Is the ocean getting darker?

More than one-fifth of the global ocean – an area spanning more than 75million sq km – has been the subject of ocean darkening over the past two decades, according to new research.

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.

For the new study, published in Global Change Biology, researchers used a combination of satellite data and numerical modelling to analyse annual changes in the depth of photic zones all over the planet.

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

However, the picture is not solely of a darkening ocean with around 10% of the ocean – more than 37million sq km – becoming lighter over the past 20 years.

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.

The study was conducted by researchers from the University of Plymouth and Plymouth Marine Laboratory, who have spent more than a decade examining the impact of artificial light at night (ALAN) on the world’s coasts and oceans.

They say that is not directly connected to ocean darkening, however, with the changes likely being as a result of a combination of nutrient, organic material and sediment loading near the coasts, caused by factors such as agricultural runoff and increased rainfall.

In the open ocean, they believe it will be down to factors such as changes in algal bloom dynamics and shifts in sea surface temperatures, which have reduced light penetration into surface waters.

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.”

Professor Tim Smyth, Head of Science for Marine Biogeochemistry and Observations at the Plymouth Marine Laboratory, added: “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 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.”

Assessing changes in the ocean’s photic zones

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 derived 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 depth of the photic zone in each location.

They also used solar and lunar irradiance models to examine particular changes that might impact marine species during daylight and moonlight conditions, demonstrating that changes in photic zone depth at night were small compared to daytime, but remained ecologically important.

A shifting global picture of ocean change

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.