Humans interact with streams in lots of ways – we like to swim in them, catch the fish that live there, and fill our water glasses and irrigate our fields with the water flowing through them. We also change rivers and streams to suit our needs – we straighten stream channels in cities, we cut down the riparian vegetation that grows on stream banks when it gets in our way, and, in some places, we put up streetlights with little regard for how that artificial light might affect the animals and plants that live in streams.
These changes influence aquatic organisms and the ecosystem overall, but sometimes it’s difficult to know what effect each change, individually, is having – something conservation officers or resource managers might want to know if they’re trying to decide which restoration project to dedicate limited funds to. In order to know how streams or rivers respond to different anthropogenic changes, it’s necessary to study systems where only one of them is occurring. And in order to experimentally test what the effects of a change are, it’s necessary to conduct the study in a place where researchers can manipulate streams.
“I was trying to find a system to work in that was basically light-naïve and would be simple to manipulate,” says Liz Perkin, currently a post-doctoral researcher at the University of British Columbia and the lead author on a paper recently published in the journal Freshwater Biology.
Perkin found a system that would work for her research – the Malcolm Knapp Research Forest, located in British Columbia. She and her team set up high-pressure sodium streetlights at four streams within the forest, then compared the insect community, leaf decomposition, and fish growth in the experimental reaches with four similar areas that weren’t lit by streetlights at night, to see what effect the streetlights had on the streams.
The scientists found that the number of aquatic macroinvertebrates drifting in the streams – a primary food source for fish – was much lower in the reaches lit by streetlights. This wasn’t surprising; as Perkin and her co-authors write, “[p]revious studies have shown that the activity of aquatic insects is at least partially controlled by light levels.” Insects are more likely to stay on the stream bottom, rather than drift through the water, when there’s enough light for fish to see and eat them.
Other macroinvertebrate activities, as well as leaf decomposition and fish growth rates, were the same at the lit and unlit streams. Perkin has some ideas about why that might be – they were only able to keep their streetlights in the research forest for one month during the summer, and it’s possible that the duration of the experiment simply wasn’t long enough to induce changes in the ecosystem, or that the effects of the streetlights would have been more pronounced during the spring, when days are shorter and nights are longer.
Another possibility is that the type of streetlight the researchers used in the experiment may explain why they didn’t see larger differences between the lit and unlit streams.
“They weren’t as bright as they could be,” Perkin says, of the streetlights they used. “We used high-pressure sodium because they’re very efficient and they’re currently the most commonly used streetlights across the globe.” But LED lights, which are gaining popularity as they become cheaper and more efficient, are brighter and have unique spectral qualities that may make them more influential in aquatic ecosystems. “If you look at the spectrum of a high-pressure sodium light, they tend to be more in the orange and red end of the spectrum, and those wavelengths are very readily absorbed by water, whereas with LEDs you have things more on the blue end of the spectrum, and those are more likely to penetrate water and be brighter in the water.”
Moving forward, Perkin plans to look at fish behavior in more detail. She suspects that even though macroinvertebrates drift less in streams lit by streetlights – meaning less food is available for fish in those streams – the extra light may make it easier for fish to spot the insects that are drifting.
“What we know from this [study] is that artificial light does have the ability to affect stream communities,” Perkin told me. “The changes [we] saw were pretty small, but because they’re added at an important level in the food web, there’s definitely the potential for there to be bigger changes on a longer timescale.”
Experimental streams with high-pressure sodium streetlights installed above them had fewer drifting insects than streams without streetlights, but didn't seem to differ in other ways.
(Image of experimental streetlight set-up by Nora Schlenker)