Ocean Darkening
Published on 26 Oct 2025
A new study, Darkening of the Global Ocean, finds that over 20% of the ocean has darkened in the past two decades.
Ocean darkening refers to the reduction in sunlight penetration into the upper layers of the ocean, known as the photic zone, the sunlit layer (up to 200m deep), which is critical for photosynthesis and marine life.
Caused by factors like algal blooms, sediment runoff, organic matter, and climate change (e.g., warmer waters, altered plankton dynamics).
It can impact marine ecosystems, food chains, carbon cycling, and fisheries, since most ocean life depends on light for survival and biological processes.
The study used Calanus copepods, a genus of small, shrimp-like zooplankton found widely in cold and temperate ocean water, to detect changes in light availability.
They are highly photosensitive and respond to faint light cues from the Sun and Moon.
Their sensitivity to light makes them an ideal indicator species to study changes in ocean light penetration and ocean darkening.
Known for their daily vertical migration—rising to the surface at night to feed and sinking during the day to avoid predators.
https://oceanservice.noaa.gov/facts/lightinocean2.jpg
