Coastal streetlights shape intertidal sessile community abundance and are associated with altered post-disturbance recovery in urban rocky shores.

Abstract

Coastal urban environments concentrate high levels of artificial light at night (ALAN), yet how persistent light pollution interacts with urban habitat conditions to shape intertidal community structure remains poorly understood. Here, we combined field surveys on natural rocky platforms and on breakwaters along the coast of northern and central Chile (20°S-33°S; eight sites per habitat type) with a 573-day field experiment at a single locality (23°S). Surveys revealed habitat-specific, non-linear declines in ephemeral and corticated algae with increasing ALAN intensity on natural rocky platforms. Experimental community removal indicated that both lit and unlit breakwater sites remained dominated by bare space, biofilm, and crustose algae, with limited colonisation by foundation species; at the rocky platforms, species richness recovery was delayed under lit compared with unlit conditions. Effect-size analyses isolating light from mechanical disturbance showed a small negative relationship between streetlight exposure and foundation species abundance at both study sites, suggesting that light pollution constrains recruitment independently of disturbance history. The findings suggest that streetlight exposure has contrasting effects between habitat types at broad spatial scales, with negative effects on algal cover at rocky platforms and no detectable effects at breakwaters. The divergent recovery trajectories observed at the local scale, while consistent with ALAN effects, cannot be unequivocally attributed to light pollution given the single site per habitat × lighting combination in the design. Rehabilitating degraded urban intertidal communities requires assisting the early establishment of foundation species while mitigating persistent light pollution alongside the other urban stressors that limit the same recovery processes.