David Balata

Spread of introduced Caulerpa species in macroalgal habitats

A short-term field experiment was designed to identify layers of Mediterranean macroalgal assemblage conducive to successful spread of two introduced Caulerpa species (Bryopsidales, Chlorophyta). By manipulation of species presence, three experimental assemblages were obtained: (1) encrusting algae, having removed the turf and erect species; (2) encrusting and turfing algae, having removed erect species; (3) encrusting, turfing and erect algae, that is, unmanipulated assemblages, which served as a control. Fragments of the two introduced species Caulerpa taxifolia (Vahl) C. Agardh and Caulerpa racemosa (Forsskal) J. Agardh were transplanted in each of the three assemblages. Width of the colony, blade density and percentage of the substratum covered by the two species were measured. The susceptibility of the indigenous community to the spread of Caulerpa species was related to type of assemblage. Blade density and amount of substratum covered by the two Caulerpa species were different between species and generally greater for C. taxifolia than for C. racemosa. Overall, the spread of these species was strongly dependent on the type but not directly on the complexity of the assemblage. Turf was more favourable than encrusting species alone, while the least advantageous habitat was where the macroalgal assemblage is composed of encrusting, turf and erect species. In other words, increased number of species in the assemblage reduces invasion of the Caulerpa species but the type of algae in the assemblage is likely to be more important than number of species. The presence of turf promotes the spread of Caulerpa species.

SEDIMENT DISTURBANCE AND LOSS OF BETA DIVERSITY ON SUBTIDAL ROCKY REEFS

How changes in environmental complexity and heterogeneity affect beta diversity is poorly known. We investigated patterns of beta diversity in subtidal assemblages of algae and invertebrates in the northwest Mediterranean in relation to inclination of the substratum and sedimentation. Vertical and horizontal substrata supported distinct assemblages under low, but not under heavy, ambient loads of sediment. To test the hypothesis that sediment reduced the dissimilarity between assemblages, sedimentation was increased experimentally in plots established on vertical and horizontal surfaces at sites experiencing low ambient levels of sedimentation. Patterns were compared to those occurring at unmanipulated sites and at sites exposed to heavy loads of sediment about 2 km apart. After one year, assemblages on vertical substrata were indistinguishable from those occurring on flat surfaces at manipulated sites and both converged toward those occurring at sites exposed to heavy loads of sediment. Control sites still supported distinct assemblages on vertical and horizontal substrata by the end of the experiment. Similar effects of sediment were observed on recovering assemblages in experimental clearings. These results show that sediment increased similarity in assemblages overriding the influence of habitat complexity on beta diversity at small and large spatial scales.

Spatial and temporal variability of Mediterranean macroalgal coralligenous assemblages in relation to habitat and substratum inclination

Abstract In the present work, coralligenous phytobenthic communities of the north-western Mediterranean Sea were studied in three deep sublittoral habitats: islands, continental coasts and offshore banks. Species and morphological groups were utilised as descriptors. A combination of univariate and multivariate analysis was used to describe variability in coralligenous macroalgal assemblages of vertical rocky cliffs between habitats and at different spatial scales within each habitat. In addition, temporal dynamics in the structure of macroalgal assemblages on horizontal and vertical surfaces were studied for one location. A total of 99 macroalgal species was identified, among them 12 Fucophyceae, 10 Chlorophyta, and 77 Rhodophyta. Results showed no major differences between assemblages of the rocky habitats studied. Spatial variability within each habitat appeared higher at the smallest scale investigated for all variables considered. In the coralligenous assemblages of the bank, patterns related to the inclination of substratum were not significant, except for filamentous species and turf layer. In contrast, the assemblages showed significant seasonal differences mostly related to the disappearance of many turf-species and the decrease in cover of most erect algae, principally foliose and corticatedterete forms, in the cold season.

Seasonal Dynamics of a Subtidal North-Western Mediterranean Macroalgal Community in Relation to Depth and Substrate Inclination

Abstract 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.

Epiphytic macroalgal assemblages of Posidonia oceanica rhizomes in the western Mediterranean

Macroalgal assemblages of Posidonia oceanica rhizomes of the western Mediterranean have been studied, in order to describe the structure of this community and to evaluate changes due to recent macroalgal invasions. To achieve these objectives, P. oceanica rhizomes were sampled from 21 beds distributed throughout the western Mediterranean Sea and macroalgal assemblages were studied using both species and functional groups as descriptors. Moreover, modifications linked to temporal fluctuations and depth were evaluated in an insular and in a continental bed. A total of 74 macroalgal species were found, among them 7 Chlorophyta, 8 Fucophyceae and 59 Rhodophyta. The species number varied from 5±0 at Vada to 18±0.58 at Panarea Island. The total percentage cover ranged from 19.5±1.8 at Cabo de Creus to 77.2±13.3 at La Maddalena Island. Filamentous species were dominant in the localities colonized by Acrothamnion preissii or Womersleyella setacea, while a high functional diversity occurred where the two introduced species were not present or they did not constitute dense turfs; the importance of crustose and foliose species was remarkable, while low cover of articulate and corticated-terete species was found at all localities. Localities where turfs were not developed were grouped together, even if they appeared more dispersed than those dominated by filamentous algae. The two beds studied over a 1 year period showed higher values of macroalgal cover in March and December, although significant differences were not detected with ANOVA in relation to either depth or season. The development of turfs, which occurs widely in western Mediterranean seagrass beds, seems to modify strongly the structure of macroalgal assemblages of rhizomes, mostly via the decrease in species and functional diversity.

Comparative study of the growth of the two co-occurring introduced green algae Caulerpa taxifolia and Caulerpa racemosa along the Tuscan coast (Italy, western Mediterranean)

Abstract Caulerpa taxifolia and C. racemosa are tropical green algae now found in the Mediterranean, where the former was accidentally introduced and the latter presumably also introduced; both species have spread rapidly and invaded a variety of habitats. Since 1996, the two algae have co-inhabited a region along the Tuscan coast near Livorno. The present study has been carried out to evaluate the invasive capacity of the two species. After one year, the increase in surface area was 67.5 % for C. taxifolia and 284.8 % for C. racemosa , while the increase in patch number was 11.4 % and 121.4 % respectively. The stolon growth of the two algae followed a similar trend, as is showing the same temporal variations with the maximum growth rate between August and October. At the end of the study period, the increase in stolon length and number was significantly higher in C. racemosa than in C. taxifolia . The results of the present study show a higher invasive potential in C. racemosa than in C. taxifolia , although further studies are necessary to evaluate the competition between the two algae and the effect of their simultaneous presence on the benthic community.

Threats to macroalgal coralligenous assemblages in the Mediterranean Sea.

Coralligenous habitat is one of the most important coastal systems of the Mediterranean Sea. This paper evaluates the main threats to macroalgal coralligenous habitats through a review of the relevant literature. Sedimentation, nutrient enrichment and biological invasions can cause severe alterations in the structure of coralligenous assemblages due to the regression of perennial structuring species and increases in populations of ephemeral algae. Under pristine conditions, mechanical disturbance seems to be easily mitigated by the recovery capability of coralligenous assemblages; however, such disturbances seriously affect coralligenous structure if they occur in concert with other stressors. Important synergetic effects among all the studied anthropogenic disturbances are also highlighted. The main consequences of the considered stressors are the loss of the complexity of macroalgal assemblages and the deterioration of both alpha and beta diversity.

Effects of nutrient enrichment on macroalgal coralligenous assemblages.

Effects of eutrophication on marine ecosystems have been widely studied, even if both the effects on deep subtidal rocky assemblages and response of different successional stages to nutrients impact are still not clear. In this context, the study aimed to evaluate the effects of nutrient enrichment on Mediterranean macroalgal assemblages associated with coralligenous habitat. A manipulative field experiment was carried out by supplying both mature and early successional stages of assemblages with nutrients. A total of 62 macroalgal species were identified. Multivariate and univariate analyses showed that the structure of both mature and early successional macroalgal assemblages of coralligenous significantly varied between areas treated with nutrients and not treated areas. Moreover, differences were stronger when macroalgal assemblages were in the early successional stage than in the mature one. Results highlighted the role played by nutrients in determining the structure of macroalgal coralligenous assemblages, furthermore suggesting possible synergetic effects with other kinds of disturbances.

Resilience of Cystoseira beds: lack of canopy effects on recruitment

Abstract The aim of the study was to evaluate the role of the presence of an adult canopy on the recruitment of the habitat-forming seaweed Cystoseira brachycarpa var. balearica, in the northwestern Mediterranean Sea. To achieve this objective, a field experiment was performed and recruitment was estimated both in the middle and at the edge of cleared patches, as well as within the bed, on ceramic tiles anchored to the rocky substrate. At the end of the study, neither the number nor the heights of Cystoseira thalli were significantly different among the positions, suggesting that there were no facilitating or inhibiting effects on recruitment.