Responses of macroalgal assemblages dominated by three Mediterranean brown macroalgae with different life strategies to nutrient enrichment

Abstract

ABSTRACT Increase of nutrient concentrations in coastal waters is a worldwide phenomenon with ecological consequences for natural assemblages. Little is known about species-specific nutrient dynamics among macroalgae in the Mediterranean Sea, which may be key to understanding competitive strategies driving the structure of macroalgal assemblages. We conducted an in situ nutrient enrichment experiment to evaluate whether increasing nutrient availability modified the relative abundance of the three dominant brown macroalgae Cystoseira brachycarpa, Halopteris scoparia and Padina pavonica and whether any such modifications were related to different nutrient dynamics of the three macroalgae under different chronic enrichment regimes. Tissue nitrogen, carbon and phosphorus were determined 6 and 18 months after the first exposure to nutrient enrichment. Under nutrient-enriched conditions, C. brachycarpa decreased in abundance over the experimental period, while P. pavonica and H. scoparia increased (albeit with different trends), the former mostly in the earlier phase and the latter in the subsequent phase. Nutrient content of the three macroalgae responded positively to enrichment in the first year, although it was exploited differently: C. brachycarpa only increased its internal reserves, while H. scoparia and P. pavonica also used available nutrients for growth. A completely different pattern was observed in the second year, as nutrient content of all macroalgae was affected very little, if at all, by nutrient enrichment. However, the different responses in terms of abundance observed during the second year indicated changing nutrient dynamics across species over the longer timescale of nutrient exposure. The results of the study confirmed the existence of species-specific response patterns of nutrient dynamics in the three macroalgae. They also suggest that nutrient dynamics are habitat-specific, since nutrient response was strongly conditioned by macroalgal tissue nutrient status, which reflects the nutrient history of the site.