Artificial light at night is gaining more attention for its potential to alter ecosystems. However, there is a noticeable lack of research on artificial light and freshwater systems. This is despite freshwaters having high biodiversity and being disproportionally affected by species loss. In addition, most of the few existing studies on freshwater systems examine the effect of outdoor lighting. In this study, we aimed to resolve if indoor lighting at night might influence adult aquatic insects. To test this, sticky traps were attached to illuminated and dark windows at three different floors of an office building near a small stream in a peri-urban area close to Zurich in August 2006. Additionally, in order to determine what adult aquatic insects made up the pool of potentially sticky-trapped insects, emergence traps were placed close to the banks of the stream and were sampled each night.
Preliminary analysis showed that a high proportion of aquatic insects such as Nematocera, Trichoptera and Ephemeroptera (approximately 80 %) was caught at lit windows. Sticky traps attached to illuminated windows caught up to several thousand insects per m2; equivalent to the number of aquatic insects emerging from more than 50 m2 of water surface area. In contrast, the number of insects caught at the dark windows was significantly lower (0 - 60 per m2). Although many attracted insects will not die at illuminated windows at night, disorientated insects lose time, energy and are exposed to predators. Therefore, we expect populations of aquatic insects living in streams near large office buildings to be negatively impacted.