Shedding Light on the Role of Artificial Light at Night in Lake Washington, WA, USA

Artificial light at night (ALAN) poses a threat to ecosystems globally. It includes both direct and indirect light, or skyglow, which occurs when ALAN scatters in the atmosphere, extending beyond its source. We analyzed ALAN trends in Lake Washington, WA, from 2014 to 2023 using Visible Infrared Imaging Radiometer Suite (VIIRS) nighttime light measurements, evaluated the relationship between in-situ and satellite measurements, and assessed juvenile salmon predation risk due to ambient light. We observed significant increases in ALAN in all open water regions, while nearshore regions primarily showed no or negative trends, revealing the significant role of skyglow on open water light levels. Although spatial correspondence between the satellite and in-situ measurements was observed, VIIRS did not capture the changes in yellow and blue light. Juvenile salmon at the shallow southern open water site experience light levels 28 times brighter than fish at the northern site. In the Ship Canal, a narrow corridor for outmigrating salmon, light levels are 65 times brighter than at the southern site. These contrasts in light exposure highlight the impact of local conditions on predation risk by visual foraging predators and emphasize the need for effective mitigation efforts targeting both nearshore and distant light sources.