Ocean Darkening Is Shrinking the World's Photic Zone

Almost all ocean life depends on the sunlit upper waters known as the photic zone. New research suggests this narrow band of valuable marine light is contracting across the world's oceans. Marine scientists have found that more than a fifth of the oceans have grown darker over the past 20 years.

Thomas Davies, a marine biologist at the University of Plymouth, and Tim Smyth, a marine biogeochemist at the University of Exeter, observed that scientists were increasingly concerned about ocean darkening, yet its extent had not really been quantified.

"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," Davies says.

"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."

By combining NASA satellite data from 2003 to 2022 with numerical modelling, the pair identified a pattern of declining photic zone depth over the years. The depth of this layer naturally limits the habitat available to most ocean life.

How the shrinking photic zone affects ocean life

The photic zone is the ocean's uppermost layer, where sunlight, including sunlight reflected by the Moon, enters the water. All marine organisms that photosynthesise, including seagrasses, kelps and, most importantly, phytoplankton, must inhabit the shallower parts of this layer to capture the Sun's energy.

Many other creatures that rely on these primary producers also live in the photic zone, among them corals, crustaceans, fish and marine mammals.

The study found that the photic zone had become more than 50 metres shallower in nearly 10 percent of the world's oceans. That substantially reduces the volume of available habitat, increasing competition for resources. More severely still, the zone has contracted by over 100 metres in 2.6 percent of the ocean.

"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 behaviour is directly influenced by light are far more sensitive to its processes and change," says Smyth.

"If the photic zone is reducing by around 50 metres in large swathes of the ocean, 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. That could bring about fundamental changes in the entire marine ecosystem."

What determines the depth of the photic zone?

Several factors influence how deep the photic zone extends. Light striking the ocean vertically, such as equatorial sunlight at midday, can travel deeper into the water because fewer photons are reflected by the surface.

Photons also penetrate further when they encounter less suspended sediment and organic matter in their path. In clear tropical waters, for example, light can reach depths of up to 80 metres.

When light arrives at an angle, more of it is reflected from the surface, while the photons that enter the water must travel further as they descend. The photic zone is consequently much shallower at the poles, reaching less than 10 metres deep in some places.

This could partly account for some of the most pronounced changes appearing in the photic zones of the Arctic and Antarctic. Sediment and other suspended material, including algal blooms, also make water less transparent and limit how far light can penetrate.

Davies and Smyth suspect these materials have contributed to the darkening of the world's waters in recent decades.

"A combination of nutrient, organic material, and sediment loading near the coasts and changes in global ocean circulation are probable causes of increases in primary and secondary productivity that have reduced light penetration into surface waters," they write in their research paper.

In extreme circumstances, such as the recent algal blooms across Australia's Great Southern Reef, insufficient light can cause mass mortality events.

Ocean darkening beyond coastal waters

The world's photic zones are not contracting only near coasts, where excess runoff from human activity would be expected to cloud the water, as seen in the Baltic Sea and along the eastern coasts of England and Scotland. Pronounced darkening also occurred in the Gulf Stream and at the poles, where climate change is rapidly altering natural systems.

"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," Davies says.

"Taking all of that into account, our findings represent genuine cause for concern."

The research was published in Global Change Biology.