How dark is a river? Artificial light at night in aquatic systems and the need for comprehensive night-time light measurements

Freshwater ecosystems are hotspots of biodiversity. They are of major importance for humans because they provide vital ecosystem services. However, as humans tend to settle near freshwaters and coastal areas, these ecosystems are also over-proportionally affected by anthropogenic stressors. Artificial light at night can occur as a form of environmental pollution, light pollution. Light pollution affects large areas on a worldwide scale, is growing exponentially in radiance and extent and can have diverse negative effects on flora, fauna and on human health. While the majority of ecological studies on artificial light at night covered terrestrial systems, the studies on aquatic light pollution have unraveled impact on aquatic organisms, ecosystem functions as well as land-water-interactions. Although monitoring of light pollution is routinely performed from space and supported by ground-based measurements, the extent and the amount of artificial light at night affecting water bodies is still largely unknown. This information, however, is essential for the design of future laboratory and field experiments, to guide light planners and to give recommendations for light pollution regulations. We analyze this knowledge gap by reviewing night-time light measurement techniques and discuss their current obstacles in the context of water bodies. We also provide an overview of light pollution studies in the aquatic context. Finally, we give recommendations on how comprehensive night-time light measurements in aquatic systems, specifically in freshwater systems, should be designed in the future. This article is categorized under: Water and Life > Stresses and Pressures on Ecosystems Water and Life > Conservation, Management, and Awareness Water and Life > Methods