The ocean is losing its light, and it is changing everything below the surface

Science · Published 7 October 2026 Swipe more like this →

In short: A massive portion of the global ocean is becoming darker. This shift in light penetration is shrinking the photic zone—the sunlit upper layer that supports 90% of all marine life—potentially forcing species into dangerous competition for resources near the surface.

A study in Global Change Biology shows that 21% of the world's oceans grew darker from 2003 to 2022. This phenomenon occurs when changes in the water's optical properties reduce the depth of the photic zone. This sunlit layer is vital for the planet, underpinning essential processes like carbon cycling and oxygen production.

The scale of this change is vast. More than 9% of the ocean—an area exceeding 32 million sq km, roughly the size of Africa—saw photic zone depths drop by more than 50 metres. Even more dramatically, 2.6% of the ocean saw reductions of over 100 metres. Interestingly, the trend is not universal; about 10% of the ocean, spanning 37 million sq km, has actually become lighter over the last 20 years.

The drivers of this darkening depend on the location. In coastal regions, scientists point to increased rainfall, sediment loading, and agricultural runoff that introduces nutrients and organic material into the water. In the open ocean, the cause is likely linked to shifting sea surface temperatures and changes in the dynamics of algal blooms, both of which reduce how deeply sunlight can penetrate.

Scientists from Plymouth Marine Laboratory and the University of Plymouth warn that these shifts could fundamentally alter ecosystems, though exact results remain under investigation. As the light-dependent zone shrinks, animals that rely on light for behavior and feeding may be forced into shallower waters, increasing competition for food and other critical resources.

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