21% of Earth's oceans are mysteriously getting darker: Here's why
28 May 2025

A recent study by scientists from the University of Plymouth and Plymouth Marine Laboratory has revealed that more than 21% of the world's oceans have become significantly darker in the last two decades.

The research, published in Global Change Biology, analyzed satellite data and advanced numerical models from 2003 to 2022.

It found that changes in ocean optical properties are shallowing photic zones, the sunlit layers supporting 90% of marine life.

Photic zones are getting shallower, threatening marine life
Impact

The study found that over 9% of the ocean, an area roughly the size of Africa, has seen photic zone depths decrease by more than 50 meters.

In some areas, this reduction exceeds 100 meters.

Although around 10% of the ocean has become lighter, the overall trend is toward darkening, raising concerns for marine species that rely on light for survival and reproduction.

Ocean darkening: A result of nutrient runoff and climate change
Causes

The researchers attribute coastal darkening mainly to increased nutrient and sediment runoff from agriculture and rainfall, which promote plankton growth and reduce light penetration.

In the open ocean, changes in algal bloom dynamics and rising sea surface temperatures are likely contributors.

Professor Tim Smyth, Plymouth Marine Laboratory, stressed that the reduction of the photic zone by around 50m in large swathes of the ocean will force animals that need light closer to the surface, where they will have to compete.

Most prominent changes seen in Gulf Stream, Arctic
Ecosystem impact

The most noticeable changes in photic zone depth in the open ocean were found near the top of the Gulf Stream, as well as in the Arctic and Antarctic, where climate change is having the greatest impact.

Darkening was also observed in coastal locations and enclosed seas, such as the Baltic Sea, where rainfall on land transports silt and nutrients into the sea, promoting plankton development while decreasing light availability.