sediment, and nutrient loading due to rainfall. In the open ocean, shifting sea surface temperatures and changes in algal bloom dynamics are the likely causes.

While some 10 per cent of ocean regions—equivalent to over 37 million square kilometers—have become lighter, the researchers note that the net trend of reduced light availability poses ecological challenges.

"If the photic zone is reducing by around 50 m 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," Professor Tim Smyth of Plymouth Marine Laboratory noted.

The most significant changes were seen in the Gulf Stream and the polar regions, as well as coastal areas such as the Baltic Sea. Around the UK, both darkening and lightening trends were observed, reflecting a complex and shifting pattern of oceanic change.

By Vafa Guliyeva