Polarised light pollution on river water surfaces caused by artificial light at night from illuminated bridges and surroundings

Bridge illumination gave rise to night-time illuminated paths across aquatic systems. However, if bridge artificial light at night (ALAN) reach waterbodies, it can result in polarised light pollution (PLP), which might alter the optical conditions of a river by night and potentially interfere with moonlight polarisation signals reflected off the water’s surface. It is a night-time phenomenon that can detrimentally change the behaviour of organisms sensitive to horizontally reflected polarised moonlight, a navigational cue and signal known to be used by flying water-seeking insects to detect suitable aquatic habitats to reproduce and lay eggs. In this study, we quantify the reflection of ALAN-induced polarisation patterns at the water’s surface near seven illuminated bridges crossing the river Spree in Berlin. The photometric data shows that bridge illumination induces PLP, which reflects from the water’s surface when measured at specific locations in space considered as potential flying paths for polarotactic aquatic insects. ALAN-induced polarisation findings at illuminated bridges suggest that PLP is a pollutant that illuminates aquatic areas. It requires better research as it can potentially affect polarimetric navigation in flying aquatic insects. As the extent of light pollution reaches riverine systems and aquatic habitats, the potential effects of PLP on freshwaters need the proper development of sustainable lighting solutions that can aid in preserving riverine nightscapes.