Socio-hydrogeological survey and assessment of organic pollutants to highlight and trace back pollution fluxes threatening a coastal groundwater-dependent ecosystem.

Despite being a vector of pollution towards connected ecosystems, groundwater is often underestimated or not taken into account in management frameworks. To fill this gap, we propose to add socio-economic data to hydrogeological investigations to identify past and present pollution sources linked to human activities at watershed scale in order to forecast threats towards groundwater-dependent ecosystems (GDEs). The aim of this paper is to demonstrate, by a cross-disciplinary approach, the added value of socio-hydrogeological investigations to tackle anthropogenic pollution fluxes towards a GDE and to contribute to more sustainable management of groundwater resources. A survey combining chemical compounds analysis, data compilation, land use analysis and field investigations with a questionnaire was carried out on the Biguglia lagoon plain (France). Results show a pollution with a two-fold source, both agricultural and domestic, in all water bodies of the plain. The pesticide analysis reveals the presence of 10 molecules, including domestic compounds, with concentrations exceeding European groundwater quality standards for individual pesticides, as well as pesticides already banned for twenty years. On the basis of both the field survey and the questionnaire, agricultural pollution has been identified as very local highlighting the storage capacity of the aquifer, whereas domestic pollution is diffuse over the plain and attributed to sewage network effluents and septic tanks. Domestic compounds present shorter residence time within the aquifer highlighting continuous inputs, related to consumption habits of the population. Under the Water Framework Directive (WFD), member states are required to preserve the good ecological status, quality and quantity of water bodies. However, for GDEs it is difficult to achieve the 'good status' required without considering the groundwater's pollutant storage capacity and pollution legacy. To help resolve this issue, socio-hydrogeology has proved to be an efficient tool as well as for implementing effective protection measures for Mediterranean GDEs.

The role of groundwater in CO2 production and carbon storage in Mediterranean peatlands: An isotope geochemistry approach.

Peatlands are permanent wetlands recognized for ecosystem services such as biodiversity conservation and carbon storage capacity. Little information is available about their response to global change, the reason why most Earth system climate models consider a linear increase in the release of greenhouse gases (GHG), such as CO2, with increasing temperatures. Nevertheless, numerous studies suggest that an increase in the temperature may not imply a decrease in photosynthesis and carbon storage rates if water availability is sufficient, the latter being under the control of local hydrology mechanisms. Mediterranean peatlands well illustrate this fact. Since they are groundwater-dependent, they are hydrologically resilient to the strong seasonality of hydroclimatic conditions, especially during the summer drought. In the present study, we demonstrate that, even if such peatlands release CO2 into the atmosphere, they can maintain a carbon storage capacity. To this end, a geochemical study disentangles the origin and fate of carbon within a Mediterranean peatland at the watershed scale. Field parameters, major ions, dissolved organic and inorganic carbon content and associated δ13C values allow for characterizing the seasonality of hydrochemical mechanisms and carbon input from an alluvial aquifer (where rain, river, shallow, and deep groundwater flows are mixing) to the peatland. The inorganic and organic content of peat soil and δ13C values of total organic matter and CO2 complete the dataset, making it possible to provide arguments in favour of lower organic matter oxidation compared to primary production. Overall, this study highlights the groundwater role in the fluxes of CO2 at the peatland-atmosphere interface, and more broadly the need to understand the interactions between the water and carbon cycles to build better models of the future evolution of the global climate.

Assessment of trace element contamination and effects on Paracentrotus lividus using several approaches: Pollution indices, accumulation factors and biochemical tools.

Among the most common contaminants in marine ecosystems, trace elements are recognized as serious pollutants. In Corsica (NW Mediterranean Sea), near the old asbestos mine at Canari, trace elements from the leaching of mine residues have been discharged into the sea for several decades. The aim of this study was to assess the levels of contamination in this area and the potential effects on Paracentrotus lividus (Lamarck, 1816) using pollution indices, accumulation factors and biochemical tools. For this purpose, the concentration of 24 trace elements was measured in sea urchins (gonads and gut content), macroalgae, seawater column and sediment collected at 12 stations nearby the old asbestos mine and at a reference site. The bioaccumulation of trace elements occurs as follows: macroalgae > gut > gonads. TEPI contribute to highlight contamination gradients which are mainly due to the dominant marine currents allowing the migration of mining waste along the coastline. This hypothesis was supported by TESVI, which identified characteristic trace elements in the southern area of the mine. High hydrogen peroxide content, associated with elevated catalase and glutathione-S-transferase enzyme activities, were also identified at these sites and at the reference site. Trace elements contamination as well as several abiotic factors could explain these results (e.g. microbiological contamination, hydrodynamic events, etc.). The results obtained in this study suggest that oxidative stress induced by contamination does not affect the health of Paracentrotus lividus. This work has provided a useful dataset allowing better use of sea urchins and various tools for assessing trace element contamination in coastal ecosystems.

Stress response to trace elements mixture of different embryo-larval stages of Paracentrotus lividus.

This study investigated for the first time the oxidative biomarkers responses in all larval stages of sea urchin. The contamination effects were reproduced by using contaminated seawater to concentrations measured in the area adjacent to an old asbestos mine at factors of 5 and 10. The results suggested that the concentrations were not sufficiently high to induce a major oxidative stress. The biometric differences make this method a more sensitive approach for assessing the effects on sea urchin larvae. Measurements of specific activities of antioxidant enzymes at each stage suggested a high capacity of the larvae to respond to oxidative stress. This normal activity of the organism must be considered in future research. This work also highlighted the importance of spawners provenance in ecotoxicological studies. These data are essential to better understand the stress responses of sea urchin larvae and provide baseline information for later environmental assessment research.

Effects of trace elements contaminations on the larval development of Paracentrotus lividus using an innovative experimental approach.

Several experiments were performed using larvae of Paracentrotus lividus (Lamarck, 1816) in order to determine the consequences of different chronic contamination with mixtures of (i) fifteen trace elements from concentrations measured in the world ocean seawater, and (ii) seven trace elements from contamination resulting from mining. To predict the impact of increased marine pollution, higher concentrations were also used. These bioassays were conducted using spawners collected from Calvi (reference site, Corsica), and Albo (mining area, Corsica). The effects of trace elements have been studied on the entire larval development. The results show wider arms and delayed development as the number and concentration of trace elements increases. Therefore, the synergy between the different trace elements is of paramount importance with regard to the impact on organisms. Probably due to a hormesis phenomenon, larvae contaminated with seven trace elements at average concentrations developed more quickly. This work also highlighted the importance of the origin of spawners in ecotoxicological studies. To our knowledge, this is the first study to investigate the effects of such a broad combination of trace elements for chronic contamination on the entire larval stage of Paracentrotus lividus.

The challenge of assessing the proper functioning conditions of coastal lagoons to improve their future management.

The sustainable management of coastal lagoon hydrosystems is a key issue for the socio-economic and environmental development of many coastal areas worldwide. Often maintained by direct or indirect groundwater supplies, they provide a large range of ecosystem services, from which human societies take advantage. Twenty years after its implementation, a large majority of the Mediterranean lagoons have still not reached the "good status" required by the WFD. Several questions then arise as to the representativeness of the WFD indicators or the relevance of the restoration objectives considering the complexity, evolutionary and unpredictable nature of lagoon hydrosystems. This study proposes an innovative, multidisciplinary, long-term approach to define the proper functioning conditions of a costal lagoon hydrosystems, i.e. all the factors that contribute to the functioning of coastal lagoon hydrosystems and the connectivity with other biodiversity reservoirs, be they ecological, hydrological, social or political. By considering the lagoons and all its hydrological, ecological and societal proper functioning conditions over almost 200 years, this approach makes it possible to assess the influence of past natural and anthropogenic disturbances and support the implementation of future relevant hydrosystem-based management plans which have to be coordinated and politically driven. Defining proper restoration and management objectives should ensure that ecological functions are maintained based on current and future ecosystem benefits and uses. Considering the highly unpredictable nature of coastal hydrosystems, the state of an ecosystem should not only be evaluated on ecological or chemical criteria but also take into account socio-economic and political indicators. The implementation of the 3rd river basin management plan of the WFD as soon as 2022 could be the appropriate occasion to reassess the restoration objectives towards more realistic goals and to give more significance to the definition of the "resilience capacity" of water bodies in place of inappropriate restricted restauration objectives.

Groundwater dependent ecosystems in coastal Mediterranean regions: Characterization, challenges and management for their protection.

Coastal lagoons deliver a wide range of valuable ecosystem goods and services. These ecosystems, that are often maintained by direct or indirect groundwater supplies, are collectively known as groundwater dependent ecosystems (GDEs). The importance of groundwater supplies is greatly exacerbated in coastal Mediterranean regions where the lack of surface water and the over-development of anthropogenic activities critically threaten the sustainability of coastal GDEs and associated ecosystem services. Yet, coastal GDEs do not benefit from a legal or managerial recognition to take into account their specificity. Particular attention should be paid to the characterization of environmental and ecological water requirements. The hydrogeological knowledge about the management and behavior of coastal aquifers and GDEs must be strengthened. These investigations must be supplemented by a stronger assessment of potential contaminations to develop local land-uses and human activities according to the groundwater vulnerability. The quantitative management of water resources must also be better supervised and/or more constrained in order to ensure the water needs necessary to maintain coastal GDEs. The transdisciplinary approach between hydrogeology, hydrology, social sciences and law is essential to fully understand the socio-economic and environmental complexity of coastal GDEs. Priority must now be given to the development of an appropriate definition of coastal GDEs, based on a consensus between scientists and lawyers. It is a necessary first step to develop and implement specific protective legislation and to define an appropriate management scale. The investment and collaboration of local water users, stakeholders and decision-makers need to be strengthened through actions to favor exchanges and discussions. All water resources in the coastal areas should be managed collectively and strategically, in order to maximize use efficiency, reduce water use conflicts and avoid over-exploitation. It is important to continue to raise public awareness of coastal aquifers at the regional level and to integrate their specificities into coastal zone management strategies and plans. In the global context of unprecedented anthropogenic pressures, hydro-food crises and climate change, environmental protection and preservation of coastal GDEs represents a major challenge for the sustainable socio-economic and environmental development of Mediterranean coastal zones.

Combinations of geoenvironmental data underline coastal aquifer anthropogenic nitrate legacy through groundwater vulnerability mapping methods.

Groundwater quality is strongly dependent on land use. Past and current anthropogenic activities can lead to the diffusion of contaminants in aquifers. This diffusion can threaten the resource exploitation for decades, thereby endangering the ecological health of groundwater dependent ecosystems. Thus, groundwater stakeholders need methods for long-term management which integrate groundwater vulnerability. This study was conducted on the shallow alluvial aquifer of the groundwater-dependent Biguglia lagoon on Corsica Island, France. The aquifer is exposed to anthropogenic contamination for many decades with nitrate contamination legacy linked to agricultural activities, uncontrolled urbanization and sewage leakages. In most cases, vulnerability mapping is done in the objective of comparing groundwater situation regarding an on-going contamination process. But the question is still pending for aquifers where contamination is inherited from past practices or contaminations and where anthropogenic influences have changed through time. To propose an effective and innovative method for territorial management in Mediterranean alluvial aquifers, four index-based groundwater vulnerability mapping methods were tested and compared: two intrinsic vulnerability mapping methods (DRASTIC and SINTACS) and two specific vulnerability mapping methods (Modified-DRASTIC and SI), the latter integrating land use in the accuracy of groundwater vulnerability. Novelty is coming from the comparison between vulnerability maps and their application and validation in a hydrosystem affected by nitrate legacy-type contamination. The specific vulnerability mapping methods are more likely to represent the current pressures to which groundwater are subject. Thus, specific vulnerability methods such as the SI one revealed here very relevant to assess groundwater quality and to react retrospectively. The comparison between groundwater nitrate legacy and intrinsic groundwater vulnerability methods appeared also useful to define priority protection areas in long-term territorial management planning (EU Water Framework Direction). In this sense, the SINTACS method seems to be the more appropriate in the Mediterranean and alluvial context of this study.

Spatial variations in trace element concentrations of the sea urchin, Paracentrotus lividus, a first reference study in the Mediterranean Sea.

A study on Trace Elements (TE) from sea urchin gonads has been conducted in the western Mediterranean Sea. Contamination data were used to determine a Trace Method Pollution Index (TEPI). TE concentrations varied considerably depending on the location of the sampling stations. The results showed that five trace elements (Zn, Fe, As, Al, Cu) are ubiquitous. The geographical area considered (Corsica) represents an important range of environmental conditions and types of pressure that can be found in the western Mediterranean Sea. TEPI was used to classify the studied sites according to their degree of contamination and allowed reliable comparison of TE contamination between local and international sites. TE contamination of the western Mediterranean Sea displayed a north-to-south gradient, from the Italian coasts down through the insular Corsican coasts to the north African littoral. Due to the increasing environmental pressure on the Mediterranean Sea, a regular monitoring of TE levels in marine organisms is necessary to prevent any further environmental deterioration.

Nodule size as predictive factor of efficacy of radiofrequency ablation in treating autonomously functioning thyroid nodules.

CONTEXT: No defined pre-treatment factors are able to predict the response to radiofrequency ablation (RFA) of an autonomously functioning thyroid nodule (AFTN). OBJECTIVE: Primary endpoint was to evaluate the success rate of RFA to restore euthyroidism in a cohort of adult patients with small solitary AFTN compared with medium-sized nodules. Secondary endpoints included nodule volume reduction and rate of conversion from hot nodules to cold using scintiscan. METHODS: This was a 24-month prospective monocentric open parallel-group trial. Twenty-nine patients with AFTN were divided into two groups based on thyroid volume: 15 patients with small nodules (<12 mL) in group A and 14 patients with medium nodules (>12 mL) in group B. All patients underwent a single session of RFA and were clinically, biochemically, and morphologically evaluated at baseline and at 1, 6, 12 and 24 months after treatment. RESULTS: After RFA, there was greater nodule volume reduction in group A compared with group B (p < 0.001 for each follow-up point). In group A, there was a greater increase in TSH levels than in group B at 6 (p = 0.01), 12 (p = 0.005), and 24 months (p < 0.001). At 24 months, the rate of responders was greater in group A than in group B (86 vs. 45%; p < 0.001). In group A, 86% of nodules converted from hot to cold compared with 18% in group B (p < 0.001). CONCLUSIONS: A single session of RFA was effective in restoring euthyroidism in patients with small AFTNs. Nodule volume seems to be a significant predictive factor of the efficacy of RFA in treating AFTN.

Delayed nitrate dispersion within a coastal aquifer provides constraints on land-use evolution and nitrate contamination in the past.

Identifying sources of anthropogenic pollution, and assessing the fate and residence time of pollutants in aquifers is important for the management of groundwater resources, and the ecological health of groundwater dependent ecosystems. This study investigates anthropogenic contamination in the shallow alluvial aquifer of the Marana-Casinca, hydraulically connected to the Biguglia lagoon (Corsica, France). A multi-tracer approach, combining geochemical and environmental isotopic data (δ18O-H2O, δ2H-H2O, 3H, δ15N-NO3-, δ18O-NO3-, δ11B), and groundwater residence-time tracers (3H and CFCs) was carried out in 2016, and integrated with a study of land use evolution in the catchment during the last century. Groundwater NO3- concentrations, ranged between 2 mg/L and up to 30 mg/L, displaying the degradation of groundwater quality induced by anthropogenic activities (agricultural activities). Comparatively high δ15N-NO3- values (up to 19.7‰) in combination with δ11B values that were significantly lower (between 23‰ and 26‰) than the seawater background are indicative of sewage contamination. The ongoing deterioration of groundwater quality can be attributed to the uncontrolled urbanization development all over the alluvial plain, with numerous sewage leakages from the sanitation network and private sewage systems. Integration of contaminant and water-residence time data revealed a progressive accumulation of pollutants with time in the groundwater, particularly in areas with major anthropogenic pressure and slow dynamic groundwater flow. Our approach provides time-dependent insight into nitrogen pollution in the studied aquifer over the past decades, revealing a systematic change in the dominant NO3- source, from agricultural to sewage contamination. Yet, today's low groundwater quality is to large parts due to legacy pollution from land-use practices several decades ago, underlining the poor self-remediating capacity of this hydrosystem. Our results can be taken as warning that groundwater pollution that happened in the recent past, or today, may have dire impacts on the quality of groundwater-dependent ecosystems in the future.

Trace element concentrations in the apex predator swordfish (Xiphias gladius) from a Mediterranean fishery and risk assessment for consumers.

Swordfish (Xiphias gladius L., 1758) is an apex predator, highly migratory meso-pelagic fish widely distributed in the Atlantic Ocean and Mediterranean. As top predators, this fish may be the end reservoir of the bioaccumulation of trace elements in a food chain because they occupy higher trophic levels and are an important food source, causing them to be potentially hazardous to consume. This study aims to investigate the concentration of 18 trace elements of Swordfish, caught in the Mediterranean Sea and to discuss human exposure risks. The mean element levels in the fish muscles were clearly below the maximum allowable concentrations established by International food safety regulations. The data suggested that the risk is minor and acceptable for human health. The findings of this study amplify the scarce database on contaminants available, especially new data on "emerging elements", for this species from the Mediterranean Sea.

Water flux management and phytoplankton communities in a Mediterranean coastal lagoon. Part II: Mixotrophy of dinoflagellates as an adaptive strategy?

Dinoflagellate proliferation is common in coastal waters, and trophic strategies are often advanced to explain the success of these organisms. The Biguglia lagoon is a Mediterranean brackish ecosystem where eutrophication has long been an issue, and where dominance of dinoflagellates has persisted for several years. Monthly monitoring of fluorescence-based properties of phytoplankton communities carried out in 2010 suggested that photosynthesis alone could not support the observed situation all year round. Contrasting food webs developed depending on the hydrological season, with a gradual shift from autotrophy to heterotrophy. Progressively, microphytoplankton assemblages became unequivocally dominated by a Prorocentrum minimum bloom, which exhibited very weak effective photosynthetic performance, whereas paradoxically its theoretical capacities remained fully operational. Different environmental hypotheses explaining this discrepancy were examined, but rejected. We conclude that P. minimum bloom persistence is sustained by mixotrophic strategies, with complex compromises between phototrophy and phagotrophy, as evidenced by fluorescence-based observations.

Water flux management and phytoplankton communities in a Mediterranean coastal lagoon. Part I: How to promote dinoflagellate dominance?

The Biguglia lagoon is a shallow Mediterranean coastal ecosystem where eutrophication is increasing for years. A channel supplying freshwater was cleared in 2009 to enhance lagoon water circulation and alleviate dystrophic crises. Monthly monitoring was started in 2010 to document the impacts of this action on abiotic characteristics and phytoplankton communities. Three stations were surveyed (by microscopy and HPLC). Evidence suggests that this operation had an unexpected outcome. Salinity footprints indicated the succession of three main hydrological sequences that depended on rainfall and circulation pattern. Diatoms and dinoflagellates dominated the first sequence, characterized by heavy rainfall, while Prorocentrum minimum became progressively the dominant species in the second period (increasing salinities) with extensive bloom over the whole lagoon (5.93×10-(5) cells·L(-1)) during the third period. These phytoplankton successions and community structures underline the risk of pernicious effects arising from remediation efforts, in the present case based on increasing freshwater inputs.

Effects of alendronate and vitamin D in patients with normocalcemic primary hyperparathyroidism.

UNLABELLED: No data on the pharmacological treatment of normocalcemic hyperparathyroidism (NPHPT) are available. We treated 30 NPHPT postmenopausal women with alendronate/cholecalciferol (treated group) or vitamin D alone (control group). Over 1 year, bone mineral density (BMD) increased significantly in treated group, but not in control group. Both treatments did not affect serum or urinary calcium. INTRODUCTION: Normocalcemic primary hyperparathyroidism (NPHPT) is defined by normal serum calcium and consistently elevated PTH levels after ruling out the causes of secondary hyperparathyroidism. It is likely that subjects with NPHPT may develop kidney and bone disease. As no data on the pharmacological treatment of NPHPT are available, we aimed to investigate the effects of alendronate and cholecalciferol on both BMD and bone biochemical markers in postmenopausal women with NPHPT. Safety of vitamin D was evaluated as secondary endpoint. METHODS: The study was a prospective open label randomized trial comparing 15 postmenopausal women with NPHPT (PMW-NPHPT), treated with oral alendronate plus cholecalciferol (treated group) and 15 PMW-NPHPT treated only with cholecalciferol (control group). Blood samples were obtained at baseline and after 3, 6, and 12 months. Bone turnover markers (BTM) were measured at baseline, 3, and 6 months, respectively. BMD was assessed at baseline and after 12 months. RESULTS: After 1 year of treatment, BMD increased significantly at the lumbar, femoral neck, and hip level in the treated group, but not in the control group (p = 0.001). No differences were found between or within groups in serum calcium, PTH, and urinary calcium levels. BTM significantly decreased in the treated group but not in the control group, at 3 and 6 months (p < 0.001), respectively. No cases of hypercalcemia or hypercalciuria were detected during the study. CONCLUSION: The results of this study indicate that alendronate/cholecalciferol increases BMD in postmenopausal women with NPHPT. Alendronate/cholecalciferol or vitamin D alone does not affect serum or urinary calcium.

Does prescribed burning affect leaf secondary metabolites in pine stands?

Prescribed burning (PB) is gaining popularity as a low-cost forest protection measure that efficiently reduces fuel build-up, but its effects on tree health and growth are poorly understood. Here, we evaluated the impact of PB on plant defenses in Mediterranean pine forests (Pinus halepensis and P. nigra ssp. laricio). These chemical defenses were estimated based on needle secondary metabolites (terpenes and phenolics including flavonoids) and discussed in terms of chlorophyll fluorescence and soil nutrients. Three treatments were applied: absence of burning (control plots); single burns (plots burned once); and repeated burns (plots burned twice). For single burns, we also explored changes over time. In P. laricio, PB tended to trigger only minor modifications consisting exclusively of short-lived increases (observed within 3 months after PB) in flavonoid index, possibly due to the leaf temperature increase during PB. In P. halepensis, PB had detrimental effects on physiological performance, consisting of (i) significant decreases in actual PSII efficiency (ΦPSII) in light-adapted conditions after repeated PB, and (ii) short-lived decreases in variable-to-maximum fluorescence ratio (Fv/Fm) after single PB, indicating that PB actually stressed P. halepensis trees. Repeated PB also promoted terpene-like metabolite production, which increased 2 to 3-fold compared to control trees. Correlations between terpene metabolites and soil chemistry were found. These results suggest that PB impacts needle secondary metabolism both directly (via a temperature impact) and indirectly (via soil nutrients), and that these impacts vary according to species/site location, frequency and time elapsed since last fire. Our findings are discussed with regard to the use of PB as a forest management technique and its consequences on plant investment in chemical defenses.

Evolution and vitality of seagrasses in a Mediterranean lagoon.

Despite their registration on the list of the Ramsar convention sites, the Mediterranean lagoons rarely beneficiate of an effective protection, and are particularly sensitive to environmental quality. A control of these wetlands needs the creation of an inventory of knowledge for the concerned environment. In this perspective, the seagrass beds were followed up in the coastal lagoon of Urbino (Corse, France) since 1990. A cartographic study was carried out by remote sensing of aerial photography. Temporal evolution of the seagrass beds (Cymodocea nodosa principally) allows to determine the vitality of these structures. A comparison of the surface areas occupied by Cymodocea nodosa, between 1990 and 1999, did not allow seeing any significant evolution. However, some variations appear like biotopes all more fragile and coveted as the Mediterranean coastal fringe is straight and is the privileged site of appear in the localization of the beds, due to the modification of environmental conditions in the lagoon.

Phenolic compounds content in Pinus halepensis Mill. needles: a bioindicator of air pollution.

Foliar phenol concentrations (total and simple phenols) were determined in Aleppo pine (Pinus halepensis Mill.) needles collected in June 2000, from 6 sites affected by various forms of atmospheric pollutants (NO, NO(2), NO(x), O(3) and SO(2)) monitored during two months. Results show an increase in total phenol content with exposure to sulphur dioxide and a reduction with exposure to nitrogen oxide pollution. p-Coumaric acid, syringic acid and 4-hydroxybenzoic acid concentrations increase with exposure to nitrogen oxide pollution, whereas gallic acid and vanillin decrease in the presence respectively of sulphur dioxide and ozone. This in situ work confirms the major interest of using total and simple phenolic compounds of P. halepensis as biological indicators of air quality.

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 'matter' 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.