Algal blooms are becoming a growing problem in lakes and rivers. These outbreaks can reduce oxygen levels in the water, damage aquatic life, and make water unsafe to drink or swim in.

The Role of Artificial Light

We’ve known for a while that light pollution—especially from cities—can disrupt natural ecosystems. But new research shows it may also be feeding the growth of harmful algal blooms, particularly those caused by cyanobacteria (blue-green algae), which can produce toxic chemicals.

Artificial light doesn’t just stay in cities. It travels into the sky and spreads as a glow—called skyglow—that reaches even remote lakes. This glow, although dim, can disrupt the balance of freshwater ecosystems.

What the Research Found

Scientists at the Leibniz Institute of Freshwater Ecology and Inland Fisheries (IGB) ran a major experiment at their LakeLab facility to study how artificial light affects freshwater life.

• They used 24 large water enclosures in a lake, each holding 1,300 cubic metres of water.
• Some enclosures were left dark, while others were exposed to low levels of artificial light—ranging from 0.06 lux (comparable to skyglow in a city) to 6 lux (a very bright level of skyglow).
• The results were clear: even the lowest level of light (0.06 lux) triggered rapid growth in cyanobacteria.

One researcher, Stella Berger, noted that these low light levels were enough to spark a visible reaction. Cyanobacteria and other microbes that rely on light grew significantly faster—by up to 32 times compared to the dark enclosures.

This growth also sped up the breakdown of organic material in the water, which can worsen water quality and increase the production of toxins. These toxins are dangerous to fish, animals, and humans—especially if they enter drinking water supplies.

Why It Matters

Until now, nutrient pollution was seen as the main cause of algal blooms. But this study shows that light pollution is another factor we need to consider—even in areas far from cities.

Franz Hölker, one of the project leads, explained that this kind of light exposure is already common in many rural areas due to the spread of city lights. And as the use of artificial lighting grows worldwide, its impact on ecosystems is becoming more serious.

The Takeaway

Managing water quality can no longer focus only on nutrients and chemicals. Artificial light—especially skyglow—can accelerate harmful algal blooms and disrupt natural cycles in lakes and rivers.

This research highlights the need for a more complete approach to protecting freshwater. That includes rethinking our use of nighttime lighting—and understanding that even a soft glow can carry big consequences.