Light pollution by coastal streetlights affects intertidal grazers and biofilm differentially in natural rocky habitats and breakwaters

Most coastal urban environments are characterized by a large concentration of shipping ports, walkways, and other built infrastructures. These are commonly associated with high levels of artificial light at night (ALAN), a pervasive anthropogenic driver that erodes natural light cycles and impacts the ecology of benthic communities. However, it is not well known whether the spatial configuration of coastal built structures may influence the effects of light pollution on community structure. Here, we conducted field surveys in natural rocky habitats and on breakwaters directly lit by streetlights (ALAN), and in matched unlit zones (without-ALAN), along the coast of northern and central Chile (20°S-32°S), to examine the influence of light pollution on the diel activity and density of the intertidal grazer guild and on the biomass of their main food resource, biofilm, in both habitat types. The patchy distribution of artificial light on the breakwaters seems to allow the co-occurrence of diurnal and nocturnal grazers at night, resulting in no major alteration of grazer densities with light pollution. The density and night-time activity of diurnal grazer species increased in parallel with an increase in the biofilm biomass under lit conditions in the topographically more homogeneous natural rocky habitat. On the lit breakwaters, biofilm also increased but no change in grazer densities was found, most likely related to the presence of dark zones. Our results indicate that the influence of coastal streetlight pollution on benthic grazers can be dampened by the presence of among-boulder interstices in the built structure. Increases in biofilm, the main food of grazers, by artificial light, may reinforce grazing pressure in both rocky habitats. Promoting a balanced mix of built habitats and conserving urban natural rocky shores while reducing coastal light pollution from streetlights could help prevent impacts on different functional groups due to accelerated urban infrastructure expansion.