Seafood shoppers might notice higher prices, recreational divers and snorkelers will find murkier waters, and even worse; possibly huge numbers of the planet’s marine species and the ecosystem services provided by the ocean as a whole could be dulled.

Those everyday effects sit upstream of a single trend: The sunlit layer that powers almost all ocean life is shrinking.

A new study led by the University of Plymouth, published in Global Change Biology, reports that during the past twenty years, light now fades sooner in roughly one-fifth of the global ocean. In patches whose combined area rivals North America, the bright “photic” zone is at least 164 feet (50 meters) shallower than in 2003. The analysis draws on two decades of NASA satellite records and computer models translating surface color into water clarity.

“We….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,” said Thomas Davies, associate professor of Marine Conservation at the University of Plymouth. “Taking all of that into account, our findings represent genuine cause for concern.”

What is the photic zone?

The photic zone is the sea’s greenhouse. This area releases nearly half the planet’s oxygen, pulls carbon from the air, and feeds the plankton that underpin the diets of tuna, salmon, and countless other species. When the roof of that glasshouse drops, less water stays bright enough for plants to grow. Fish must crowd closer to the surface, competing within a thinner slice of habitat. That squeeze can push stocks toward cooler or deeper grounds, forcing fishing boats to burn extra fuel and reducing the catch that reaches markets and kitchens.

The research team found that about 90% of marine species inhabit the photic zone, yet that living space has contracted by 164 feet (50 meters) or more across an estimated almost 20 million miles (32 million square kilometers). The retreat exceeds 328 feet (100 meters) in roughly 1.8 million miles (three million square kilometers). A separate slice of the sea, equal in area to Canada, has grown lighter, but the net picture tilts toward darkening.

“There has been research showing how the surface of the ocean has changed color over the last 20 years, potentially as a result of changes in plankton communities,” Davies said. “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.”

Changes near shore arise mainly from soil, fertilizer, and organic matter washed off the land, which tint the water and feed algal blooms that block sunlight. Farther offshore, the culprits shift to warmer surface layers and altered currents that change where algae grow or how colored dissolved substances circulate. The steepest declines in clarity cluster were the top of the Gulf Stream and in polar waters already under rapid climate stress.

Study co-author Tim Smyth of Plymouth Marine Laboratory notes that many sea creatures cue their daily movements to faint shifts in light.

“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 behavior is directly influenced by light are far more sensitive to its processes and change.”

The study team calls for quick wins near land—restoring wetlands, planting river buffers, improving wastewater treatment—and broader action on greenhouse gases for the open sea. They also promote routine tracking of ocean clarity, arguing that absent regular checks the problem will stay hidden until economic pain forces attention.

“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,” said Smyth. “That could bring about fundamental changes in the entire marine ecosystem.”