A creeping darkness is settling over the ocean’s photic zone. While the full consequences are not yet clear, scientists say the trend is real and “a genuine cause for concern.”
The photic zone is the wonderful in-between world where sunlight provides energy to the ocean before it plunges into absolute darkness. Even at night, the photic zone is dazzled by moonlight, guiding the behavior of countless creatures and lifeforms.
It’s home to around 90 percent of all marine life and provides the foundation of the ecological interaction that underpins much of the ocean. But in a new study, scientists have identified a worrying trend: this vibrant zone has become increasingly dim and lightless in just a few decades.
Scientists at the University of Plymouth and Plymouth Marine Laboratory used satellite data and numerical modeling to analyze changes in the depth of photic zones all over the planet.
They found that around 21 percent of the global sea surface had become darker between 2003 and 2022. This includes both coastal regions and, perhaps more unexpectedly, areas of open ocean.
Along the coasts, increased flushes of nutrients, organic matter, and sediments – likely from agricultural runoff and more intense rainfall – are clouding the water. In the open ocean, changes in algal bloom patterns and rising sea surface temperatures appear to be playing the key role.
Changes in global photic zones between 2003 and 2022 are shown with red areas to indicate ocean darkening and blue lightening.
Image credit: Thomas Davies/University of Plymouth
The darkening acts like a hazy veil over the ocean, reducing the amount of light that can penetrate to deeper waters. As a result, the photic zone – the sunlit upper layer of the ocean where photosynthesis can occur – has been growing increasingly shallow in many regions. In about 9 percent of the ocean, the depth of the photic zone has decreased by more than 50 meters (164 feet), and in 2.6 percent of the ocean, it has shrunk by over 100 meters (328 feet).
Small parts of the oceans have been seeing the opposite, though. The team’s work suggests that around 10 percent of the ocean surface has actually become lighter over the past 20 years.
These are all important changes to study as the photic zone is where sunlight supports photosynthesis, the base of the marine food chain. With less light, there could be less life, or at least some significant changes to it.
Deep-sea ecosystems that exist below the photic zone in pitch darkness can draw upon chemosynthesis, a process where organisms use chemical energy from sources like hydrothermal vents to produce food. However, even the darkest portions of the ocean will still depend on the continuous rain of organic material generated in sunlit waters. A dead whale, for instance, could support a deep-sea ecosystem for years, if not decades.
But as surface waters grow darker and less productive, that vital supply of nutrients could dry up, threatening even the most remote corners of the marine world – and even life on the surface.
“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,” study author Dr Thomas Davies, Associate Professor of Marine Conservation at the University of Plymouth, said in a statement.
“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,” he added.
The study is published in the journal Global Change Biology.
Source Link: The World's Oceans Are Getting Darker, Raising "A Genuine Cause For Concern"
