Great swathes of the planet’s oceans have become darker in the past two decades, according to researchers who fear the trend will have a severe impact on marine life around the world.
Most marine life thrives in the photic zones of the world’s oceans, the surface layers that allow sufficient light through for organisms to exploit.
Many fish, marine mammals and other creatures hunt, feed and reproduce in the warmer waters of the photic zones where food is most abundant.
Darkening affected 21% of the global ocean in the 20 years to 2022.
In 9% of the ocean, this led to photic zones being 50 metres shallower, while in 2.6% of the ocean, the zones were 100 metres shallower.
According to researchers, large portions of the world’s oceans have darkened over the last 20 years, which they fear will negatively affect marine life worldwide.
According to satellite data and numerical modeling, between 2003 and 2022, over a fifth of the world’s oceans darkened, reducing the area of water where life dependent on sunlight and moonlight can flourish.
Across 75 million square kilometers (30 million square miles) of ocean, which is the same land area as all of Europe, Africa, China, and North America combined, the effect is noticeable. It disrupts the top layer of water, which is home to 90% of marine life.
With potentially serious ramifications for marine ecosystems, global fisheries, and the vital turnover of carbon and nutrients in the oceans, Dr. Thomas Davies, a marine conservationist at the University of Plymouth, called the findings a “genuine cause for concern.”.
The photic zones, or the ocean surface layers that let enough light in for organisms to take advantage of, are where the majority of marine life is found. In actuality, there is not much sunlight below 200 meters, even though it can reach a kilometer beneath the waves.
Photosynthesis is carried out by microscopic organisms called phytoplankton in this upper band of water. Nearly half of the oxygen on Earth is produced by the organisms that support almost all marine food webs. Since food is more plentiful in the warmer waters of the photic zones, many fish, marine mammals, and other animals hunt, eat, and breed there.
Davies and his colleagues determined the depths of photic zones worldwide using satellite data and an algorithm that measures light in seawater. Over the 20 years leading up to 2022, 21% of the world’s oceans experienced darkening. Photic zones became 50 meters shallower in 9% of the ocean as a result, and 100 meters shallower in 2–6% of the ocean. This study’s specifics can be found in Global Change Biology.
When light has a harder time penetrating the oceans, the oceans become darker. It frequently occurs along coastlines where rainfall carries sediments and nutrients from the land into the water and upwellings of cold, nutrient-rich water rise to the surface.
Although the exact causes of darkening far offshore are unknown, changes in ocean currents and global warming are believed to be involved. Davies stated, “The Southern Ocean and up through the Gulf Stream past Greenland appear to be darkening, as these regions are experiencing significant changes in ocean circulation or ocean warming brought on by climate change.”.
According to the study, over the past 20 years, roughly 10% of the ocean, or 37 million sq km, became lighter despite an overall darkening. For instance, the ocean has darkened farther out while a vast expanse off the west coast of Ireland has brightened.
Light serves a multitude of functions for marine life. They use it to hunt, mate, and schedule reproductive activities. “They use it for almost every aspect of their biology,” Davies said. They must ascend the water column as the ocean gets darker, and since there is less room, they are all being compressed closer to the surface. “”.
A “worrying trend,” according to Prof. Oliver Zielinski, director of the Leibniz Institute for Baltic Sea Research in Germany, is the darkening of large ocean areas.
He stated that these kinds of changes have the potential to upset marine food webs, change the distribution of species, and impair the ocean’s ability to sustain biodiversity and control the climate. Because they are closest to human activity, coastal seas are especially vulnerable, and maintaining their resilience is essential for both ecological and human well-being. “.”.
