Oceanica Beds Research Articles

Alterations of the structure of <em>Posidonia oceanica</em> beds due to the introduced alga <em>Caulerpa taxifolia</em>

The impact of Caulerpa taxifolia on the structure of shallow Posidonia oceanica beds was studied in permanent quadrats from 1995 to 2005 at the invaded site of Cap Martin and the control site of Cap d’Antibes (French Riviera, France). The cover of C. taxifolia , shoot density, number of orthotropic and plagiotropic shoots and proportion of ramifications of P. oceanica were measured yearly. The cover of C. taxifolia in the invaded zone rapidly reached a maximum of infestation in 2000 with 93% of the quadrats covered by the alga. In 2001 an unexplained phenomenon led to a sharp decrease in the infestation and in the following years the colonisation remained low. Within the 10 years of the study, P. oceanica did not disappear from the permanent quadrats, but we observed a drastic change in the structure of the meadow invaded by C. taxifolia . Between 1999 and 2000 a decrease in the shoot density observed at both sites was probably related to the warm temperature event recorded in 1999 (from 636 to 143 shoots m -2 at the invaded site and from 488 to 277 at the control site). At the invaded site, the seagrass never recovered its initial density even after a sharp decrease in C. taxifolia . The orthotropic/plagiotropic shoot ratio was strongly modified at the invaded site, where plagiotropic shoots became dominant because of an increase in their ramification.

Evaluating the effects of protection on Paracentrotus lividus distribution in two contrasting habitats

The sea urchin Paracentrotus lividus is common in the Mediterranean in shallow subtidal rocky habitats and in Posidonia oceanica beds. The aim of this study is to investigate whether protection has the same effect on the population structure of P. lividus occurring in rocky reef habitats and in P. oceanica beds. These results are important to generate hypotheses about the influence of human harvesting, predatory pressure and migration processes on P. lividus in the two habitats. Paracentrotus lividus was sampled at seven locations within the Gulf of Alghero (North West Sardinia) where the Capo Caccia–Isola Piana MPA (Marine Protected Area) is sited: 1 location was sited in Zone A, where no harvesting of P. lividus is allowed (NH), 3 locations were sited in Zone B, where harvesting is restricted (RH), and the other 3 were located outside the MPA where no restrictions apply to sea urchin harvesting (UH). For each combination of habitat × location, P. lividus density was assessed in 10 replicates using quadrats of 1 × 1 m and the size of 20 individuals (test diameter without spines) was measured. Finally, the specimens were grouped into size-classes to examine frequency distributions at each location. Sampling was performed at the end of the sea urchin harvesting period (April–May 2006). Analyses of data have highlighted significant variability among locations for both response variables. In both habitats, no differences were found in Paracentrotus lividus abundance among levels of protection (NH vs. RH vs. UH), while a significantly higher size was found in NH rather than in RH and UH locations. Differential direct and indirect effects of protection on P. lividus size is discussed. Also, P. lividus size seemed dependent on the habitat being quite larger in Posidonia oceanica than in the rocky reefs. This finding suggests that settlement and recruitment could be more highly successful events in rocky habitats, and that in P. oceanica meadows large-sized immigrants coming from the rocky habitat contribute to the population structure. The need to define the role of the P. oceanica habitat is also discussed.

Posidonia oceanica in the Marmara Sea

The seagrass Posidonia oceanica is a stenohaline species endemic to the Mediterranean Sea, where it normally lives at a salinity of between 36.5 and 39.5 ppt. Surveys carried out at the North-eastern distribution limits revealed large beds in the Dardanelles Strait and isolated beds in the Marmara Sea, where the salinity ranges between 21.5 and 28 ppt. Microsatellite analysis of these low-salinity tolerant P. oceanica beds, show different signs of genetic isolation: excess of heterozygosity and a presence of fixed alleles. These particularities are rarely found in the whole distributional range of the species. Moreover, all the populations considered in the analysis have a very low genetic diversity in comparison with most of the meadows sampled throughout the Mediterranean Sea. Taking into consideration the genetic data, rhizome expansion rate and the actual extent of the isolated beds in the Marmara Sea and knowing the reproductive rate and dissemination characteristics of P. oceanica, we hypothesize that the isolated population of the Marmara Sea has been established since the Middle Holocene, before the catastrophic intrusion of brackish water into the Marmara Sea and the strong and persistent flow coming from the Black Sea.

Impact of fish farming facilities on Posidonia oceanica meadows: a review

AbstractThe impact of fish farming facilities on Posidonia oceanica meadows was assessed from studies of intensive facilities carried out over the last few years. The disturbances caused by these fish farms were measured by means of both abiotic (light, sediment, interstitial water) and biotic variables (meadow density, leaf biometry, lepidochronology, primary production, epiphytes, reserve carbohydrates in the rhizomes), in function of increasing distance from cages and/or inside a geographically close reference site. The results showed significant degradation of these seagrass meadows in all the sectors investigated. When fish farming cages were placed above a P. oceanica bed, the meadow was severely degraded or disappeared and the sediment showed a strong increase in organic matter that could lead to anoxia phenomena. The irreversible impact of fish farming projects on P. oceanica meadows requires the application of the precautionary principle. Several recommendations (site selection, preliminary studies and monitoring over time) are suggested in order to enable piscicultural activities to be incorporated in a global process of Integrated Coastal Zone Management.

Occurrence of mucous aggregates and their impact on Posidonia oceanica beds

Mucous macro-aggregates of both pelagic and benthic origin are recurrently observed in the meadows formed by the seagrass Posidonia oceanica around the island of Ischia (Gulf of Naples, Italy). In the past two decades, major events occurred in 1991, 1993 and 2000, when a thick layer of mucilage covered vast areas of the meadows. To investigate the environmental triggers for mucilage formation and the effects of macro-aggregates on the functional and structural status of P. oceanica, a number of abiotic and biotic parameters were monitored over three years within the frame of a research project on Adriatic and Tyrrhenian mucilages (MAT). Basic environmental parameters (salinity, temperature, irradiance, dissolved oxygen concentration) in the water column and inorganic nutrient concentrations above and within P. oceanica meadows were measured. As descriptors of the status of the seagrass, shoot density and the nitrogen content of the leaves were monitored. Moreover, a reconstructive technique (lepidochronology) was employed to track back annual plant production. During the three-year study, mucous aggregates produced by benthic algae were observed in spring–summer and a major event of macro-aggregates of pelagic origin affecting P. oceanica canopy was observed in autumn 2000. Each of these episodes was observed for a few weeks to one month. We hypothesize nutrient limitation to explain the short duration of the benthic algal blooms that were responsible of macroaggregate formation; a highly dynamic circulation resulted in the dispersion of the pelagic aggregates deposited on the meadows. None of the descriptors of the structure of the meadow and of plant performance showed any obvious alterations in relation to the occurrence of mucilage coverage. Presumably, the short duration of the mucilage episodes recorded were not enough to induce such alterations, at least at the temporal and spatial scales considered.

Colonisation process of vegetative fragments of Posidonia oceanica (L.) Delile on rubble mounds

Seagrass colonise new areas via the dispersion of seeds or vegetative fragments. Independent of the manner of colonization, habitat requirements need to be met for the successful establishment of seagrasses. Here we report on the colonization process of Posidonia oceanica in a highly disturbed area: a gas pipeline trench at Capo Feto (SW Sicily, Italy). A trench dredged through a P. oceanica bed was back-filled with rubble added from dump barges leading to the formation of a series of rubble mounds on the seabed. Over time, these mounds became colonised with P. oceanica. I>. In order to understand the pattern of P. oceanica colonization, shoot density was quantified over 3 years (2001–2003) on different mound locations (crests, sides, valleys). Seagrass coalescence was observed only in valleys between mounds where shoot density averaged 133±50 shoots m−2, while values for sides and crests were significantly lower (30.5±14 and 5.8±2.6 shoots m−2, respectively). Although sediment accumulated on both crests and valleys, a significantly thicker sediment layer was recorded in the valleys (9.8±0.4 cm) than on crests (1.1±0.2). Plaster dissolution rate (an indicator of the hydrodynamic regime) tended to decrease from crests to valleys but even in the valleys, the currents were still higher than in the adjacent vegetated control location. This pattern was constant over time and depths. This is the first study to report on P. oceanica vegetative recruitment on artificial rubble after a disturbance event. It appears that the valleys between the rubble mounds are suitable for seagrass recruitment as sediment deposited between the rubble provides the necessary resources for plant settlement and growth. Once the seagrass patches are established, they may start a positive feedback of attenuation of currents, sediment accumulation and seagrass patch expansion.

Taxonomic diversity and distribution of polychaete and isopod borers on the sheaths of the seagrass Posidonia oceanica: analysis at regional scale along the coast off Sardinia (Italy)

Polychaete and isopod borers of Posidonia oceanica leaf tissues have been identified as a peculiar guild of detritus consumers, colonizing the seagrass sheaths (remains of the leaf bases which persist along the rhizome). Taxonomic diversity and distribution of borer organisms have been in 89 stations along the coast off Sardinia during summer 2001: 34 in shallow Posidonia stands (5–12 m depth), 50 in deep stands (22–26 m depth), and 5 at the lower limit (36–38 m). Shoot density was measured at each station. Borers and their traces were recorded for each rhizome. In each station relative abundance of species (%), and the Indices of Borer (IB) and of traces were calculated for each borer species. A total of 6643 vertical (orthotropous) P. oceanica shoots were examined (mean 75 shoots per station), 26.6% of which hosted borers, while 42.5% showed only traces of their previous colonization. Four species of polychaete Eunicidae (Lysidice collaris Grube, Lysidice ninetta Audouin & Milne‐Edwards, Nematonereis unicornis Grube, Marphysa fallax Marion & Brobretzky), and a single species of isopod Limnoriidae (Limnoria mazzellae Cook‐son & Lorenti) were identified. Polychaetes were collected in all the 89 stations sampled, while L. mazzellae was restricted to 32 stations, mainly in shallow stands. The mean IB values of L. mazzellae were significantly higher in shallow stands, with respect to deep ones. The species L. collaris and M. fallax showed higher frequencies in deep stands, while L. ninetta and N. unicornis had similar IB values, regardless of depth. The lepidochrono‐logical analysis, performed on most of the shoots, revealed that L. mazzellae selects more recent sheaths (first 2 lepidochronological years), while eunicid polychaetes are more frequent in aged scales (>2 years). Most of the significant differences found in IB indices and relative abundances of borer species, were recorded between normal density or high sub‐normal density meadows, and the “altered” density ones, regardless of depth. The Spearman's analysis revealed various significant correlations of IB indices of borers with shoot density, although with relatively low coefficients. The analysis of some stations selected in potentially human‐impacted and in pristine areas showed that while shoot density was significantly higher in un‐impacted areas with respect to impacted ones, the IB indices of borer groups and individual borer species did not show differences among the two kinds of sites, except for that of polychaetes. The studied data set demonstrates that borer organisms occur in P. oceanica beds under different ecological situations (e.g., depth range, bed typology, shoot density, potential human impact), with different distribution among species that are discussed also in relation to some biotic factors (life habit, habitat partitioning, competition).

Architectural characteristics of two bed types of the seagrass Posidonia oceanica over different spatial scales

Seagrass beds occur in various morphological forms, ranging from small patches to continuous meadows. The endemic Mediterranean seagrass Posidonia oceanica forms dense and extensive stands that occur in several different morphotypes, including reticulate (seagrass interspersed with a different habitat type, such as bare sand) and continuous beds. This study, undertaken in the Maltese Islands, examined whether reticulate and continuous P. oceanica beds, located adjacent to each other and at similar depths, had different within-bed architectural characteristics. Five commonly used architectural measures (shoot density, number of leaves per shoot, mean leaf length, mean leaf width and shoot biomass) were measured from P. oceanica shoots collected from the two bed types at three different spatial scales: (1) tens of metres (‘small’ scale); (2) hundreds of metres (‘medium’ scale); and (3) kilometres (‘large’ scale). Results of 2-factor ANOVA (factor 1=bed type; factor 2=sampling locality) carried out at the three spatial scales indicated significant differences between the two bed types in shoot density ( P < 0.01 ) and leaf length ( P < 0.05 ) at the small scale, and in leaf number ( P < 0.05 ) at the large scale. Significant interactions were also apparent for shoot density (at the large scale) and for shoot biomass (at the medium scale). However, the results obtained did not indicate consistent architectural differences between the two P. oceanica bed types over the spatial scales considered. Spatial variations in within-bed architectural characteristics observed were therefore thought to be attributable mainly to the influence of local environmental factors. The findings are discussed with reference to the conservation and management of P. oceanica habitat.

Underwater acoustic positioning systems as tool for Posidonia oceanica beds survey

The study of the dynamics of Posidonia oceanica beds, and more particularly, the position of their lower limit, are usually carried out by the installation and monitoring of markers set on the sea bottom. Acoustic telemetry could constitute an interesting alternative solution to the marking of the lower limit seagrass beds. In order to test the potentialities of this system, an experiment was conducted in the Natural Marine Reserve of Cerbère–Banyuls (eastern Pyrenees, France). The possibility of very precise mapping offers many potentialities with regards to the work of monitoring networks in the marine environment (species and assemblages of importance), but also in the monitoring of competition dynamics between the species. To cite this article: P. Descamp et al., C. R. Biologies 328 (2005).

Lipid composition of Symphodus ocellatus (Perciforme: Labridae) in the north-western Mediterranean: influence of two different biotopes

The lipid content and fatty acid composition of the small common wrasse Symphodus ocellatus was analysed within two populations located in very different biotopes on the French shore: a Caulerpa taxifolia meadow located in Cap Martin and a Posidonia oceanica bed located in Villefranche. Polar lipids represented 82–90% of the total lipid in adult female livers, gonads and in eggs. Cholesterol in gonads and eggs and triacylglycerols in livers were the dominant neutral lipids. Differences in lipid classes were found in livers between the two populations, but not in their gonads and eggs. Moreover, a quantitative difference in total lipid content was evident. Young fish living on the C. taxifolia meadow in winter had 21% less lipid than those living on the P. oceanica bed. The populations of S. ocellatus showed differences between triacylglycerol versus phospholipid fatty acid compositions and between gonad versus liver fatty acid compositions. Moreover, a significant difference was found between both populations in their liver triacylglycerol fatty acid compositions, suggesting a difference in diet. Gut content analysis supported this hypothesis as it showed important differences in the ingested preys between the two populations in January (planktonic vs. benthic, frequency of isopods) and in June (frequency of gastropods). Thus, we found that the fish populations from Villefranche and Cap Martin showed significant differences in body lipids due to different nutritional habits.

A one-century suite of seagrass bed maps: can we trust ancient maps?

To assess the dynamics of seagrass beds, it is necessary to be able to compare their present day and former distribution patterns. In most cases, ancient data are available in the form of maps whose reliability must be estimated before comparison. In the study area (near Marseilles, France), a series of maps of the seagrass Posidonia oceanica beds spanning more than one century (dating from 1883, 1897, 1958, 1975, 1979, 1991 and 2000, respectively) are available. In addition, a new map was established, using aerial photography (shallow areas), side scan sonar (deeper areas) and data validation by ground truth. In order to help assess the value of the ancient maps, a Reliability Index (RI) is proposed. The RI, rated from 0 to 50, weighs three parameters: (i) the initial scale of the map (source map) and the working scale (target map); (ii) the method of data acquisition (e.g. dredges, grabs, aerial photography, side scan sonar, scuba diving); and (iii) the method of data georeferencing. Dramatic differences in the distribution of seagrass beds are apparent between the eight maps. In view of the biological features of P. oceanica (e.g. slow rate of change of the meadow limits under natural conditions and the persistence of rhizomes after its death), it is possible that some of these differences may not reflect change over time but may be due to errors in the ancient maps. In contrast, other differences are plausible and validated by field data, e.g. the regression of the lower limit of the meadow. It was thus possible to compare the calculated RI for each map and the plausibility of observed differences as a basis for calibrating the RI.

Boat anchoring on Posidonia oceanica beds in a marine protected area (Italy, western Mediterranean): effect of anchor types in different anchoring stages

Seagrasses worldwide are noted for suffering from mechanical damage caused by boat anchoring. This is particularly so in sites highly frequented by boaters (marine protected areas or coastal urbanised areas). In the last decades, different strategies have been put into practice to reduce such impacts on seagrasses (i.e. by anchoring bans or by deploying boat moorings). More recently, in consideration that few marine protected area (MPA) management bodies or local administrations have the resources to enforce their anchorage regulations, the self-regulatory approach based on education and information of boaters has been preferred in several cases. At present, however, very little is known on the correct anchoring practices to ensure the safeguarding of seagrasses. The aim of the present study was to experimentally quantify in the field the damage caused to Posidonia oceanica shoot density by anchoring. A multifactorial experiment was designed to test whether the damage is dependent on (1) different anchor types (Hall, Danforth and Folding grapnel), (2) the use of a chain vs. a rope, (3) the three anchoring stages (anchor fall, dragging/lock-in and weighing), and finally (4) whether the pattern is consistent among different locations of the meadow. As expected, the three anchor types employed in the present study differed in the levels of damage inflicted on the P. oceanica meadows of the Ustica Island MPA. In particular, the use of the Hall type anchor seems to be preferable to minimise this impact in comparison with the other two anchor types. Moreover, the effect on the meadow of the three anchor types is greatly dependent on the anchoring stage. These results confirm that the weighing stage is the critical stage of the anchoring process. The number of damaged shoots of P. oceanica was not affected by the presence of the chain. These patterns were consistent between locations. In the long term, even anchoring on P. oceanica by small boats using low-impact anchors may potentially have detrimental consequences. For this reason, we suggest that in vulnerable sites, it is preferable to implement an educational program based on information of boaters on correct anchoring practices and anchor typology to use, rather than adopting strong restrictions to boat anchoring or deploying mooring buoys. Although the use of these management strategies is still recommended in the case of anchorage frequented by bigger vessels using heavier anchors and chains.

Biodiversity and trophic structure of nematode assemblages in seagrass systems: evidence for a coupling with changes in food availability

Nematode assemblage composition, trophic structure and biodiversity were followed over an annual cycle in a Posidonia oceanica bed of the NW Mediterranean to test the response to temporally changing food availability (sediment organic matter, bacterial and microphytobenthic biomass). The sediment–water interface of the seagrass meadow was characterised by high particulate organic matter concentrations. Also, seagrass sediments had high organic matter content, chloropigment concentrations and bacterial biomass. All organic matter components (i.e. carbohydrates, proteins and lipids) changed temporally, with higher concentrations in winter and spring and lower concentrations in summer; however, overall, large amounts of organic compounds were potentially available for (seagrass) benthic consumers throughout the year. Nematode assemblage in the P. oceanica bed maintained a high genus number (88 genera), and a trophic structure tightly coupled with the composition of the potential food sources. In agreement with the relevance of microphytobenthos, epigrowth-feeder nematodes (2A) represented the dominant trophic guild. The biomass of predator nematodes (2B) was significantly correlated with the biomass of other nematodes. Non-selective deposit feeders (1B) displayed a close relationship with the concentration of proteins, carbohydrates and biopolymeric carbon (i.e. labile organic detritus). Conversely to what was expected, epigrowth feeders were not correlated with chloropigment concentrations, but showed a significant relationship with the number of dividing bacteria, possibly suggesting a trophic plasticity of the 2A feeding guild. The coupling between temporal changes in food sources and temporal variability of the nematode trophic structure was highlighted by a CANOCO analysis, which allowed us to identify and associate, at each sampling time, nematode genera and their feeding habits, with specific environmental variables and food indicators. Furthermore, species diversity (H′) and evenness (J) calculated on nematodes identified to the genus level displayed evident temporal changes, also reflected by the index of trophic diversity (ITD). Both structural and functional diversity were coupled with high concentration and highly heterogeneous composition of the food sources (including organic detritus, microphytobenthic algae and bacteria) potentially available to nematodes. These data suggest that temporal changes in quantity and quality of food sources do not only influence nematode dynamics and trophic composition, but also influence nematode structural and functional diversity.

First record of the Indo-Pacific gastropod Melibe fimbriata in the Adriatic Sea

The specimens of Melibe fimbriata were found during October 2001 in Stari Grad Bay (Island of Hvar, Croatia) in Cymodocea nodosa and Posidonia oceanica beds on sandy and sandy–muddy bottoms at depths of 2 to 15 m. Presently, this is the northernmost record of this lessepsian immigrant in the Mediterranean basin.

Meiofaunal production and energy transfer efficiency in a seagrass Posidonia oceanica bed in the western Mediterranean

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 234:95-104 (2002) - doi:10.3354/meps234095 Meiofaunal production and energy transfer efficiency in a seagrass Posidonia oceanica bed in the western Mediterranean Roberto Danovaro*, Cristina Gambi, Simone Mirto Marine Biology Section, Faculty of Science, University of Ancona, Via Brecce Bianche, Monte D'Ago, 60131 Ancona, Italy *E-mail: danovaro@popcsi.unian.it ABSTRACT: Measurements of secondary production of meiofaunal assemblages are limited, and for most seagrass systems no information is available yet. In the present study meiofaunal production was estimated in a Posidonia oceanica bed in the NW Mediterranean Sea using the summation of biomass increments and different equations based on the production:biomass ratio (i.e. biomass turnover). Total meiofaunal production ranged from 7.5 to 13.2 gC m-2 yr-1, according to the approach utilised (mean value 10.0 ± 1.9 gC m-2 yr-1, median value 9.6 gC m-2 yr-1). These values are among the highest reported worldwide, indicating that seagrass systems are hot spots of meiofaunal production. Despite the numerical dominance of nematodes, harpacticoid copepods were responsible for about 50% of total meiofaunal production, followed by polychaetes and nematodes (34 and 12%, respectively). Daily meiofaunal production rates were highest in February-March (21.3 mgC m-2 d-1), late May-June (31.4 mgC m-2 d-1) and October-December (44.6 to 52.0 mgC m-2 d-1). Analysis of daily secondary production of the dominant taxa suggested that different meiofaunal components dominated in different periods. During February-March, May-June and September-October harpacticoid copepods displayed highest production rates, but in November and December daily meiofaunal production was largely accounted for by polychaetes and nematodes. Temporal analysis of total meiofaunal production also revealed that, conversely to what is expected for temperate environments, only ca. 15% of the meiofaunal carbon was produced in summer, whereas 23% was observed in spring and 45% in autumn. Temporal changes in total meiofaunal daily production were apparently not dependent upon temperature, but rather reflected changes in the content and bio-availability of sediment organic matter in the seagrass bed. The seagrass P. oceanica was responsible, together with microphytobenthos, for high primary production values (169 to 300 gC m-2 yr-1). The fraction of total benthic primary production potentially consumed by meiofauna (as a measure of seagrass trophic efficiency) ranged from 10 to >50%. These estimates provide evidence of the relevance of meiofauna in benthic energy flows. At the same time, the production efficiency in the Posidonia bed (calculated by the ratio of total meiofaunal production to total primary production) was only 3.0 to 5.3%. These values, comparable to those observed in estuarine/organic-rich systems, are 2 to 10 times lower than those estimated in Cymodocea nodosa or Zostera noltii seagrass meadows, indicating that the Posidonia system is less efficient than other seagrass systems in channelling primary production to higher trophic levels. KEY WORDS: Meiofauna · Secondary production · Energy transfer efficiency · Posidonia oceanica · Mediterranean Sea Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 234. Online publication date: June 03, 2002 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2002 Inter-Research.

Particulate Organic Matter Composition in A Semi-Enclosed Marine System

Spatial and temporal changes in the biochemical composition of particulate organic matter in a semi-enclosed marine system (Stagnone di Marsala, Mediterranean Sea) were studied, on a monthly basis, from January to December 1994, in order to assess nutritional value of suspended particles for benthic suspension feeders. According to previous findings, the study site displayed a strong oligotrophy. Chlorophyll-a accounted for a very low fraction of the total suspended matter pool (0.1%), whereas at least 75% of POC was of detrital/heterotrophic origin. POC: PON ratio values indicate that bacterioplankton biomass accounted for a significant fraction of the total POC pool, displaying values comparable to those of the phytoplankton biomass (phytoplankton to bacterial biomass ratio was about (1). Temporal and spatial changes in the biochemical composition of particulate organic matter were rather limited and related to its sources, the main of which is represented by detrital particles released by the Posidonia oceanica (L.) beds. the comparison between our results and those encountered in other coastal lagoons indicates that the low abundance of suspension-feeding organisms observed in the study area is related to the “quality depression” of particles due to the dilution of high quality compounds (i.e., biopolymeric carbon) in a largely inorganic matrix. This result leads us to conclude that, to reach the same amount of high quality particulate food, a suspension feeder mollusc in the Marsala lagoon would need to filter a sea water volume around 3 times higher than in other Mediterranean coastal lagoons.

Interpopulation variability of the reproductive cycle of Paracentrotus lividus (Echinodermata: Echinoidea) in the south-western Mediterranean

The spawning of Paracentrotus lividus has been determined on the basis of the annual gonad index cycle at three sites in the Algiers area presenting low, intermediate and strong hydrodynamism. Three biotopes (Posidonia oceanica beds, rocky substrate with photophilous algae and overgrazed rocky substrate) and two depths (1–3 m and 6–10 m) were considered at these sites, the sex-ratio was always strongly in favour of females (2:1 to 3:1). In all sites but one, there were marked differences in the gonad index and the spawning period between sites; the gonad index reached its maximum value at the site with the weakest hydrodynamism, and the minimum value at the site with the strongest hydrodynamism. Spawning took place in April–May and August–September at the site with the weakest hydrodynamism, in April–June and October–December at the site with intermediate hydrodynamism and only in winter at the site with maximum hydrodynamism. The coexistence, on regional scale, of these reproductive cycles might enable Paracentrotus lividus to compensate the mortality during planktonic larval phase by the fact that spawning of this sea urchin occurred in all seasons.

Mapping of Posidonia oceanica beds around Elba Island (western Mediterranean) with integration of direct and indirect methods

Direct and indirect methods were used for mapping Posidonia oceanica beds around Elba Island. Side scan sonar and a visual infra red scanner were used to detect the extension of beds. Also, direct observations were adopted to assess the bed density and to calibrate the indirect methods. Samples of P. oceanica were collected and phenological variables and macroalgal epiphytes were studied to assess the health of the beds. P. oceanica around Elba Island covered an area of 3 680 ha. The largest beds occupied the four main gulfs of the island, characterized by gentle slopes and sandy bottoms. There, the beds showed uniform cover while a patchy structure was common on the rocky and steep bottoms of the western and south-eastern coasts of the island. P. oceanica beds at Elba Island demonstrated a good state of health. The epiphytic algal assemblage of the leaves was dominated by species generally found on this habitat, while, on the rhizomes, we observed a high covering of turf-forming species. The methods adopted allowed accurate maps at a scale of 1:10 000. The integration of data obtained both by direct and indirect methods and the precision of the positioning system were particularly important. © 2000 Ifremer/CNRS/IRD/Éditions scientifiques et médicales Elsevier SAS Posidonia oceanica / seagrass / algae / mapping / airborne teledetection / Mediterranean

Differences Among Fish Assemblages Associated with Nearshore Posidonia oceanica Seagrass Beds, Rocky–algal Reefs and Unvegetated Sand Habitats in the Adriatic Sea

Fish assemblages associated with nearshore Posidonia oceanica seagrass beds, rocky–algal reefs and unvegetated sandy substrates were studied at two sampling localities, Otranto (Apulian coast) and S. Domino (Tremiti Islands), located in the Southern and Central Adriatic Seas (Mediterranean Sea), respectively. Data were collected in situ by using non-destructive diver visual census methodology. A higher species richness and fish density were observed over thePosidonia seagrass and, in turn, the rocky–algal reef and unvegetated sand habitats. Planktivorous fish species (Spicara maena, Spicara smaris and Chromis chromis) showing patchy distributions were dominant in terms of abundance at both localities. The two former species did not show any particular habitat preference, while C. chromis was common over both the Posidonia and rocky–algal habitats and was only occasionally recorded over sand.Symphodus ocellatus , Diplodus annularis and Spondyliosoma cantharus were the most common species in theP. oceanica seagrass beds. Symphodus roissali, Symphodus tinca and Diplodus sargus were mainly associated with the rocky–algal reef habitats, while other species, such as Gymnammodytes cicerellus,Lithognathus mormyrus , Gobius geniporus, Mullus barbatus and Uranoscopus scaber, were censused over the unvegetated sand habitats. A cluster analysis performed on the entire fish density data set showed distinct groupings for seagrass, rocky–algal and bare sand fish assemblages, regardless of season and sampling site. Fish assemblages in the more structured (seagrass and rocky–algal reef) habitats were relatively similar to each other but quite different from those of the unstructured bare sands. With regard to small-sized specimens (juvenile stages including recruits), those of S. ocellatus, Symphodus mediterraneus, Serranus cabrilla, D. annularis, S. cantharus andSarpa salpa were mainly censused over P. oceanica beds, while juveniles of C. chromis inhabited predominantly rocky–algal bottoms rich in crevices. Small individuals of Coris julis were censused over both P. oceanica and rocky–algal habitats. No juveniles of any species were observed over bare sand. These results suggest that differences in fish species richness and abundance are primarily related to habitat structure. Different habitat preferences were evidenced for the juveniles and adults of several fish species. The ecological importance and need for protection of such shallow inshore habitats are discussed in relation to their crucial role as nurseries for many fish species, and their overall importance in maintaining littoral fish populations and species diversity is emphasized.

Distribution and dynamics of Posidonia oceanica beds along the Albères coastline

The location and dynamics of Posidonia oceanica beds were studied in the marine natural reserve of Cerbère–Banyuls-sur-Mer (eastern Pyrenees, France), through the mapping of bottom assemblages (transects and image processing) and the monitoring of fixed markers installed at the Posidonia oceanica lower limit. The surface area occupied by continuous beds is relatively limited and localized, whereas the presence of important areas of dead ‘matte’ would seem to indicate that bottom cover by Posidonia oceanica was more extensive in the past. Although the limited extension of the seagrass beds seems to be linked to both a high level of turbidity and severe hydrodynamic conditions, it is hypothesized that the regressions observed are due to lack of sediment.