Somewhere in the open Atlantic, the water starts to move at dusk. No one has ever accurately counted the number of tiny creatures that emerge from the darkness below. Lanternfish, zooplankton, and krill. They feed on phytoplankton in the thin layer of water that is still exposed to light as they follow the setting sun upward. They sink once more by morning. It has been occurring for millions of years on every ocean, every night. However, there seems to be a problem with that nightly migration these days.
Tim Smyth, a marine scientist at Plymouth Marine Laboratory, had no intention of learning that the oceans were becoming darker. Mostly, he was researching light pollution. The artificial type, the type that emanates from cities along the coast. Then, after compiling two decades’ worth of satellite data, he and his colleague Thomas Davies discovered something unexpected. Light was becoming more difficult to get through the water. Not in isolated areas, but over vast, interconnected expanses of ocean. about one-fifth of the world’s seas. It’s not a small detail.
| Topic Snapshot | Details |
|---|---|
| Phenomenon | Ocean Darkening — reduced light penetration into seawater |
| Lead Researcher | Tim Smyth, Plymouth Marine Laboratory |
| Co-Author | Thomas W. Davies, University of Plymouth |
| Study Published | 27 May 2025, Global Change Biology |
| Area Affected | 21% of the global ocean (about 75 million km²) |
| Time Frame Studied | 2003 to 2022 |
| Photic Zone Loss | Over 50 metres lost across 32 million km² |
| Marine Life in Photic Zone | Roughly 90% of all ocean species |
| Likely Drivers | Climate change, nutrient runoff, algal blooms |
| Data Source | MODIS Aqua satellite, NASA |
Sitting with the numbers makes you uncomfortable. Over an area about the size of Africa, photic zones—the top layer where sunlight penetrates deeply enough to support photosynthesis—shrank by more than 50 meters between 2003 and 2022. Over 100 meters of light were lost in some areas. It’s difficult not to imagine what that actually means. Once bathed in gentle daylight ninety meters below, the creature now floats in something more akin to dusk.
This is peculiar in part because the causes appear to differ depending on the location. Rainfall carries organic matter, fertilizer runoff, and sediment into shallow water near coasts, such as the Baltic and portions of the North Sea, where it feeds plankton blooms that cloud everything. The situation is different in the open ocean. Algal blooms seem to be occurring more frequently due to warmer surface temperatures, and changes in global circulation may be agitating nutrients in ways that are still unclear. Researchers believe that multiple issues are compounding at once.
Scientists are more concerned about more than just the dimming. When light retreats, it is squeezed. The photic zone is home to 90% of marine life. Fish, plankton, predators, and prey are all crammed closer to the surface if that zone contracts. more rivalry. greater visibility. less space for concealment. Although “severe” is the word the published research consistently strives for, the ecological knock-on effects are currently unknown.
All of this has a subtle unnerving quality. The oceans are thought to be the planet’s slow, steady portion. Instead of changing in twenty years, they do so on geological timescales. And yet here we are, observing a worldwide change that virtually no one on the ground noticed. According to Smyth, there is still hope that some of these trends could be reversed if warming and nutrient pollution are controlled. Perhaps. Whether there is political will or if the ocean is moving in this direction more quickly than we would like to acknowledge is still up for debate.
The great migration continues for the time being. Fish follow, krill rise, and the moon works. Just in a little bit less light than before.
