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SCIENCE OBSERVER

Night Lights

Aili Petersen

Thanks to the creators of the incandescent filament lamp, we are able today to illuminate our streets, homes and businesses with the flip of a switch, thereby extending the "day" long past sundown. Yet turning night into day has not been without its drawbacks. Astronomers decry excessive light levels for obstructing the view of the skies. The International Dark-Sky Association seeks to educate the public about light pollution—the unnatural luminescence created by a combination of urban sky glow and glare—which it calls a "growing threat to the nighttime environment."

Behavior of the freshwater zooplankterClick to Enlarge Image

Nonhuman organisms can be affected as well. Research in Florida has shown that near coastal cities, sea turtle populations are often threatened by unnatural light. New hatchlings, which instinctively head toward the brightest spot on the horizon, confuse urban lights with the ocean's reflection of moonlight, starlight and bioluminescence in the water. Led inland, the young turtles frequently die from unfortunate encounters with cars or predators.

Some migratory birds that use the constellations to navigate at night are attracted to brightly lit skyscrapers or towers; collisions with or exhaustion suffered from hovering around such objects in confusion can lead to a bird's death or injury. The Fatal Light Awareness Program, a Canadian organization established to care for injured birds and to educate others about the problem, estimates that "every year, some 10,000 birds are killed or injured in Toronto's downtown core alone" from such disasters.

For all the buzz, little research has been done on other ecological effects of light pollution—for instance, its impact under water. One aquatic ecologist, however, is currently investigating the effects of light pollution in lakes. Marianne V. Moore of Wellesley College says these areas, along with coastal waters, are at higher risk than other habitats in developed areas because they are unshielded and openly exposed to light.

Beginning in 1997 with the freshwater zooplankter Daphnia, Moore and her research team explored the relation between light pollution and the species' daily vertical migration. Samples were taken from enclosures in Lake Waban, a suburban lake 16 kilometers southwest of Boston, at night during a new moon period (when no moonlight was present). Black plastic enclosures designed to block 96 percent of nighttime downwelling irradiance as well as control enclosures of clear plastic were used to compare a dark and light nighttime environment, respectively.

This study, forthcoming in the Proceedings of the International Association of Theoretical and Applied Limnology, found that "the movement of Daphnia was significantly greater in both amplitude (2 meters higher) and magnitude (10–20 percent more individuals) in the black enclosures than in control enclosures within the lake." Because the zooplankton responded more normally in the black enclosures simulating a dark night, Moore and her colleagues concluded that light pollution—as seen in the clear control enclosures—reduces both the amplitude and magnitude of Daphnia's movements near the water's surface. As a result, increased amounts of surface algae left unconsumed by the zooplankton could potentially lead to algal blooms and poor water quality.

The same fate could result if augmented light levels in urban areas cause changes in the behavioral cues of organisms. Daphnia, for instance, have typically relied on reductions in light at dusk as a signal to begin vertical migration. Nighttime light levels also determine the height to which they ascend. Yet as nighttime lighting becomes brighter, Daphnia may not be triggered to rise as near to the water's surface.

Are light levels within lakes really increased significantly by light pollution? Moore finds that they are. Her group has recently quantified how incident levels of light intensity vary among five lakes along a rural to urban gradient. Testing during the new moon period, Moore and her research team "found that the intensity of light pollution striking the surface of the most urban lake (Jamaica Pond in Boston), is similar to that emitted by a full moon and 100 times that striking the rural lake (Mirror Lake in central New Hampshire)."

In a related experiment, Moore studied the spectral quality of light pollution reaching lakes. The most prevalent wavelength found, Moore claims, is yellow light, which reaches the deepest in a typical freshwater lake (and interestingly enough "matches almost perfectly the spectrum of the most common type of street lamp—high pressure sodium lamps"). Moore will use this information to determine the depths to which various wavelengths of light pollution can permeate throughout a typical lake. Here's hoping continued work will shed further light on the ecological effects of light pollution.—Aili Petersen


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