An international team of scientists has discovered that over the past 20 years, more than 21% of the surface of the World Ocean — approximately 75 million square kilometers — has significantly darkened. This phenomenon, which affects the photic zone (the sunlit upper layer of the ocean that supports life), poses a serious threat to marine ecosystems. The findings, published in Global Change Biology, highlight the scale and complexity of the problem.

Importance of the Photic Zone

The photic zone, stretching from the ocean’s surface to the depth where light still penetrates, is home to 90% of marine life. Sunlight fuels photosynthesis in phytoplankton, which produces half of the Earth’s oxygen and supports entire food chains. Light also regulates animal behavior, such as migration and feeding. A decline in light availability disrupts these processes, shrinking habitable space and destabilizing food webs.

Scope of the Changes (2003–2022)

  • 21% of the ocean has lost sunlight — an area twice the size of Russia.
  • 9% lost more than 50 meters of photic depth; 6% lost over 100 meters — equivalent to entire ecosystems disappearing.
  • 10% of the ocean surface has brightened, likely due to regional factors such as decreased algae concentration.

The most affected regions include coastal zones and open waters. Notable darkening has been observed in the North Atlantic and Indian Oceans, while parts of the Pacific have seen increased light levels.

Causes of Ocean Darkening

Researchers have identified several key factors:

  • Coastal areas: Increased land runoff, including fertilizers, organic matter, and sediments. Climate change-driven heavy rainfall washes soil and pollutants into the ocean, reducing water clarity.
  • Open ocean: Warming surface waters disrupt phytoplankton blooms. Warmer water limits vertical mixing, cutting off nutrient supply to algae. It also strengthens ocean stratification, trapping particulate matter near the surface and increasing turbidity.
  • Global drivers: Higher levels of dissolved organic carbon and microplastics, both of which absorb light and reduce visibility.

These changes are closely linked to human activity, such as agriculture, deforestation, and greenhouse gas emissions.

Ecological Consequences

Darkening of the photic zone threatens marine biodiversity:

  • Phytoplankton: Reduced light lowers photosynthesis, decreasing plankton biomass — the foundation of oceanic food chains.
  • Fish and marine mammals: Light-dependent species lose orientation and face disrupted migration and feeding patterns. Coral reefs, which rely on light for their symbiosis with algae, are especially vulnerable.
  • Carbon cycle: Less phytoplankton means the ocean absorbs less CO₂, weakening its role as a carbon sink and exacerbating global warming.

Conversely, brightening in some regions may cause overheating of surface waters, stressing marine species and degrading ecosystems like coral reefs.

What Needs to Be Done

Scientists are calling for urgent action to safeguard the ocean:

  • Monitoring: Expand satellite surveillance and underwater sensors to track water transparency in real time.
  • Pollution control: Regulate agricultural runoff and industrial waste, including fertilizers and plastics.
  • Climate mitigation: Reduce greenhouse gas emissions to slow ocean warming and preserve natural circulation.
  • Local restoration: Rehabilitate coastal ecosystems — such as mangroves and seagrass meadows — which help filter runoff and protect water clarity.

Researchers stress that ocean darkening, while less visible than acidification or plastic pollution, is an equally critical threat requiring global attention and coordinated solutions.