Why Are Ocean Waters Getting Darker?

A recent study from the University of Plymouth in the United Kingdom has revealed a concerning trend: around 21% of the global ocean surface has darkened between 2003 and 2022. The research, published in the journal Global Change Biology, highlights significant changes in how sunlight penetrates the ocean.

This phenomenon primarily affects the ocean’s upper layer — known as the photic zone — which is home to about 90% of marine life. It’s also where crucial processes take place, such as photosynthesis by phytoplankton, microscopic organisms that form the base of the marine food chain and produce a significant portion of the Earth’s oxygen.

What’s Causing the Ocean to Darken?

Multiple factors contribute to the decrease in water clarity. These include shifts in the distribution and behavior of microscopic algae, changes in sea surface temperatures, and an increase in extreme rainfall events around the world — all linked to climate change.

In coastal areas, rainfall carries sediments and nutrients from land into the sea, fueling the overgrowth of plankton. This excess growth can block sunlight. In open oceans, warmer surface waters may encourage the spread of microscopic life, which also reduces the depth that sunlight can reach.

Which Areas Are Most Affected?

The study found that over 9% of the ocean experienced a significant drop in light penetration — in some places, deeper than 50 meters. In more extreme cases, the reduction surpassed 100 meters. Some of the most affected regions include the Gulf Stream, the Arctic, and the Antarctic — areas already known to be under pressure from climate change.

Coastal zones and enclosed seas, like the Baltic Sea, also showed noticeable darkening. However, the study noted that not all parts of the ocean are getting darker: around 10% of it actually became clearer over the same period, reflecting the complexity of ocean systems and the wide range of environmental influences on water clarity.

What Does This Mean for Marine Life?

While the full impacts of these changes are still being studied, scientists warn that many marine species and ecosystems may be affected. Numerous marine organisms depend on sunlight — and even moonlight — for orientation, hunting, and reproduction.

Phytoplankton, which require sunlight to perform photosynthesis, are especially vulnerable. As light diminishes, their ability to thrive may be reduced — with serious consequences for the entire marine food web, oxygen production, and the global carbon cycle.

The darkening of the oceans serves as another warning about the rapid environmental changes affecting marine ecosystems. For divers, researchers, and ocean advocates, understanding these shifts is essential for building a more sustainable and balanced future.

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