Light Pollution in Lakes: Effects of Artificial Light at Night on Freshwater Systems

Light Pollution in Lakes is quietly reshaping freshwater systems. The glow from cities, streets, and bridges doesn’t just brighten the sky—it spills into lakes and rivers, altering how they function. Lakes support diverse species, yet artificial light at night (ALAN) disrupts natural rhythms. Fish swim at the wrong times, algae grow excessively, and tiny organisms at the base of the food chain lose balance. Drawing from recent research, including studies from Lake Erie and Lake Washington, this article explores the growing problem and the need for urgent action.

People often settle near rivers and lakes. As urban areas expand, lights stay on all night. For example, in Chicago, city lights create intense “skyglow” over Lake Michigan, making nights unnaturally bright. Even lakes far from cities are not fully dark anymore. Because blue light penetrates water most deeply, sometimes reaching 20 meters in clear lakes, Light Pollution in Lakes alters habitats well below the surface.

How ALAN Enters Lakes and Rivers

Artificial light spreads from multiple sources. Bridges, for instance, act as hotspots. German researchers observed that bridges create light barriers, forcing fish to avoid certain river stretches. Rivers are often more exposed than lakes, receiving little to no complete darkness. Riparian zones, the banks that connect land and water, also light up—changing animal behaviors. In the Rhine River, eels were observed swimming toward illuminated areas, shifting their migration routes unnaturally.

The color of light matters too. Red light fades quickly, while blue light travels much deeper. Many modern LEDs emit blue wavelengths, worsening ecological impacts. Studies in Lake Washington show even low levels of light—around 1 lux—are enough to disrupt fish activity. Unlike moonlight, which is natural and cyclical, artificial light is constant, confusing cues that animals depend on.

Scientists mapping ALAN in aquatic systems have noted shifts in insect movement, nutrient cycles, and species interactions. In the Wadden Sea, for instance, researchers found that shoreline lighting reduced cross-boundary insect transfers, starving terrestrial predators such as bats and birds. Similarly, Italian studies have shown that bridges alter the polarization of reflected light on water surfaces, misleading insects that rely on optical signals.

Zooplankton: Tiny Players with Big Impacts

Zooplankton, though microscopic, are central to freshwater balance. They feed on algae and in turn are eaten by fish. Normally, they rise toward the surface at night (a process called diel vertical migration) and retreat during the day to avoid predators. ALAN interrupts this rhythm.

In Lake Erie, scientists observed reduced nightly movement of zooplankton, leaving them exposed to predators for longer. Even extremely low light levels, just 0.1 lux, were enough to alter their hiding behavior in controlled experiments. As zooplankton decline, algae grow unchecked, leading to blooms. These blooms can deplete oxygen, release toxins, and disrupt the food web. Large-scale tests at LakeLab in Germany confirmed that Light Pollution in Lakes can trigger these cascading effects, mimicking moonlight but without the natural variability.

Fish: Survival and Behavior Changes

Many fish species rely on natural darkness to feed, spawn, and migrate. ALAN disrupts their hormonal cycles, notably reducing melatonin, which influences reproduction and growth. Laboratory studies on perch, for example, revealed reduced egg production under continuous light exposure.

In the wild, the impact is even clearer. Juvenile salmon in Lake Washington were found to linger near illuminated shorelines, increasing their mortality by 20–30 percent due to predation. Other research from the Rhine River showed that wild fish were drawn to lights, where they became more vulnerable. Long-term monitoring revealed survival rates dropped by more than a third in consistently illuminated habitats.

Bridges again pose barriers. Dutch studies on eels found that bright crossings prevented migration, fragmenting their natural routes. Crayfish, another keystone species, also showed reduced activity under artificial light—slowing the breakdown of organic matter and disrupting bottom food webs.

Algae and Plants: Overgrowth Issues

Algae are highly light-sensitive. Extended exposure from ALAN lengthens their active period, driving overgrowth. In Lake Müggelsee, continuous lighting spurred toxic cyanobacterial blooms, turning the water green and lowering oxygen levels.

These shifts alter nutrient cycles. Some algae increase nitrogen fixation, creating imbalances that favor harmful species. Aquatic plants, too, are affected. Constant light disrupts their growth patterns, though algae usually gain the competitive edge, further destabilizing ecosystems.

Insects and Amphibians: Collateral Damage

Insects are drawn to artificial lights, often fatally. Mayflies, for example, normally swarm rivers at night, but ALAN diverts them toward streetlamps, where they die in large numbers. This reduces food availability for fish and bats.

Caddisflies, which drift downstream at night, are also disrupted, impacting trout that rely on them. Amphibians suffer similarly. Frogs in artificially lit wetlands show reduced mating calls, while tadpole development becomes erratic. These patterns highlight how ALAN impacts ripple through multiple species and food webs.

Broader Ecosystem Shifts

The cumulative impact is profound. Short-term gains for some predators give way to long-term declines in biodiversity. Food webs unravel as prey species collapse and ecological interactions weaken. Land-water links also suffer, with fewer insects transferring nutrients across boundaries.

Global studies show ALAN increasing by 2–6 percent annually. Researchers now recognize it as a significant driver of ecosystem change—on par with chemical pollution or habitat loss.

Ways to Reduce ALAN

Solutions are available and effective for light pollution in lakes

• Use shielded lights that direct illumination downward, preventing skyglow.
• Switch to warmer bulbs with reduced blue light.
• Install motion sensors and timers, ensuring lights are only active when needed.
• Adapt bridge lighting, allowing fish to migrate naturally.
• Monitor light levels with sensors to identify problem areas.

Communities that adopt dark-sky policies, such as around Lake Erie, have already seen benefits. By taking action now, we can protect freshwater ecosystems from further harm.

Conclusion

Artificial light at night is more than a human convenience—it’s an ecological stressor with wide-reaching consequences. From tiny zooplankton to migratory fish, the impacts cascade across food webs, altering entire ecosystems. Yet with practical steps, from better lighting design to policy changes, we can restore darkness where it’s needed most. Protecting lakes and rivers from light pollution is not only possible—it’s essential for biodiversity and the health of our freshwater systems.