More than one-fifth of the global ocean – an area spanning more than 75 million square kilometres – has been the subject of ‘ocean darkening’ over the past two decades, a process researchers suggest could affect huge numbers of marine species and ecosystems.

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

Conducted by researchers from the University of Plymouth and Plymouth Marine Labs, a new study has now combined satellite data and numerical modelling to analyse the annual changes in the depth of photic zones all over the planet.

It’s found that between 2003 and 2022, 21% of the global ocean – including large expanses of both coastal regions and the open ocean – had become darker.

In addition to this, more than 9% of the ocean – an area of more than 32 million square kilometres (similar in size to the continent of Africa) had seen photic zone depths reduce by more than 50 metres, while 2.6% saw the photic zone reduce by more than 100 metres.

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

The researchers are also quick to add, that the picture of the ocean is not solely of this darkening. In fact, around 10% of the ocean – more than 37 million square kilometres – has become lighter over the last 20 years.

Among factors such as agricultural runoff and increased rainfall, scientists have put these changes observed over the last two decades potentially down to changes in plankton communities.

Dr Thomas Davies, associate professor of marine conservation at the University of Plymouth, said: “There has been research showing how the surface of the ocean has changed colour 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.

“We also rely on the ocean and its photic zones for the air we breathe, the fish we eat, our ability to fight climate change, and for the general health and wellbeing of the planet. Taking all of that into account, our findings represent genuine cause for concern.”

To assess changes in the photic zone, the researchers used data from NASA’s Ocean Colour Web, which breaks the global ocean down into a series of 9km pixels. This satellite data enabled them to observe changes on the ocean surface for each of these pixels, while an algorithm developed to measure light in sea water was used to define the depths of the photic zone in each location.

The most prominent changes in photic zone depth in the open ocean were observed at the top of the Gulf Stream, and around both the Arctic and Antarctic; areas of the planet experiencing the most pronounced shifts as a result of climate change.

Darkening is also widespread in coastal regions and enclosed seas – such as the Baltic Sea – where rainfall on land brings sediment and nutrients into the sea, stimulating plankton growth and reducing light availability.

Around the UK, the picture was mixed.

The study indicated that areas of the North Sea and Celtic Sea, the eastern coasts of England and Scotland, the coastline of Wales, and the northern elements of the Irish Sea have all become darker over the past two decades.

However, much of the English Channel and regions stretching from the north of Scotland to the Orkney and SHetland Islands have become lighter.

Professor Tim Smyth, head of science for marine biogeochemictry and observations at the Plymouth Marine Laboratory, said: “The ocean is far more dynamic than it is often given credit for. For example, 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 50 metres 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.”

The full study: ‘Darkening of the Global Ocean’ has now been published in the scientific journal, Global Change Biology. 

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