Habitat effect on spatio-temporal variability in size and density of the introduced alga <i>Caulerpa taxifolia</i>

Caulerpa taxifolia is an introduced green seaweed in the Mediterranean; it is a very fast-spreading species, is able to invade all kinds of substrata and causes regression of some seagrasses. C. taxifolia was investigated in a small bay of the northwestern Mediterranean, where it occupies 3 distinct habitat types: (1) at the edge of Posidonia oceanica, (2) within Cymodocea nodosa beds, and (3) on sand and cobbles. To provide a basis for further experimental investigations of the factors affecting its performance, a descriptive study was carried out at this site in which both size and density of blades were measured on 20 dates in all habitats from October 1994 to September 1996. Great temporal fluctuations within the same season were evident for both variables, despite the fact that high variability was found at small spatial and temporal scales (areas within each habitat and time within each season). However, habitat effect in combination with season was evident as a major factor affecting both length and density of blades, suggesting a positive effect of seagrasses on C. taxifolia. The greatest blade length was found in individuals at the edge of P. oceanica, while within C. nodosa beds, blade length was intermediate. Reduction of blade density occurred in spring in the sand and C. nodosa habitats, but not in the P. oceanica one. Patterns were similar throughout the 2 years. Experimental evidence is needed both to highlight mechanisms regulating this kind of interaction (nurse effect vs shade-induced changes) and to investigate whether it affects patterns of invasion and replacement of existing species with C. taxifolia in these habitats. The facilitative effect of seagrasses on the alga observed in the study is also likely to cause an indirect negative effect on seagrasses themselves

The Role of vegetative fragmentation in dispersal of the invasive alga Caulerpa taxifolia in the Mediterranean

This study evaluated the importance of fragmentation in the recruitment of the fast-spreading, introduced green alga Caulerpa taxifolia at the margins of beds of the seagrass Posidonia oceanica. A multifactorial experiment was designed to test the hypotheses that there are seasonal differences in patterns of establishment of vegetative fragments, whether this process changes with depth and whether these patterns were consistent at different spatial and temporal scales. Our experimental approach consisted of dispersing drifting fragments of C. taxifolia along the margin of a bed of P. oceanica and recording the number of fragments established after 1 mo. The results show that a surprisingly large number of fragments become established in this habitat and that numbers varied in space and time: the probability of establishment of fragments was greatest during summer especially at the shallow sites, but smaller in spring and smallest in winter. Differences among areas were also found: a great variability in establishment of fragments depended on the site and time within season. Results indicate that dispersal by fragmentation can greatly contribute to a very wide spread of the alga in the Mediterranean. We predict that spread will be greatest during summer when a large proportion of fragments can re-attach to the substratum, even at shallow sites. Such information is important for the understanding of the ecology of this species and, with the help of hydrographic studies, in the prediction of its patterns of geographic dispersal

Condizione ecologica di <i>Ruditapes decussatus</i> (<i>Veneridae</i>) e sua presenza in aree di raccolta nel Golfo di Olbia = Ecological status and occurrence of <i>Ruditapes decussatus</i> (<i>Veneridae</i>) in harvested sites within the Gulf of Olbia

A pilot study was performed in order to evaluate the ecological status of the clam Ruditapes decussatus and its occurrence in specific harvested areas within the Gulf of Olbia. Indeed, since 1998, stocks of this clam seems to be decreased, due to intense human harvesting

Investigating on the occurrence of <i>Paracentrotus lividus</i> in rocky and <i>Posidonia oceanica</i> habitat

The sea urchin Paracentrotus lividus (Lamarck) is the most common grazer in the Mediterranean infralittoral that at high densities overgrazes complex algal assemblages turning them into barren areas. This study has the aim to investigate whether abundance and population structure of P. lividus is consistent between rocky boulders and Posidonia oceanica habitat. At this aim, we have sampled P. lividus at six sites in the Gulf of Alghero (North West Sardinia), 3 fished (sea urchin are harvested) and 3 controls (no harvest is allowed) and at each site the two habitats, 6-10 m deep, were considered. For each combination site x habitat 10 replicates were taken. Density of P. lividus was assessed using quadrats of 1 x 1 m. The size of 20 individual (test diameter without spines) per site was measured by means of a calliper (1#1 0.1mm). Sea urchins, finally were grouped into size classes to examine frequency distributions. Sampling was performed at the end of a harvesting period (April-May 2006). Analyses of data have highlighted a significant variability for both response variables among sites while in Posidonia oceanica habitat a lower density and a higher size were found rather than in rocky habitat. Further data collected at three types of Posidonia oceanica (patches close to rocky habitat, far from rocky habitat and patches bordered by sediment) suggest that the abundance of adults in the seagrass is probably sustained by immigration from the rocky habitat

Different structure of assemblages understoried by three <i>Cystoseira</i> species occurring on the same upper-infralittoral platforms

Canopy algae of the genus Cystoseira are considered important habitat formers providing shade and shelter for a diversified assemblage of animals and plants. Some evidence suggested that these algae are highly sensitive to anthropogenic disturbances and their presence could be used as an `ecological quality' indicator. In this paper we tested the hypothesis that structure of assemblages understoried by these algae is dependent on Cystoseira species. To achieve this aim the composition of benthic assemblages under the canopy of three Cystoseira species was estimated at Sinis-Mal di Ventre MPA (western Mediterranean), where at some sites these algae occur on the same upper-infralittoral platforms. At one site 2 areas have been sampled and in each area 5 replicates were taken per Cystoseira species. The percent coverage of benthic species was estimating in the field using a 20X20 cm multiple-scale quadrats. A multivariate analysis was performed to highlight differences in taxa composition. Results have evidenced significant variations in assemblages depending on Cystoseira species (ANOSIM R=0.585 p=0.1%). These results suggest that the three Cystoseira species co-occurring on the same platforms do structure different habitats supporting diversified species assemblages. Hence, given the high variability on small spatial scale of habitats it is necessary to perform well designed experiments to correctly interpret results collected during monitoring program

L'Effetto del calpestio sulla meiofauna e macrofauna dell'infralitorale superiore roccioso nell'AMP dell'Isola dell'Asinara (Sardegna nord occidentale)

La ricerca qui descritta si è svolta nell'AMP dell'Isola dell'Asinara e ha cercato di valutare l'effetto del calpestio sulla comunità zoobentonica associata ad alghe del medio- ed infralitorale superiore di fondo roccioso. È stato condotto un esperimento multifattoriale di simulazione del calpestio: in due cale `no entry, no take' sono state scelte 6 aree e, 3 intensità di calpestio sperimentale (0, 60 e 120 calpestii per quadrato di 20x20 cm), sono state attribuite a random nelle aree. I prelievi sono stati compiuti con cilindri di plastica (diametro 40 mm) e un raschietto metallico; sono stati raccolti 2 campioni per quadrato. Il materiale è stato fissato in formalina. In laboratorio sono state separate le due componenti mediante adeguati setacci; gli animali raccolti sono stati identificati e contati. Dall'analisi multivariata risulta che, per entrambe le componenti, vi è differenza significativa tra i controlli e le aree dove è stato effettuato il calpestio (ANOSIM macrofauna p=0,4% R=0,45, meiofauna p=0,5% R=0,50). Inoltre dai confronti a coppie tra i livelli di calpestio risulta una differenza significativa tra i controlli e le due intensità di trampling (60 e 120), mentre non vi è differenza significativa tra le due intensità di calpestio. Sebbene non si possa fare nessuna previsione sul recupero della comunità, questi risultati suggeriscono che sia la macrofauna sia la meiofauna siano estremamente vulnerabili a questo tipo di disturbo

S'Ena Arrubia lagoon, western Sardinia

Generally, through the LOICZ model application to the lagoon, it is observed that there is prevalence of nutrient storing processes rather than that of nutrient mobilisation. Nitrogen fixation always prevailed over denitrification. The productive processes in the lagoon thus seem to be prevalent over most of the year. These results confirm that S’Ena Arrubia Lagoon is to be considered an autotrophic system as far as the net total metabolism is concerned

Habitat and scale shape the demographic fate of the keystone sea urchin <i>Paracentrotus lividus</i> in Mediterranean macrophyte communities

Demographic processes exert different degrees of control as individuals grow, and in species that span several habitats and spatial scales, this can influence our ability to predict their population at a particular life-history stage given the previous life stage. In particular, when keystone species are involved, this relative coupling between demographic stages can have significant implications for the functioning of ecosystems. We examined benthic and pelagic abundances of the sea urchin Paracentrotus lividus in order to: 1) understand the main life-history bottlenecks by observing the degree of coupling between demographic stages; and 2) explore the processes driving these linkages. P. lividus is the dominant invertebrate herbivore in the Mediterranean Sea, and has been repeatedly observed to overgraze shallow beds of the seagrass Posidonia oceanica and rocky macroalgal communities. We used a hierarchical sampling design at different spatial scales (100 s, 10 s and &lt;1 km) and habitats (seagrass and rocky macroalgae) to describe the spatial patterns in the abundance of different demographic stages (larvae, settlers, recruits and adults). Our results indicate that large-scale factors (potentially currents, nutrients, temperature, etc.) determine larval availability and settlement in the pelagic stages of urchin life history. In rocky macroalgal habitats, benthic processes (like predation) acting at large or medium scales drive adult abundances. In contrast, adult numbers in seagrass meadows are most likely influenced by factors like local migration (from adjoining rocky habitats) functioning at much smaller scales. The complexity of spatial and habitat-dependent processes shaping urchin populations demands a multiplicity of approaches when addressing habitat conservation actions, yet such actions are currently mostly aimed at managing predation processes and fish numbers. We argue that a more holistic ecosystem management also needs to incorporate the landscape and habitat-quality level processes (eutrophication, fragmentation, etc.) that together regulate the populations of this keystone herbivore

Efectos a corto plazo de un pisoteado experimental sobre las poblaciones de poliquetos que habita un sustrato rocoso intermareal en la Isla de la Asinara (Área Marina Protegia del Mediterráneo Noroccidental)

Rocky intertidal platforms at Asinara Island Marine Protected Area (Italy, Mediterranean Sea) were experimentally trampled to assess the impact of human visitation on polychaetes. Trampling at two different experimental intensities (60,120 steps/20x20 cm quadrat) and controls were applied at two locations (&ldquo;no-entry, no-take&rdquo; zones) of the MPA. One day after the experimental trampling, benthic samples were collected and the polychaetes counted and identified. Overall 1146 individuals were collected; Salvatoria clavata, Polyophthalmus pictus, Syllis prolifera, Amphiglena mediterranea, Fabricia stellaris, Platynereis dumerilii and Sphaerosyllis taylori were the most abundant species. Analyses of variance on all polychaetes and on each of the important polychaete species showed a significant higher abundance in controls than in trampled plots. Given the short-term decline in abundance, these results suggest that polychaetes are vulnerable even to the lowest experimental level of trampling.Las plataformas rocosas intermareales del &Aacute;rea Marina Protegida (AMP) de la Isla de la Asinara (Cerde&ntilde;a, Italia), fueron experimentalmente pisoteadas para evaluar el impacto de la presencia humana sobre las comunidades de poliquetos. Se aplicaron dos diferentes intensidades experimentales (60,120, pasos sobre una superficie de 20x20cm) y control en dos zonas (zonas definidas como &ldquo;no-entry, no-take&rdquo;) de la AMP. Un d&iacute;a despu&eacute;s de la perturbaci&oacute;n experimental, se recog&iacute;an muestras bent&oacute;nicas identificando y cuantificando los poliquetos. Un total de 1.146 poliquetos fueron recogidos; Salvatoria clavata, Polyophthalmus pictus, Syllis prolifera, Amphiglena mediterranea, Fabricia stellaris, Platynereis dumerilii y Sphaerosyllis taylori fueron las especies m&aacute;s abundantes. El an&aacute;lisis de la variaci&oacute;n global de los poliquetos y de sus especies permitieron observar una abundancia significativamente superior en las &aacute;reas de control que en las parcelas pisoteadas. Considerando en el corto plazo la disminuci&ograve;n de la abundancia, estos resultados sugieren que los poliquetos son vulnerables, includo en el nivel experimental m&aacute;s bajo probado. &nbsp

Variabilità spazio-temporale della microalga <i>Chrysophaeum taylorii</i> Lewis &amp; Bryan lungo le coste nord-orientali della Sardegna = Spatio-temporal variabilty of the microalga <i>Chrysophaeum taylorii</i> Lewis &amp; Bryan along North-Eastern Sardinian coasts

In the summer months of 2010 the cell density of the alien benthic microalga Chrysophaeum taylorii Lewis &amp; Bryan (Pelagophyceae) was assessed on hard benthic substrates in sixteen study sites along the north-eastern coast of Sardinia in order to obtain useful information on spatial and temporal variations of the species in the area during summer