Great swathes of the world’s oceans have become significantly darker over the past two decades, shrinking the critical photic zone where the majority of marine life thrives, a new study warns. Researchers used satellite data and advanced modelling to reveal that between 2003 and 2022 more than one-fifth of the global ocean has lost enough light penetration to push its photic zone shallower by at least 50 metres in some regions.
The Photic Zone: Ocean Life’s Sunlit Realm
Surface Layer Vital for Photosynthesis
Most marine organisms—from microscopic phytoplankton to fish and marine mammals—depend on the upper “photic zone,” the layer of water where sunlight supports photosynthesis and fuels ocean food webs. Although sunlight can penetrate to about 1,000 metres, effective photosynthesis generally occurs in the top 200 metres. Phytoplankton living here generate nearly half of the planet’s oxygen and form the base of nearly all marine ecosystems.
Darkening Trends Across 75 Million Square Kilometres
Study Quantifies Global Light Loss
Dr Thomas Davies of the University of Plymouth and colleagues processed two decades of ocean colour data from satellites using a light-attenuation algorithm. They found that 21% of the world’s oceans have darkened, equivalent to 75 million sq km—an area the size of Europe, Africa, China and North America combined. In 9% of ocean waters, the photic zone has retreated by 50 metres; in 2.6%, it has lost 100 metres of depth.
Drivers of Ocean Darkening
Coastal Upwellings to Climate-Driven Currents
Along coastlines, darkening is often linked to natural upwellings of nutrient-rich water or runoff carrying sediments and organic matter. Far offshore, however, drivers are less understood. Davies points to global warming and shifts in ocean circulation—such as changes in the Southern Ocean and the Gulf Stream—reducing water clarity and light penetration.
Impacts on Marine Ecosystems
Compressed Habitat and Altered Behavior
“Marine organisms use light for every aspect of their biology—hunting, mating, timing reproductive events,” Davies explains. As waters darken, species must migrate closer to the surface where conditions may not suit them, compressing habitats and heightening competition. Such shifts could reverberate through food webs, diminishing fish stocks and undermining the ocean’s role in carbon and nutrient cycling.
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Bright Spots Amid a Darkening Sea
Regions of Increased Clarity
Not all ocean regions are dimming. The study notes that roughly 10% of the ocean (37 million sq km) has brightened over the same period. Off Ireland’s west coast, for instance, clearer waters now reach deeper, illustrating that local factors—currents, climate oscillations, or reduced runoff—can enhance light penetration even as global trends point toward darkening.
Expert Warnings and Future Research
Urgent Need to Understand and Mitigate Trends
Professor Oliver Zielinski of the Leibniz Institute for Baltic Sea Research calls the widespread darkening “a worrying trend” that could destabilise marine biodiversity and the climate regulatory services oceans provide. He stresses coastal seas’ vulnerability, where human activity exacerbates nutrient loading and sediment runoff.
Davies and his team advocate for expanded monitoring of ocean light profiles and research into the ecological consequences of darkening. “We must understand how these changes interact with warming, acidification, and pollution to safeguard marine ecosystems and the fisheries and coastal communities that rely on them,” he argues.
As the planet warms and ocean currents shift, preserving the sunlit zones of the sea will be critical to maintaining the health of marine life and the global benefits they deliver.
