Almost all ocean life depends on the upper layers of water where sunlight reaches, called the photic zone. A new study shows that this narrow window of light is shrinking across the world’s oceans. Thomas Davis, a marine biologist at the University of Plymouth, and Tim Smith, a marine biogeochemist at the University of Exeter in the UK, pointed out that although scientists have worried about ocean darkening, nobody had yet measured how widespread the problem really is. Earlier work already showed that over the past 20 years the color of the ocean surface has changed, likely because plankton communities have shifted. Davis says the new study indicates those changes have led to broad darkening. As a result, the portion of the ocean available to animals whose survival and reproduction depend on the sun and moonlight has decreased substantially.
What else did the scientists find? By combining NASA satellite data from 2003 to 2022 with numerical modeling, the authors identified a clear pattern: the depth of the photic zone, which sets the habitat for most ocean organisms, is gradually shrinking. The photic zone is the upper ocean layer where sunlight (including light reflected from the moon) penetrates the water. All photosynthetic marine organisms — seagrasses, algae, and especially phytoplankton — must live in the shallower parts of this layer to capture solar energy. Many other species that rely on those primary producers — corals, crustaceans, fish, and marine mammals — therefore also live in the photic zone.
The study found that in nearly 10% of the world’s oceans, the photic zone depth has decreased by more than 50 meters. That reduces the three-dimensional habitat available to species, increasing competition for food and space. Alarmingly, in 2.6% of the global ocean the photic zone has shrunk by more than 100 meters. ‘The ocean is much more dynamic than previously thought,’ Smith said. ‘Light levels in the water column can change a lot over a 24-hour period, and animals whose behavior is directly influenced by light are more sensitive to those shifts.’ He added that if the photic zone shrinks by 50 meters across large areas, light-dependent animals will be pushed closer to the surface and forced to compete for food and other resources. That could trigger fundamental changes throughout marine ecosystems, Science Alert reported.
Why is the photic zone shrinking? Several factors control how deep light penetrates the ocean. When sunlight hits the surface more directly it can penetrate deeper, because fewer photons reflect off the surface. The fewer suspended sediments and organic particles the photons encounter, the deeper the light can go. That’s why in clear tropical waters light can reach depths of up to 80 meters. But when sunlight strikes the surface at a low angle, more light is reflected, and the photons that do enter the water must travel farther downward. That creates a much shallower photic zone at the poles, where in some places it is less than 10 meters deep. This helps explain why some of the most noticeable changes show up in the Arctic and Antarctic photic zones.
Suspended sediments and other particles, such as algae from blooms, make the water less transparent and limit how far light can travel. Davis and Smith suspect those factors have darkened many of the world’s waters in recent decades. The authors attribute the global shrinking primarily to human activities that drive climate change, along with increased precipitation, warming surface waters, shifts in algal bloom dynamics, and greater runoff from agriculture and other sources. In extreme cases, like the recent algal bloom on the Great Southern Reef, reduced light can cause mass fish die-offs. Photic zones are shrinking not only near coasts: researchers have also observed darkening in the Gulf Stream and at the poles, where climate change is rapidly altering natural systems. ‘Our findings are a real cause for concern,’ the scientists wrote. The results of this study were published in the journal Global Change Biology.
