Artificial light at night alters seaweed reproductive phenology

Artificial light at night (ALAN) is a growing, globally prevalent environmental stressor. It is known to disrupt biological processes across taxa and biomes, including reproductive phenology in terrestrial plants, but its potential to alter reproductive timings in marine macroalgae remains unexplored. We used reflectance spectrometry to quantitatively assess changes in receptacle ripeness of three fucoid macroalgae species at four field sites along an ALAN gradient in Plymouth Sound, UK over a six-month period. At sites with elevated ALAN (measured using Sky Quality Meters, range 16.15–18.76 mag arcsec ⁻², equivalent to 3.75 × 10 ⁻²–3.38 × 10 ⁻³ cd/m ²), expected seasonal patterns of receptacle ripening in Ascophyllum nodosum were reversed, causing receptacles to continue ripening into the winter months as opposed to peaking during the summer. Fucus serratus also continued to ripen in winter when exposed to the highest ALAN levels (16.15 mag arcsec ⁻², 3.75 × 10 ⁻² cd m ⁻²). Our results provide some of the first evidence that ALAN disrupts reproductive timings in fucoid macroalgae. Given the critical role of fucoids in coastal ecosystems worldwide, ALAN should be recognised as a potential driver of ecological change in these species. Incorporating ALAN into conservation strategies is essential for protecting these foundational habitats.