In News:

A recent study titled "Darkening of the Global Ocean", led by researchers from the University of Plymouth, has revealed that over 21% of the global ocean has darkened between 2003 and 2022, marking a significant environmental concern. The phenomenon, known as ocean darkening, is increasingly disrupting marine ecosystems and global climate regulation.

What is Ocean Darkening?

Ocean darkening refers to the reduction in the photic zone — the upper layer of the ocean (up to ~200 meters deep) where sunlight penetrates to support photosynthesis. This zone is foundational to:

• ~90% of marine biodiversity
• Climate regulation
• Ocean productivity
• Global fisheries

The study used satellite data and modeling based on the Diffuse Attenuation Coefficient (Kd 490), which measures how rapidly light fades through seawater. It found:

• 21% of global oceans experienced darkening in two decades.
• 9% saw photic depth decline by over 50 meters.
• 2.6% saw a reduction exceeding 100 meters — an area roughly equal to the size of Africa.

Geographic Distribution

• High darkening: Arctic, Antarctic, Gulf Stream, North Sea, eastern UK coast.
• Lesser darkening or even brightening: Some parts of the English Channel.
• The open ocean and climate-sensitive zones have witnessed the most pronounced declines.

Causes of Ocean Darkening

• Coastal Zones:
  • Runoff of agricultural nutrients, organic matter, and sediments.
  • Leads to algal blooms that block sunlight.
• Open Ocean:
  • Shifts in plankton dynamics
  • Rising sea surface temperatures
  • Altered ocean circulation patterns

These changes may be linked to climate change, land-use modifications, and increased rainfall-driven erosion.

Impact on Marine Ecosystems

Ocean darkening leads to:

• Shrinking habitats for light-sensitive species like Calanus copepods (key zooplankton and food web base).
• Disrupted feeding, migration, and reproduction cycles due to reduced solar and lunar light cues.
• Increased crowding in shallower waters, intensifying competition and predation.
• Collapse of marine food chains, even in areas with minimal fishing pressure.

Experts warn that this could represent one of the largest habitat losses in recent history, with implications for:

• Biodiversity
• Carbon cycling
• Oxygen production
• Ocean buffering against climate change