Artificial Light At Night (ALAN) is an emerging anthropogenic stressor in coastal ecosystems, particularly in urbanised areas. ALAN can disrupt the natural diel cycles of light-sensitive foundation species, such as seagrasses, altering key physiological processes. Although ALAN can affect marine primary producers, its effects on seagrasses under realistic field conditions and in interaction with other stressors remain poorly understood. In urbanised coastal systems ALAN frequently co-occurs alongside nutrient enrichment and consequent epibiotic accumulation, which could mediate seagrass responses to ALAN. We assessed the synergistic effect of ALAN and epibiota on the seagrass Cymodocea nodosa using an in situ field experiment in a low light polluted location of the Venice Lagoon, Italy, where artificial illumination was experimentally introduced and epibiota presence was manipulated through biweekly leaf cleaning. Night-time oxygen fluxes and photosynthetic performance were measured to observe immediate physiological responses. Further, photosynthetic efficiency (ΦPSII) was assessed in the lab, together with leaf length and specific leaf area traits, to evaluate longer-term functional responses. ALAN and epibiota had no detectable effects on oxygen fluxes or photosynthetic efficiency measured at night. However, leaf length was reduced in ALAN-exposed plants compared to controls and under standardised daylight conditions in the lab, ΦPSII was significantly higher, an effect observed only in the absence of epibiota. Our results demonstrate that realistic ALAN disturbance does not directly affect seagrass photosynthesis, however long-term physiology and morphology can be affected. This long-term effect is modulated by epibionts, underscoring how frequently co-occurring stressors interact in coastal ecosystems.
