A study titled “Darkening of the Global Ocean” finds that over 21% of global oceans darkened between 2003 and 2022.

What is Ocean Darkening ?

• Ocean darkening refers to the reduction in light penetration through ocean water, resulting in a shrinking of the photic zone. 
• When this layer becomes darker or shallower, it limits the biological productivity of oceans and threatens organisms that depend on light for survival and reproduction.
• This phenomenon is emerging as a serious ecological concern, potentially disrupting marine ecosystems, biodiversity, and key climate-regulating functions.

Key Findings of the Study

• 21% of global ocean area has darkened between 2003 and 2022.
• 9% of ocean area (equal to the size of Africa) saw a loss of photic depth over 50 meters.
• 2.6% saw a reduction of over 100 meters in light penetration.
• The Arctic, Antarctic, Gulf Stream, and North Sea experienced the most intense darkening.
  • Some areas (e.g., English Channel) became slightly lighter, showing regional variability.
  • Coastal areas like the Baltic Sea have also lost photic depth because of erosion of sediments and nutrients from lands.

Causes of Ocean Darkening

• In Coastal Regions:
• • Runoff of sediments, organic matter, and agricultural nutrients.
  • These promote algal blooms that block sunlight from penetrating deeper.
• In Open Oceans:
  • Changes in plankton dynamics.
  • Rising sea surface temperatures.
  • Shifts in ocean currents and circulation.

Impacts of Ocean Darkening

• On Marine Ecosystems:
  • Light-sensitive species are forced into shallower waters, increasing competition, predation, and stress.
  • Disruption of food webs and migration patterns.
  • Decline in populations of indicator species like Calanus copepods, crucial to marine food chains.
• On Climate and Humans:
  • Reduced photic zones impair photosynthesis, weakening the ocean’s role in carbon sequestration and oxygen production.
  • Threatens fisheries, biodiversity, and the ocean’s climate-buffering capabilities.
  • Considered a potential case of global-scale habitat loss.

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