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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. [ABSTRACT FROM AUTHOR]

Abstract The ability of species to recover from disturbance through natural recruitment is an important factor in determining the resilience of assemblages. The identification of patterns of resilience compatible or favoured by human activities has become a compelling need to address environmental management. In marine coastal systems, barrens are characterized by a dominance of encrusting calcareous algae and by high density of sea urchins and are considered an alternative stable state of systems collapsed due to overgrazing by sea urchins that originally were dominated by canopy-forming macroalga. Sea urchins also represent an important commercial resource, as some species are exploited worldwide. The present study aims at evaluating the effect of the harvesting of the sea urchin Paracentrotus lividus on promoting the shift from barren towards macroalgal beds. Three treatments were considered within a manipulative experiment in a Cystoseira -barren mixed habitat: natural sea urchins density, removal of all sea urchins and removal of only commercial P. lividus (natural density of the non-harvested sea urchin Arbacia lixula and juvenile P. lividus are maintained). Results showed that the removal of commercial P. lividus can promote Cystoseira recruitment. In fact, even if both A. lixula and juvenile P. lividus were present, the barren area decreased due to the spread of fleshy algae and the recruitment of the Cystoseira from the edge of vegetated patches. Therefore, removal of commercial P. lividus urchin is inferred as a reliable strategy to return ecosystems dominated by canopy-forming macroalgae. Highlights • The study evaluated sea urchin harvesting on promoting macroalgal beds. • A manipulative experiment was performed in a Cystoseira -barren mixed habitat. • The removal of the commercial Paracentrotus lividus can promote macroalgal beds. • The non-harvested Arbacia lixula did not prevent the recruitment of Cystoseira. • Sea urchins management should be considered to recovery macroalgal canopy. [ABSTRACT FROM AUTHOR]

Highlights • Structural complexity role in the barren formation. • The hysteretic path between kelp forest and barren. • Coexistence of three steady states (tristability) for kelp forest, sea urchin and fish density. Abstract In contrast to ecosystems that change smoothly and continuously in response to various stressors, some transitions between states with radically different properties can occur abruptly. An example are the sea urchin barrens and canopy algae (e.g. kelp beds) which represent alternative stable states. More precisely, the variation in grazing intensity in coastal rocky system may drive switches between one complex state into the barren state, the former dominated by erect algae and the latter by encrusting coralline algae and bare rock. Identifying the causes that drive a complex system towards a phase-shift becomes crucial for implementing strategies for the successful conservation and/or recovery of marine forests. Mathematical models that aim to assess effects of fisheries and sea urchins-seaweeds interaction may contribute to understand mechanisms driving transitions between alternative states. Fisheries exploitation has been considered the main driver of urchin population density transitions, with consequent effects on canopy algae distribution. The major novelty of the model here presented is the incorporation of habitat structural complexity, which explains the effect of algal biomass loss on coastal fish assemblages and the strong irreversibility of the system. We have found that as some critical parameters change macroalgae are more resilient and this may give rise to new scenarios, such as the emergence of new stationary states. [ABSTRACT FROM AUTHOR]

In the present study we describe, throughout a single year, subtidal macroalgal assemblages present on a shallow platform and on a deep cliff, using species and functional groups as descriptors. The study site is located south of Leghorn in the north-western Mediterranean Sea. Univariate and multivariate analyses were performed to detect spatial and temporal variations. A total of 101 macroalgal species were found: 17 Chlorophyta,13 Phaeophyta and 71 Rhodophyta. The shallow platform assemblage was more structured than that on the cliff, in fact the number of species was higher and a well developed stratified structure, consisting of turf, encrusting and erect vegetation layers was present. On the platform, functional groups showed qualitative and quantitative seasonal variations, while the cliff assemblage exhibited a dominance of crustose species throughout the year. The species richness found in the present study is lower than that observed in other Mediterranean macroalgal assemblages. This condition may be linked both to the spread of Womersleyella setacea turf, that may exclude other macroalgae by overgrow and pre-emption, and to the stress caused by sediment deposition and burial, that may damage macroalgal growth and recruitment. The multidimensional scaling analysis, performed both on the samples-species matrix and on the samples-functional groups matrix, showed a similar pattern, even if the use of species-level approach permitted a more accurate description of the assemblages. [ABSTRACT FROM AUTHOR]

The occurrence of filamentous turf-forming macroalgae assemblages has been widely reported in tropical and temperate oceans. In the Mediterranean Sea the development of dense and persistent turfs has recently started and has been linked to the spread of the two introduced filamentous Rhodophyta Acrothamnion preissii and Womersleyella setacea. In this work algal assemblages on rocky bottoms and rhizomes of Posidonia oceanica were studied during one year in three localities of the northwestern Mediterranean Sea (Leghorn, Vada Shoals and Gorgona Island) characterised by different sedimentation rate and composition of particulate matter. Dead matte of P. oceanica was also studied on Vada Shoals. Results showed that turfs are able to develop on all habitats investigated, but different patterns are found. Turfs were abundant on rhizomes of P. oceanica in all localities studied while on rocks they reached high cover at Leghorn and Vada but not on Gorgona Island. The dead matte of P. oceanica showed patterns similar to those found on rocky bottoms. Turfs seem positively influenced by sedimentation while neither the nature of particulate matter nor other polluting substances had any important result on their spread. The development of turfs negatively affects species number and diversity of invaded communities, while cover did not show significant variations. Womersleyella setacea replaced Acrothamnion preissii on rocky bottoms and on dead matte, but not on Posidonia oceanica rhizomes where Acrothamnion preissii was dominant everywhere. In spite of the occurrence of competition between the two algae, it did not seem to limit the development of turfs as they reached similar cover values independently of the dominant species. [ABSTRACT FROM AUTHOR]