A new study from the University of Helsinki reveals that artificial light at night significantly affects freshwater invertebrates and their ecosystems.
Artificial light at night is one of the fastest-growing global pollutants, yet its ecological effects are often overlooked compared to issues such as chemical pollution or climate change. It disrupts the natural light-dark cycles to which organisms have adapted over evolutionary time, and which they depend on for critical biological processes.
"Our research highlights how seemingly small human actions-like switching on lights-can reshape natural ecosystems. As cities expand and nights grow brighter, recognizing and mitigating light pollution becomes essential for preserving healthy freshwater ecosystems and the vital services they provide to humans," says Yuhan He, doctoral researcher at the Faculty of Biological and Environmental Sciences, University of Helsinki.
The study examined how artificial light at night influences individual behavior, population traits, and community diversity on aquatic invertebrates. Overall, the findings show that artificial light at night not only disrupts nocturnal activities but also has long-term consequences that extend to daytime behaviors, evolutionary adaptation, and ecosystem functioning.
Nighttime lighting messes with the mating of shrimps and reproduction of water flea
The study found that artificial light at night alters not only nocturnal mating behavior of freshwater shrimp (Gammarus pulex) but interfered with their pairing status even during the day. Additionally, water fleas (Daphnia pulicaria) with different ancestral exposure to artificial light at night responded differently to experimental light conditions. Water fleas from urban areas exposed to light pollution for decades showed higher survival and reproductive performance than their counterparts from rural areas without artificial illumination, suggesting possible adaptation. However, artificial light at night generally had a negative effect on reproduction and lifespan across all groups.
Yuhan He found that artificial light at night changes the composition of freshwater invertebrate communities, with measures based on the variety of traits and functions of species were better at detecting changes in the ecosystem than the usual methods that only look at which species are present and how many there are. This highlights how artificial light at night influences ecosystem functionality and interactions between species.
Reducing light pollution can make a real differences
Yuhan He emphasises that the effects of artificial light at night reach far beyond simply brightening the darkness.
"Artificial light at night affects animal behaviour, population survival, and community structure, ultimately changing how natural ecosystems function," Yuhan explains. "Our findings show that light pollution should be recognised as an important ecological stressor in freshwater environments."
Yuhan adds that reducing light pollution can make a real difference. Simple actions-such as avoiding unnecessary lighting, using shields or dimmers, and choosing wavelengths less harmful to wildlife-can help protect aquatic biodiversity. These insights can also support better urban planning and conservation efforts by pinpointing the species and traits most vulnerable to artificial light.
