Eutrophication affects the resistance of fucoids to an introduced alga spread

•The study aimed to evaluate the effects of nutrients on Cystoseira assemblages.•An experimental field study manipulated nutrients and seaweed invasion.•Nutrient enrichment changed the macroalgal assemblages dominated by Cystoseira.•Cystoseira decreased in nutrient enriched conditions.•Caulerpa cylindracea was advantaged by nutrient enrichment.

This study investigates whether eutrophication can affect the capacity of the canopy alga Cystoseira brachycarpa to impede or limit the spread of the introduced species Caulerpa cylindracea. By means of a manipulative field study (16 months long), the effects of nutrient enrichment and C. cylindracea removal were tested on the canopy-alga and the associated macroalgal community. Results highlighted deep changes through time due to nutrient enrichment, as C. brachycarpa decreased and Halopteris scoparia increased in cover. Furthermore, C. brachycarpa was also affected by the presence of the introduced species Caulerpa cylindracea which, in turn, was found significantly advantaged by nutrient enrichment. Overall, our findings suggest that eutrophication can drive the substitution of Cystoseira with H. scoparia, leading to the shift from canopy to opportunistic species, which are unable to avoid the spread of C. cylindracea.