Why the world's oceans are getting darker
For World Ocean's Day, we're taking a look at how human activity could be contributing to darker oceans, and what that may mean for marine life.
A recent worldwide study led by researchers from the University of Plymouth and Plymouth Marine Laboratory finds that more than 20 per cent of the world’s oceans, equating to about 75 million square kilometres, have gotten darker in the past two decades, a phenomenon known as ocean darkening.
What is ocean darkening?
Ocean darkening refers to changes in water clarity that reduce the depth of the photic zone, i.e., the sunlit layer of the ocean where 90 per cent of marine life lives and where sunlight and moonlight trigger ecological interactions.
For their paper, scientists used satellite and numerical data to track changes in the photic zone between 2003 and 2022.
They found that 21 per cent of the global ocean has darkened, and more than 9 per cent -- or 32 million sq km -- saw the photic zone shrink by more than 50 metres.
In 2.6 per cent of the world's oceans, the depth of the photic zone was reduced by 100 metres.
Significant darkening was found in both coastal waters and major open-ocean areas, particularly, the Gulf Stream, Arctic and Antarctic regions, and coastal and enclosed seas like the Baltic Sea, where agricultural runoff is high.
But not everything is getting dark. Around 10 per cent of the area studied, which is about 37 million sq km, got lighter.
A map focused on the UK and North Atlantic Ocean showing changes identified through this project – red areas indicate regions where the oceans are getting darker, while blues indicate regions where they are getting lighter. Image and caption courtesy of the University of Plymouth
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Human activity contributing to darker oceans
The paper outlines several factors that may be causing oceans to darken.
Near coastlines, runoff from agriculture, increased rainfall, and sediment flow bring nutrients and organic materials into the water. This encourages plankton growth and reduces water clarity.
In the open ocean, algal blooms and rising sea surface temperatures are likely driving ocean darkening.
“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,” Dr. Thomas Davies, Associate Professor of Marine Conservation at the University of Plymouth, says in a statement.
“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 well-being of the planet. Taking all of that into account, our findings represent genuine cause for concern.”
A 50-metre reduction in a photic zone means animals that need light will be forced to congregate closer to the surface, which could result in more competition for food and resources.
"That could bring about fundamental changes in the entire marine ecosystem,” says Professor Tim Smyth, Head of Science for Marine Biogeochemistry and Observations at the Plymouth Marine Laboratory.
The study offers new insight into how our oceans are changing over time, and what that could mean for marine ecosystems.
While the full ecological impact of ocean darkening is unknown, the findings highlight the importance of continued monitoring and research into the connections between ocean light, climate change, and human activity.