Artificial light at night is prevalent in human-dominated landscapes, and streams in these landscapes can be expected to be affected by artificial lights. We hypothesised that artificial light at night would reduce the activity of aquatic insects, resulting in reduced drift rates, lower fish growth rates and lower leaf litter decomposition rates.
We tested these hypotheses by installing street lights to reaches in four forested, natural streams of coastal British Columbia each paired with a control reach. Cutthroat trout (Oncorhynchus clarkii) are the top predators in these streams and feed mostly on terrestrial and drifting aquatic invertebrates.
We found that the night-time drift of aquatic invertebrates in lit reaches was Ëœ50% of the drift in dark reaches. However, the density of emerging aquatic insects, the density of insects falling into reaches, leaf litter decomposition rate and the number and growth rate of trout caught were not significantly different between the dark and experimentally lit reaches.
We conclude that, while short-term exposure to artificial light during the summer changes invertebrate behaviour, it does not significantly alter other trophic levels in forested headwater streams. Our results suggest that low levels of artificial light do not strongly influence stream ecosystems, but future research should determine whether this is true for all seasons and longer-term exposure to light.