'Skyglow' from light pollution causes more problems than we knew

Faint glows in the night sky, often seen above distant city skylines, can do more than reveal urban sprawl. Recent findings show that even extremely low levels of artificial light can affect freshwater systems, sparking rises in certain algae while changing how carbon flows through lake ecosystems.

Researchers at the Leibniz Institute of Freshwater Ecology and Inland Fisheries (IGB) investigated these effects, demonstrating how skyglow – artificial light scattered in the atmosphere – can reach remote lakes and subtly disrupt the balance of microscopic organisms.

“The effects of skyglow on freshwater ecosystems were largely unknown until recently. However, we have now learned that many of the organisms in lakes follow a day-night rhythm.,” said IGB researcher Hans-Peter Grossart, who led the research.

“In our study, we have shown that artificial light at night promotes the proliferation of cyanobacteria, also known as blue-green algae, which can produce toxins. Skyglow also stimulates carbon cycling in freshwaters.”

Studying light pollution in lakes

The team used the IGB’s LakeLab, a one-of-a-kind experimental platform set within a larger lake. This setup consists of 24 separate enclosures, each containing 1,300 cubic meters of lake water and isolating its contents from the rest of the environment.

At the start of the experiment, the researchers ensured that algae, bacteria, fungi, and small crustaceans were evenly distributed throughout all of these enclosures.

Ten of these sections were lit at night for a month using a specially designed system that introduced levels of illumination ranging from 0.06 lux, resembling typical skyglow in urbanized areas, to 6 lux, the highest documented skyglow level ever measured. Five enclosures were left dark as controls.

Artificial illumination and cyanobacteria

The experiment was designed to test whether low-intensity nighttime light would alter the composition and metabolism of lake organisms.

The focus was on microorganisms, especially the bacteria and algae crucial to cycling nutrients like carbon. The scientists wanted to see if weak night lighting could trigger noticeable changes in how the lake community produced and processed organic material.

Study co-author Mark Gessner is an IGB researcher one of the coordinators of the project on light pollution in lakes.

“The IGB LakeLab offers ideal conditions for such a large-scale experiment where cause-and-effect relationships can be ascertained in realistic field settings by comparing responses of lit and unlit control enclosures,” said Gessner.

Bacteria respond to low light levels

During the trial, the scientists tracked how bacterial populations grew and responded, with a particular focus on cyanobacteria and other bacteria that rely on light to generate energy.

The results were significant: these light-using microorganisms increased on average by a factor of 32 in the lit enclosures compared to the dark ones.

Many of them, including cyanobacteria and certain anaerobic phototrophs, rose to levels high enough to suggest a direct response to even the weakest artificial glow.

“In our experiments, even very low light intensities of 0.06 lux were sufficient to elicit a response,” explained co-author Stella Berger, a phytoplankton expert at IGB.

These findings highlight how extremely subtle amounts of artificial light may have bigger ecosystem effects than scientists once assumed.

Disrupted carbon cycling and toxins

When more light-favoring microbes thrive, the processes driving organic matter production, consumption, and decomposition shift accordingly.

Freshwater systems depend on a balanced cycle: algae, aquatic plants, and photosynthetic bacteria convert inorganic elements such as carbon dioxide into organic matter, which eventually serves as food for other creatures.

Bacteria that function as decomposers break down leftover material, returning nutrients to the environment. The presence of extra cyanobacteria can throw off this balance, sometimes producing toxins harmful to fish, wildlife, and even humans if they accumulate in drinking water sources.

Light pollution changes lakes

Laboratory analysis of water samples from the lit enclosures revealed that under conditions resembling typical urban skyglow, bacterial decomposition of organic substances ramped up considerably.

Genetic evidence confirmed that community composition shifted, with certain species thriving under low artificial light and altering the rate at which carbon and nutrients circulated in the lake.

“An illuminance of 0.06 lux is roughly the illuminance to which organisms can be exposed over large urban areas,” added IGB researcher Franz Hölker, co-author of the study and second coordinator of the project.

This subtle glow, in other words, matches the brightness that rural or distant locations may experience simply because of distant city lights scattering in the atmosphere.

Implications for water quality

The study’s conclusions carry weight for managing freshwater resources. Cyanobacteria blooms can disrupt recreation, kill fish, and release toxins that pose public health risks.

Even if other conditions including temperature and nutrient levels remain stable, the presence of faint artificial light at night might be one more factor tipping the scales toward damaging algae blooms.

The researchers suggest that for cases where algal blooms are not easily explained, light pollution should be considered as a contributing cause.

Global increase in light pollution

The growing use of satellite imagery and drone technologies can help track which regions might be most affected, offering early warning and mitigation strategies. Still, prevention remains challenging.

The broader global increase in artificial lighting could spark far-reaching changes in habitats never intended to be illuminated.

By showing that conditions once deemed too dim to matter can trigger noteworthy ecological responses, this research shifts the conversation about artificial light pollution.

Though it is easy to overlook faint skyglow, lakes teeming with life are more sensitive to these gentle, scattered illuminations than previously realized.

The study is published in the journal Water Research.

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