Francesco Cinelli

The Introduced Green Alga Caulerpa Taxifolia Continues to Spread in the Mediterranean

The tropical green alga Caulerpa taxifolia in the Mediterranean has spread steadily since its introduction in 1984. At the end of 2000, approximately 131 km2 of benthos had been colonized in 103 independent areas along 191 km of coastline in six countries (Spain, France, Monaco, Italy, Croatia and Tunisia). Large regions neighboring the invaded areas appear favorable to further colonization, and there is thus no reason to believe that spreading will slow down in the years to come.

Short-term effects of nutrient enrichment of the sediment and interactions between the seagrass Cymodocea nodosa and the introduced green alga Caulerpa taxifolia in a Mediterranean bay

Abstract Nutrient addition experiments provide a means for testing ecological theories concerning the effects of nutrient availability on community composition and development. Here we present the results of two reciprocal short-term (4 months) experiments testing for the effects of nutrient addition in the sediment and competitive interactions between the seagrass Cymodocea nodosa and the tropical green alga Caulerpa taxifolia . This study was conducted from 1 June to 30 September 1995 in a bay on the south coast of Elba Island (Italy) recently impacted by the alga. Each experiment consisted of the manipulation of the level of nutrients (addition vs. control) and the manipulation of neighbours (presence vs. removal). Response variables were blade density and length of C. taxifolia in one experiment and shoot density and leaf length of C. nodosa in the other. Results indicate that during the growing season the canopy of C. nodosa did not have any effect on C. taxifolia growth while nutrient addition in the sediment favoured colonization (higher blade density) of this alga independently of the presence of the seagrass. Conversely, C. taxifolia had a negative effect on shoot density of C. nodosa independently of nutrient availability. Hence, colonization by the introduced alga C. taxifolia may provide an explanation for the regression of C. nodosa beds occurring at the site. Furthermore, we demonstrated that C. taxifolia was favoured by high nutrient loads in the sediment, while C. nodosa was not; this might affect the outcome of the interactions among these species in the long-term.

Effects of sedimentation on subtidal macroalgal assemblages: an experimental study from a mediterranean rocky shore

Abstract The effects of sedimentation on the development and structure of macroalgal assemblages were investigated from June 1992 to December 1993 on a rocky subtidal shore South of Livorno (Ligurian Sea). Experimental procedures were specifically designed to address: (1) which species recruit in patches of bare rock under different sedimentation rates; (2) whether small-scale variations of sediment deposition influence the local diversity of the assemblage; (3) whether the effects of sedimentation are independent from the time of the year when succession begins. Depositional environment was manipulated in the field by means of transparent Plexiglas panels which on average reduced the amount of sediment sinking over the algal assemblage by 35%. Possible effects on the penetration of light and flow microenvironment were assessed. Small-scale variations of sedimentation rates significantly influenced the local structure and diversity of the algal assemblage. Such effects were particularly evident during the recolonisation of patches of bare rock and were dependent upon the time at which succession was initiated. The erect algae were the ones more affected by depositional environment and showed different responses to the experimental reduction of sediment supplies. All the plots were quantitatively dominated by a filamentous turf mostly consisting of Polysiphonia setacea , whose growth was potentially enhanced by reduced sedimentation rates. At the end of the experiments, the overall diversity and evenness of the assemblage were lower in treatments with lessened sediment supplies. It is suggested that small-scale spatial variability of the depositional environment may affect the within-habitat diversity of algal assemblages either through direct effects on individual species or on their propagules, and through indirect effects mediated by competitive outcomes.

Grazing by the sea urchins Arbacia lixula L. and Paracentrotus lividus Lam. in the Northwest Mediterranean

The sea urchins Arbacia lixula and Paracentrotus lividus are common on shallow subtidal reefs in the Mediterranean. Previous studies on the ecology of these species reported that P. lividus is generally more abundant on horizontal or gently sloping substrata, where it forages mainly on erect algae. In contrast, A. lixula is more common on vertical substrata and it is considered a main grazer of encrusting coralline algae. Observations on some rocky shores in the Ligurian sea indicated that P. lividus occurs mainly in crevices at the bottom of the vertical walls, and that neither species is present on horizontal or sub-horizontal substrata. In this study we investigated the distribution and abundance of the two species of sea urchins on vertical substrata at different spatial scales and through time. A field experiment was used to test whether A. lixula constrained the distribution of P. lividus on vertical substrata, and to test for the predicted differences in the effects of the 2 species on assemblages of algae and invertebrates. Four treatments were used: (1) control (sea urchins left untouched); (2) A. lixula removed, P. lividus present; (3) A. lixula present, P. lividus removed, and (4) both species removed. The effects of sea urchins on colonising algae and invertebrates were examined after 6, 12 and 18 months. A. lixula was consistently more abundant than P. lividus on patches of vertical substrata, but the removal of A. lixula did not affect the abundance of P. lividus. There was no effect of A. lixula on encrusting corallines, but the cover erect algae increased and the number of limpets decreased significantly where A. lixula was removed. P. lividus had only a minor impact on the assemblage, probably due to its low abundance. A. lixula had effects close to those predicted for P. lividus, suggesting that the ecological role of these herbivores may be more similar than previously thought. The implications of these results for the management of the edible sea urchin, P. lividus, are discussed.

Spatial and temporal variability in the distribution of algae and invertebrates on rocky shores in the northwest Mediterranean

Abstract This study examines the distribution of algae and invertebrates on rocky shores in the northwest Mediterranean. The main objectives were to: (1) examine the consistency of vertical patterns of distribution in space and time; and (2) determine the relative importance of vertical position on the shore with respect to other sources of spatial and temporal heterogeneity. These patterns were investigated by repeatedly sampling two shores (about 70 km apart) in four different areas (10s to 100s of m apart) at each of three different heights. These areas were sampled eight times between April 1995 and August 1996. At each time, the percentage cover of sessile organisms and the density of molluscan grazers were estimated in three replicate plots (50×10 cm) in each area. The patterns of distribution and abundance of mobile animals were also examined in relation to season and topography of the substratum. At each level on each shore, sampling was done inside crevices in two areas randomly chosen among the four available, and outside crevices in the remaining two areas. This sampling was repeated in summer and in winter, at two randomly chosen dates in each season. There were large spatial and temporal fluctuations in patterns of abundance and distribution. There were inconsistencies in temporal changes from one shore to the other and among areas within levels for several of the organisms considered. Differences from one level on the shore to another were usually large but, in most cases, were not consistent from shore to shore or from time to time. The magnitude of these spatial and temporal effects was at least as large as that due to vertical position on the shore. Season and topography of the substratum were related to differences in the distribution and abundance of molluscan grazers but, again, these effects were variable in space and time. The results were, however, consistent with the hypothesis that limpets may benefit from the presence of crevices in proximity to their upper limit of distribution. Overall, results indicate that the structure of these assemblages cannot be predicted reliably on the basis of vertical position on the shore alone.

The influence of canopy algae on vertical patterns of distribution of low-shore assemblages on rocky coasts in the northwest Mediterranean

Abstract Canopy-forming algae have been shown to play a fundamental role in the maintenance of understorey assemblages on rocky shores. In the Mediterranean, low-shore habitats are often monopolised by canopy algae of the genus Cystoseira and, in particular, by the species Cystoseira amentacea var. stricta . Alternatively, low-shore habitats are dominated by turf-forming algae and/or mussels. Previous studies showed that loss of Cystoseira , by natural or anthropogenic disturbances, resulted in the increase of turfing algae. Here, we propose that turf-forming algae may act as colonisers from nearby habitats, specifically from lower portions of the shore. The following hypotheses were tested to investigate this proposition: (1) in presence of Cystoseira , the assemblage living under its canopy will be distinct, in terms of composition and structure, from that found lower on the shore, (2) if the canopy of Cystoseira was removed, the differences between the two assemblages would decrease or disappear, (3) the effects of the canopy would be more important than other causes of variability at the spatial scale investigated (hundreds to thousands of metres), and (4) that effects of removal of the canopy would be consistent through time. These hypotheses were tested by means of a canopy removal experiment, involving several spatial and temporal repetitions of the manipulation. The assemblage underneath the canopy of Cystoseira was distinct from that found immediately lower on the shore, but when the canopy was removed there were no differences between the two. The effects of the canopy were consistent at the spatial and temporal scales investigated. Several species living under the canopy disappeared and were replaced by turf-forming species, resulting in a homogenisation of the two habitats and in a drastic loss of habitat diversity. This study shows that Cystoseira can be considered a habitat-forming species, responsible for the maintenance of two distinct low-shore assemblages. In this light, its importance should be taken into account by policies aiming to preserve biodiversity on rocky shores.

Structure, Seasonal Dynamics and Reproductive Phenology of a Filamentous Turf Assemblage on a Sediment Influenced, Rocky Subtidal Shore

Seasonality and reproductive phenology of marine macroalgae were investigated from June 1992 to June 1993 on a subtidal rocky shore south of Livorno (Western Mediterranean Sea) that was characterized by high rates of sediment deposition and movement. The assemblage was strongly dominated by a persistent filamentous turf of Polysiphonia setacea that covered the rocks and encrusting algae, while Halimeda tuna was the only abundant erect species. During the year the thickness of the turf fluctuated in response to changes in weather conditions and in the intensity of depositional environment. Conversely, the composition and the structure of the assemblage remained rather constant, and its diversity was quite low. It is suggested that both sediment stress and competitive interactions among P. setacea and erect species might play an important role in maintaining and structuring the algal turf in the study area.

Response of the non-indigenous Caulerpa racemosa (Forsskål) J. Agardh to the native seagrass Posidonia oceanica (L.) Delile: effect of density of shoots and orientation of edges of meadows

Abstract Caulerpa racemosa is a tropical green alga introduced into the Mediterranean as an immigrant from the Red Sea which has successfully fast-spread in the south-eastern and in the north-western part of the basin. C. racemosa occurs mostly in shallow but also in deep subtidal habitats colonising hard and soft substrata where turfs, erect algae and even seagrasses are present with the potential to profoundly alter indigenous communities. However, the extent to which biotic interactions influence the spread of the alga is not well known. In this study the effects of the presence of the native seagrass Posidonia oceanica on the non-indigenous alga Caulerpa racemosa are examined: a multifactorial experiment was designed to test (1) the importance of the seagrass canopy structure and (2) of orientation of seagrass edge on algal performance along the edge and inside the meadow of P. oceanica and (3) whether patterns of algal growth are consistent at different spatial scales (few centimetres to several metres). The aim of this study is to provide a basis for further experimental investigations of the factors and mechanisms affecting the performance of this alga in the Mediterranean. The results of this study indicated that where Caulerpa racemosa is at the edge of Posidonia oceanica meadow, the vertical growth of the alga (blade length) is sensitive to the combination of time, seagrass density and edge-meadow orientation, that the spread of the alga along the edge of the seagrass meadow is dependent on the characteristics of the area and that the growth of the alga inside the meadow is influenced by seagrass density. The findings of this study suggest that the susceptibility of the indigenous P. oceanica community to invasion of the introduced alga C. racemosa is related to the availability of sand habitat ground created, since low invasion of the very dense edges of the seagrass was observed compared to the less dense ones.

Sexual reproductive success in Posidonia oceanica

Abstract To identify the factors that might contribute to limiting the sexual reproduction of Posidonia oceanica we examined: (1) flowering and fruiting phenology; (2) variability in flowering frequency, seed production and reproductive success (i.e. the proportion of flowers setting mature fruits per unit area) over a 2-year period; and (3) losses of potential seed production to fruit abortion and/or pre-dispersal seed predation. The flowering frequency of P. oceanica varied among years, ranging between 7.9 and 19.8%. Despite the large number of flowers and ovules, few of these produced mature fruits owing to abortion. Moreover, about 84% of developing inflorescences were damaged by herbivores. Fruit production varied among years, ranging from 4.7 to 13.5 fruits per m2, but reproductive success remained constant (2.0–2.4%). Exclusion of herbivores showed that abortion alone was reponsible for the loss of about 87% of the reproductive potential. Since reduced fertilization did not completely explain the observed frequency of abortion, it was hypothesized that post-fertilization factors could affect seed formation. The higher reproductive success of protected plants (11.2%) compared to herbivore-exposed plants (3.1%) indicated that predation significantly reduced the number of seeds available for establishment. We concluded that pre-dispersal seed losses to abortion and predation may seriously reduce the reproductive success of the species.

Effects of Posidonia oceanica canopy on Caulerpa taxifolia size in a north-western Mediterranean bay

Abstract Caulerpa taxifolia (Vahl) C. Agardh is a green tropical seaweed recently introduced in the Mediterranean. Since 1984, when for the first time it was recorded along the French Riviera, it has rapidly spread into a wide range of sublittoral habitats and has contributed to the decay and regression of the seagrasses Cymodocea nodosa (Ucria) Ascherson and Posidonia oceanica (L.) Delile. Descriptive field studies have indicated a strong habitat effect on C. taxifolia performance and suggested a positive influence of seagrasses, particularly of P. oceanica, on both blade density and size of the alga. Here we present two experimental investigations that have been carried out in situ to highlight mechanisms that regulate this kind of interaction. The aim of this study was to elucidate factors inducing the performance of this alga in the Posidonia oceanica habitat, to provide a basis to detect determinants of success for the invader non-indigenous species, and to predict vulnerability to invasion for different conditions of the seagrass beds. In the first experiment Posidonia oceanica shoot density and canopy height were manipulated along the edge of the meadow at 2 and 10 m depth and the response of Caulerpa taxifolia was assessed after four different elapsed times. The second experiment, designed to investigate the mechanism responsible for the canopy influence on blade size, employed mimic plants made of transparent plastic strips able to protect but not shade the alga. Because of the epiphyte growth which progressively attenuated the transparency effect of the treatment, only short-term results could be obtained. Overall results have indicated that the positive effect of Posidonia oceanica on Caulerpa taxifolia is greater at the deepest edge of the seagrass stand where the shoot density is lower and have suggested contrasting effects of the canopy on the alga. This has been partially explained by the fact that at the deeper site algal blades were greater where they occurred at the edge of mimic transparent plants indicating that the protection has a cost. Therefore, we predict that dense P. oceanica meadows are likely to be less vulnerable to seaweed invasion, whereas sparse meadows represent an optimal compromise of protection and shading.