‘Over 75 million sq km’: Scientists warn darkening of ocean threatening survival of marine species

Deep under the waters, a silent change is taking place. Over the past two decades, scientists have detected a steady dimming in vast swaths of the world’s oceans — a shift that could disrupt marine life from the seafloor to our dinner plates. As sunlight’s reach into the sea shrinks, so does the living space for countless creatures that depend on light to survive.

Researchers at the University of Plymouth and the Plymouth Marine Laboratory analyzed ocean data from 2003 to 2022. Their findings reveal a startling trend: more than 21% of the world’s oceans are darkening. That’s over 75 million square kilometres — an expanse larger than any single continent.

The team used satellite imaging and computer models to study how light penetrates the ocean. They focused on the photic zone — the sunlit layer that supports much of marine life. In 9% of ocean areas, this zone has shrunk by more than 50 metres. In another 2.6%, it has thinned by over 100 metres.

While up to 10% of ocean regions have slightly brightened, the dominant trend is dimming. This limits visibility and reduces the ability of marine species to find food, reproduce, and thrive. “Loss of light prevents many animals from surviving,” said Thomas Davies, a marine conservation specialist. “As the lit area gets smaller, organisms are driven to the surface, where the food is more limited.”

Causes of the darkening differ by region. Along coastlines, fertiliser runoff and heavy rains stir up sediments, spurring plankton blooms that cloud the water. In the open ocean, factors like shifting algal blooms and warming temperatures are contributing to the reduced sunlight.

Tim Smyth, professor at Plymouth Marine Laboratory, warned that shrinking photic zones could unbalance entire marine ecosystems. “When animals are pushed up, they fight more for food and space,” he said. This crowding can shift fish populations, disrupt the food web, and even affect how oceans absorb carbon.

The research underscores a growing need to monitor this stealthy transformation. Ocean darkening may be invisible from land, but its consequences could ripple through climate systems and global food chains.