Posidonia Oceanica Beds Research Articles

Modelling distribution patterns and habitat preference of the invasive green alga Caulerpa racemosa in the Saronikos Gulf (Eastern Mediterranean)

AB Aquatic Biology Contact the journal Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections AB 10:57-67 (2010) - DOI: https://doi.org/10.3354/ab00267 Modelling distribution patterns and habitat preference of the invasive green alga Caulerpa racemosa in the Saronikos Gulf (Eastern Mediterranean) Stelios Katsanevakis1,*, Maria Salomidi2, Aliki Panou3 1Institute of Marine Biological Resources, and 2Institute of Oceanography, Hellenic Centre for Marine Research, 46.7 km Athens-Sounio Avenue, 19013 Anavyssos, Greece 3Archipelagos – Environment and Development NGO, 28100 Lourthata, Kefalonia, Greece *Email: skatsan@ath.hcmr.gr or stelios@katsanevakis.com ABSTRACT: Although the invasive alga Caulerpa racemosa has exhibited an excessive rate of proliferation and has invaded the entire Mediterranean Sea basin within the last 18 yr, its spatial distribution patterns remain largely unknown. A 2-component modelling approach based on generalized additive models was applied to model the density of C. racemosa fronds in relation to spatial and environmental variables. A bimodal distribution of presence probability in relation to depth was observed, with higher probabilities of occurrence at very shallow bottoms (<4 m) and at depths between ~15 and 30 m. Presence probability steadily decreased at greater depths. Frond density was markedly higher in rocky habitats than in Posidonia oceanica beds or on sandy/muddy bottoms. The highest frond densities were observed in the southeastern part of the Saronikos Gulf, which is considered to be the area of initial establishment of the species in the Gulf; this indicates that time favours the steady expansion and further dominance of this species. KEY WORDS: Caulerpa racemosa · Generalized additive models · Greece · Invasive macrophytes · Spatial distribution patterns Full text in pdf format PreviousNextCite this article as: Katsanevakis S, Salomidi M, Panou A (2010) Modelling distribution patterns and habitat preference of the invasive green alga Caulerpa racemosa in the Saronikos Gulf (Eastern Mediterranean). Aquat Biol 10:57-67. https://doi.org/10.3354/ab00267 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in AB Vol. 10, No. 1. Online publication date: July 21, 2010 Print ISSN: 1864-7782; Online ISSN: 1864-7790 Copyright © 2010 Inter-Research.

Vulnerability of marine habitats to the invasive green alga Caulerpa racemosa var. cylindracea within a marine protected area

The relative vulnerability of various habitat types to Caulerpa racemosa var. cylindracea invasion was investigated in the National Marine Park of Zakynthos (Ionian Sea, Greece). The density of C. racemosa fronds was modelled with generalized additive models for location, scale and shape (GAMLSS), based on an information theory approach. The species was present in as much as 33% of 748 randomly placed quadrats, which documents its aggressive establishment in the area. The probability of presence of the alga within randomly placed 20 × 20 cm quadrats was 83% on ‘matte morte’ (zones of fibrous remnants of a former Posidonia oceanica bed), 69% on rocky bottoms, 86% along the margins of P. oceanica meadows, 10% on sandy/muddy substrates, and 6% within P. oceanica meadows. The high frond density on ‘matte morte’ and rocky bottoms indicates their high vulnerability. The lowest frond density was observed within P. oceanica meadows. However, on the margins of P. oceanica meadows and within gaps in fragmented meadows relative high C. racemosa densities were observed. Such gaps within meadows represent spots of high vulnerability to C. racemosa invasion.

Occurrence and distribution of different bed types of seagrass Posidonia oceanica around the Maltese Islands

The small-scale distribution of Posidonia oceanica bed types were mapped at four locations off the northern coast of the Maltese Islands, using aerial photography supplemented by surveys using SCUBA diving. Results showed a similar pattern of occurrence of the seagrass at all locations surveyed. In shallow waters (2 m – 4 m), P. oceanicaoccurred as patches of variable size on a rocky and/or sandy substratum. In deeper waters (5 m – 10 m), the patches of seagrass were often replaced by reticulate beds consisting of P. oceanicainterspersed with areas of bare sand. Deeper still (11 m – 13 m), a transition from reticulate to continuous beds occurred. Continuous beds extended to depths of around 25 - 30 m and eventually became reticulate or patchy in deeper waters (&gt;25 m). Values of total seagrass percentage cover increased, while the ratio of fragmented:continuous bed cover decreased for the four study locations on moving southwards (Ramla Bay to St Thomas Bay), indicating that P. oceanica habitat was more abundant and less fragmented in the south-eastern parts of the Maltese Islands. However, values calculated using an exposure index did not did indicate a relationship between exposure and the observed decease in fragmentation of seagrass beds on moving northwest to southwest along the north-eastern coast. Data from the four sites surveyed, together with data from other surveys, were used to show the large-scale distribution of P. oceanica beds around the Maltese Islands. The implications of the study findings for the conservation and management of P. oceanica habitat around the Maltese Islands are discussed.

Alterations of the structure of &lt;em&gt;Posidonia oceanica&lt;/em&gt; beds due to the introduced alga &lt;em&gt;Caulerpa taxifolia&lt;/em&gt;

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.

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.

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.

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.

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.

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

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.

NO DELETERIOUS ALTERATIONS IN POSIDONIA BEDS IN THE BAY OF MENTON (FRANCE) EIGHT YEARS AFTER CAULERPA TAXIFOLIA COLONIZATION

High‐resolution multispectral imagery, aerial photography, and SCUBA diving surveys were used to determine if there had been any significant alteration in the dimensions of Posidonia oceanica beds in the Bay of Menton, French Riviera, since colonization of the green alga Caulerpa taxifolia began in the late 1980s. After 8 years of direct contact, there was no evidence of a decrease in the abundance of P. oceanica. On the contrary, the integrity of a seagrass bed had increased in the first area colonized by Caulerpa taxifolia on the eastern side of Cape Martin and, in a more recently colonized central part of the bay, established tufts of Posidonia were isolated from the alga by well‐defined exclusion zones.

Effects of boat anchoring inPosidonia oceanica seagrass beds in the Port-Cros National Park (north-western Mediterranean Sea)

1. A study was set up in the Port-Cros National Park in order to determine the effects of boat anchoring on Posidonia oceanica seagrass beds. 2. Experiments on the effects of anchors on the seagrass meadows revealed that, on average, 34 shoots were destroyed during an anchoring cycle (lock-in and retrieval), especially when the seagrass mat compactness is weak and the extent of rhizome baring is high. 3. Five parameters of the Posidonia oceanica beds (meadow cover, shoot density, extent of rhizome baring, proportion of plagiotropic rhizomes, degree of meadow fragmentation) were considered and it was shown that the extent of rhizome baring was not correlated with anchoring pressure. Meadow cover and mean shoot density were positively correlated with high anchoring pressure. 4. The proportion of plagiotropic (i.e. horizontally growing) rhizomes and the degree of meadow fragmentation were positively correlated with moderate anchoring pressure. Copyright © 1999 John Wiley & Sons, Ltd.

Preliminary data on the impact of fish farming facilities on Posidonia oceanica meadows in the Mediterranean

While the extension of aquaculture along the coasts of the Mediterranean islands is today an economic priority, no data is available on the impact of these facilities on the littoral environment in general, and in particular on the Posidonia oceanica meadows that are responsible for the richness and diversity of these biotopes. The impact of fish farming facilities in the marine littoral environment is assessed at three Mediterranean sites in Corsica and Sardinia. Various parameters are considered: (i) the amount of light available, (ii) the nature of the sediment, and (iii) the vitality of the Posidonia oceanica beds. The findings confirm the impact of the aquaculture facilities within the water column (increase in turbidity, enrichment of the sediment in organic matter and nutrients), and it would appear that the Posidonia oceanica beds also undergo significant alterations. The decline in density of the meadows in the area influenced by aquaculture farming and their total disappearance beneath the facilities is particularly disturbing, even if the areas affected are not very extensive, at least for the smaller farms (a few hundred square metres), and if certain beneficial effects may be recorded (increase in benthic primary production). Because of its ability to record environmental alterations caused by these facilities (light, nutrients, trace metals), the Posidonia oceanica meadow is a good bioindicator for use in monitoring studies.