The effects of artificial lighting on adult aquatic and terrestrial insects ELIZABETH K. PERKIN* ,†,1 , FRANZ H € OLKER* AND KLEMENT TOCKNER* ,† *IGB, Leibniz-Institute of Freshwater Ecology and Inland Fisheries, Berlin, Germany † Institute of Biology, Freie Universit€ at Berlin, Berlin, Germany SUMMARY 1. There is a growing concern that artificial light might affect local insect populations and disrupt their dispersal across the landscape. In this study, we investigated experimentally the effect of artificial light on flying insects in the field, with an emphasis on aquatic insects. We asked whether lights prevented the ability of insects to disperse across the landscape, a process that is crucial in col- onising restored habitats. 2. We set up six, c. 3.5 m high downward facing high-pressure sodium streetlights along a perma- nently connected oxbow in the Spree River of eastern Germany. We collected insects using 12 flight intercept traps, each with trays at three different heights (0.5, 1.5 and 2.5 m), placed at distances 0, 3, 40 and 75 m from the lights and 5, 8 and 80 m from water. The number of emerging aquatic insects in the study area was measured with six emergence traps. We emptied the traps 22 times between June and September 2010; the lights were on for 11 of these nights and off for the other 11. 3. In total, we caught almost 27 times as many insects at traps 0 m from the lights when the lights were on than when they were off. Most insects caught when the lights were on were aquatic, with Diptera being the most common order. Furthermore, the proportion of aquatic insects caught at traps 0, 3 and 40 m from the lights when they were on was significantly higher than when they were off. On lit nights, more aquatic insects were captured per hour and m 2 (area in which flying insects were intercepted) at traps 0 m from the lights than emerged from per square metre per hour from the Spree River. 4. Our results suggest that adult aquatic insects can be negatively affected by artificial light and that city planners should take this into account when designing lighting systems along rivers. Keywords: biodiversity, connectivity, dispersal, high-pressure sodium lamp, light pollution, river Introduction Artificial light at night occurs virtually everywhere there are permanent human settlements. Because people tend to live around fresh water (Kummu et al., 2011), it is likely that artificial light is present and could adversely affect freshwater systems. Indeed, a recent study found that linear freshwater environments are more brightly lit than other natural spaces in the same area. For example, streams and canals in the Berlin area are five times brighter than forests and 1.4 times brighter than meadows and pastures (Kuechly et al., 2012). However, it has only been in the last 10–20 years that ecologists have turned their attention to the possible influences of artificial light on organisms and ecosystems (Longcore & Rich, 2004), and in this respect, freshwater ecosystems have received less attention than terrestrial and marine systems (Perkin et al., 2011). This is despite the fact that artificial light at night has the potential to affect biodiversity negatively (H€ olker et al., 2010). In addition to light being used to trap both aquatic and terrestrial adult insects, it is well known that Correspondence: Elizabeth K. Perkin, IGB, Leibniz-Institute of Freshwater Ecology and Inland Fisheries, 12587 Berlin, Germany. E-mail: eperkin@mail.ubc.ca 1 Present address: Department of Forest & Conservation Sciences, University of British Columbia, Vancouver, BC Canada V6T 1Z4. 368 © 2013 John Wiley & Sons Ltd Freshwater Biology (2014) 59, 368–377 doi:10.1111/fwb.12270