The deep blue sea is becoming deeper — in color, that is. Human development is a primary cause for the reduction in light that can filter through the water, potentially disrupting the marine food chain significantly. Climate change is also contributing to — and being fueled by — the ocean’s growing opacity.
Ocean darkening occurs when “changes in the optical properties of the oceans reduce the depth to which sufficient light penetrates to facilitate biological processes guided by sunlight and moonlight,” said a 2025 study published in the journal Global Change Biology. The part of the ocean that sunlight can penetrate, called the photic zone, is “home to 90% of marine species,” said the World Economic Forum.
Rather than just some patches of darkening, the phenomenon has affected “large, connected regions” of the ocean, said Tim Smyth, a marine scientist at Plymouth Marine Laboratory and co-author of the study, to New Scientist. “Roughly one‑fifth of the world’s oceans have darkened in some way.” The depth of the photic zone has also shrunk by more than 10% “across 9% of the global ocean,” said the study.
Some of the main culprits of ocean darkening are “sediment runoff from agriculture, deforestation and development,” especially in coastal regions, said the World Economic Forum. Improved land management can play a large role in reducing the level of darkening. The good news is that the ocean has a “remarkable capacity to heal itself,” said Smyth. “Give marine ecosystems a little room to recover, and they often respond with surprising speed.”
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