Over the past two decades, 21% of the global ocean has experienced a reduction in the depth of its photic zones — the sunlit layers that support around 90% of all marine life. This finding comes from a study by researchers at the University of Plymouth and the Plymouth Marine Laboratory, published in the journal Global Change Biology. The research analyzed changes in photic zone depth between 2003 and 2022, revealing a widespread trend of ocean “darkening.”
Using NASA satellite data and advanced numerical models, the study found that more than 75 million square kilometers of ocean — equivalent to 21% of the global ocean surface — have seen a measurable decline in light penetration. In more than 9% of these areas, the photic zone depth decreased by over 50 meters, while 2.6% experienced a reduction of more than 100 meters. At the same time, approximately 10% of the ocean surface became clearer, indicating a complex and uneven pattern of change.
The causes of this darkening vary between coastal and open-ocean regions. Along coastlines, increased runoff from agriculture, along with higher rainfall, has led to greater concentrations of nutrients, organic matter, and sediments — all of which reduce water clarity. In the open ocean, the phenomenon is linked to changes in algal blooms and sea surface temperatures, both of which can affect how deeply sunlight penetrates the water column.
These changes can significantly impact marine ecosystems. Many light-dependent organisms are being forced to move closer to the surface, potentially intensifying competition for resources and altering ecosystem dynamics.
The most affected areas include the upper Gulf Stream, the Arctic and Antarctic regions — all particularly sensitive to climate change — and coastal seas like the Baltic, where sediment and nutrient inflows further limit light availability.
The study underscores the critical role of photic zones in ocean health and the delivery of global ecosystem services such as oxygen production and climate regulation. In short, vast areas of the ocean are becoming darker, a trend that serves as a clear signal of ongoing environmental changes with potentially far-reaching ecological consequences.
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