Shading by invasive seaweeds reduces photosynthesis of maerl from the Ría de Vigo (NW Spain).
The overgrowth and shading of several alien species along the European Atlantic coast are expected to reduce photosynthesis of maerl, decreasing its growth and fitness. In this work, three shade levels (0, 20 and 50%) were set up under laboratory conditions to simulate different competitive scenarios potentially affecting maerl beds. Live individuals of Sargassum muticum and Undaria pinnatifida were placed over maerl cultures, and the effects of shading were assessed by chlorophyll a fluorescence using a pulse-modulated fluorescence monitoring system. Photosystem II efficiency was measured as the ratio of variable to maximal fluorescence (Fv/Fm) as a proxy for the stress experienced by maerl. According to our data, irradiance reduction results in a small, yet significant, impact on the PSII efficiency of maerl, which could have fitness consequences.