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Everyone is familiar with the light dome that shows from afar where cities are brightly lit at night. Artificial light that is scattered in the atmosphere and brightens the night sky can have an effect far from where it is emitted. This phenomenon is known as artificial skyglow. Skyglow can affect biodiversity over long distances.

"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. 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," said IGB researcher Prof. Hans-Peter Grossart, who co-authored and led the study in Water Research.

The researchers conducted their experiment in IGB's LakeLab. This unique facility can be described as a suite of experimental lakes in a lake: 24 enclosures, each holding a water volume of 1,300 cubic meters and separating it from the rest of the lake. At the start of the experiment, the plankton organisms—i.e., algae, bacteria and other single-celled organisms, fungi and small crustaceans—were equally distributed among all enclosures.

Ten of the 15 enclosures were dimly lit at night for one month using a specifically designed lighting system, with illuminance levels ranging from 0.06 lux (typical skyglow) to 6 lux (highest documented skyglow); five control enclosures were left unlit. This was the largest field experiment on light pollution in lakes to date.

"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 IGB researcher Prof. Mark Gessner, co-author of the study and one of the two coordinators of the project on light pollution in lakes.

Significant increase of cyanobacteria in response to light at night

The team studied the composition of bacterial communities and their metabolism in the water. In lakes, organic matter cycling involves biomass production, consumption and decomposition. As primary producers, algae, aquatic plants and certain bacteria utilize sunlight for photosynthesis to produce biomass from inorganic substances such as carbon dioxide or, mostly, hydrogen carbonate.

Some of the produced biomass serves as food for various organisms and is converted back into inorganic matter by so-called decomposers. This cycling of carbon and other elements maintains resource availability in lake ecosystems, which is altered by artificial light at night.

Bacteria play an important role as both primary producers and decomposers in ecosystems. The abundance of cyanobacteria and other bacteria that use light energy—primarily anaerobic oxygenic phototrophs (AAPs)—was on average 32 times higher under lit conditions than under dark control conditions. Although numbers varied among enclosures, the result was unambiguous.

"We found the observed increase surprising because light levels were too low to stimulate photosynthesis of cyanobacteria and other phototrophs. In our experiments, even very low light intensities of 0.06 lux were sufficient to elicit a response," explained IGB researcher Dr. Stella Berger, co-author and phytoplankton expert.

Stimulation of the lake carbon cycle

Exposure to artificial skyglow during the experiment changed the composition of the bacterial communities and thus also lake metabolism, as shown by genetic analyses of the bacterial community and mass spectrometric analyses of dissolved organic matter in the water samples. Clearly, skyglow stimulated, for example, the bacterial decomposition of organic matter produced by algae and hence overall carbon cycling 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 Dr. Franz Hölker, co-author of the study, and second coordinator of the project on light pollution at IGB.

Thus, one of the consequences of the dramatic increase in light pollution observed worldwide could be an increasing risk of potentially toxic cyanobacterial blooms. Therefore, for algal blooms that cannot be explained at present, light pollution may be considered more than before as a possible cue. Remote sensing methods using satellites, airplanes and drones provide options for early detection.