JUST over a fifth of the ocean has darkened over the past twenty years, research by the University of Plymouth has shown.

Published in Global Change Biology, the research found that between 2003 and 2022, 21 percent of the global ocean – including large expanses of both coastal regions and the open ocean – had become darker.

More than nine percent of the ocean – an area of more than 32million sq km, similar in size to the continent of Africa – had seen photic zone depths reducing by more than 50metres, while 2.6 percent saw the photic zone reduced by more than 100m.

Ocean darkening occurs when changes in the optical properties of the ocean reduce the depth of its photic zones, home to 90 percent of all marine life and places where sunlight and moonlight drive ecological interactions.

The new study saw researchers use a combination of satellite data and numerical modelling to analyse annual changes in the depth of photic zones all over the planet.

While the precise implications of the changes are not wholly clear, the researchers say it could affect huge numbers of the planet’s marine species and the ecosystem services provided by the ocean as a whole.

Professor Tim Smyth, Head of Science for Marine Biogeochemistry and Observations at the Plymouth Marine Laboratory, said: ‘We know the light levels within the water column vary massively over any 24-hour period, and animals whose behaviour is directly influenced by light are far more sensitive to its processes and change.

‘If the photic zone is reducing by around 50m 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’.