Artificial light from cities, visible as a glow in the night sky, can shape the biology of lakes in ways that go beyond mere aesthetics. Researchers from the Leibniz-named Fresh Water Ecology and Fisheries Institute investigated whether even faint, widespread night illumination could perturb freshwater ecosystems. Using a Lakelab platform on Lake Sharmutselsey, they built 24 closed micro-ecosystems to monitor responses under different lighting conditions. Over a month, ten of the fifteen experimental tanks were illuminated at night to simulate urban light pollution while the remaining five were kept in darkness. The setup provided precise control over light exposure and nutrient supply, enabling a clear comparison between lit and dark treatments. The aim was to understand how nighttime brightness from cities might influence microbial communities, carbon cycling, and overall lake health in North American environments.
Cyanobacteria, also known as blue green algae, are tiny photosynthetic organisms that can bloom when nutrients are abundant. In lakes rich in nitrogen and phosphorus, cyanobacteria can dominate and form visible masses. Some species produce toxins that are harmful to fish and wildlife and can pose risks to humans through contaminated water and recreational contact.
Scientists have long noted that many aquatic organisms follow daily rhythms tied to light and dark. The study shows that even low levels of night light can shift lake biology when they mirror urban brightness. The result suggests that city lighting may extend daytime cues and influence growth, metabolism, and interactions within microbial communities.
Among the dark tanks, conditions remained constant with no nighttime illumination, providing a baseline against which to measure the effects of light pollution.
Results were striking. In the lit tanks, cyanobacteria and other light sensitive bacteria grew much more rapidly, increasing about 32 times compared with the dark tanks. The lighting used in the experiment was deliberately gentle, yet it triggered photosynthesis enough to drive ecological change. This demonstrates how even modest artificial illumination can initiate shifts in lake microbe populations.
Beyond population increases, the light also altered the carbon cycle within the water. The composition of bacterial communities shifted and the decomposition of organic matter accelerated. These changes can alter nutrient balance, oxygen dynamics, and energy flow, potentially affecting a wide range of organisms and the resilience of freshwater ecosystems.
Such shifts raise concerns about the risk of toxic algal blooms that threaten natural habitats and human health. When cyanobacteria become dominant, toxins can contaminate drinking water sources and recreational waters, prompting advisories and costly mitigation measures. The findings highlight light pollution as a factor in bloom dynamics alongside nutrient concerns.
Experts emphasize that light pollution should be considered alongside climate and land use changes when assessing freshwater health. A project coordinator commented that nighttime illumination near cities may be an overlooked driver of algal blooms and their geographic patterns.
Taken together, the results point to a broader view of how urbanization interacts with natural systems. For communities in Canada and the United States, reducing nighttime lighting can benefit water quality and ecosystem health. Strategies include shielding outdoor lights, using warmer color temperatures, and timing lighting to minimize spillover into rivers and reservoirs.
These findings contribute to a growing understanding of how modern light environments affect microbial ecology in freshwater. They suggest practical steps that cities and lake managers can take to balance energy use with the health of aquatic ecosystems now and in the years ahead.