Ocean Darkening: Over 21% Global Oceans Lost Light Penetration (2003–2022 Study)

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.

Darkening of the Global Ocean

The photic zone is the upper layer of the ocean (extending up to 200 meters) where sunlight can penetrate. This zone:

Supports photosynthesis, driving oceanic food chains.
Covers areas inhabited by 90% of marine life.
critical for oxygen production, carbon cycling, and climate regulation.
A healthy photic zone sustains marine food webs and supports global fisheries.

The aphotic zone lies below the photic zone, where sunlight cannot penetrate.
In this region, photosynthesis is impossible; only respiration and decomposition occur.
It is typically cold, dark, and supports only light-independent organisms like deep-sea bacteria and certain invertebrates.

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.

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.

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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