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The Posidonia oceanica Matte: a Reservoir of Environmental Information
Abstract and Figures
Seagrass meadows have long been recognized for their high ecological and economie value. More recently, a potential role in climate regulation due to their ability to fix and sequester carbon has been the focus of intensive study. In the Mediterranean Sea, the matte, a specifie structure constructed by the seagrass Posidonia oceanica is of particular interest because it keeps buried for thousands of years massive amounts of carbon.
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... Three devices with distinct emission frequencies were deployed for data collection: a Sparker (1 kHz) and two sediment profilers; the Manta EDO (2.5 kHz) and the Pesk Avel (3.5 kHz). In total, 510 seismic profiles were acquired corresponding to 3,095 km of data (Monnier et al. , 2019b. Mapping of matte thicknesses is done after integrating the data into the ArcGIS® 10.2.2. ...
... Analysis of samples were undertaken after drying, sieving (< 2 mm) and separating sediment slices in different fractions (calcium carbonates, mineral and organic material). The total organic matter content (%TOM) and the elemental analysis (%C) were carried out on the fine fraction and the coarse organic fraction following Monnier et al. (2019b). The standing organic matter and carbon stocks per unit area (cumulative stocks; kg C m -2 or Mg C ha -1 ) were calculated according to Howard et al. (2014). ...
Seagrass meadows have long been recognized for their high ecological and economic
value (ecosystem services). More recently, a potential role in climate regulation, due to
their ability to fix and sequester carbon, has been the focus of intensive study. In the Mediterranean Sea, the matte, a specific structure built by the seagrass Posidonia oceanica, is of particular interest because it keeps buried for thousands of years massive amounts of carbon. Recent studies carried out along the Corsican coasts show a mean fixation of 1.62 Mg C ha–1 yr–1, with a sequestration rate of between 27 and 30 %, a mean matte thickness of 210 cm and 711 Mg C ha–1 of organic carbon trapped in the matte. That is to say, a stock corresponding to 1,580 years of P. oceanica carbon sequestration, confirmed by radiocarbon analysis. An extrapolation to the Mediterranean basin (1.0 to 1.5 million hectares covered by P. oceanica meadow; mean matte thickness: 210 cm) shows that the total stock of organic carbon sequestered in the P. oceanica matte might be as much as 711 to 1,067 million Mg C. The conservation of the P. oceanica meadows thus constitutes an issue of major importance since any degradation of the matte, which has been built up over the past millennia, would very likely result in the release of considerable
quantities of carbon. Rather than playing a major role in the attenuation of the impact of
climate change (blue carbon sequestration), the P. oceanica meadow would then become a
source of carbon that would be likely to amplify the greenhouse gas emissions. Management of P. oceanica meadows should take into account not only their role in carbon sequestration, but also the whole the full range of their ecosystem services, in relation with the functioning of the ecosystem.
Despite its very small surface area, coastal vegetation fixes a large amount of the Blue Carbon and plays a major role in its sequestration. These carbon sinks contribute to the mitigation of the effects of climate change. Among them, Posidonia oceanica, a Mediterranean endemic seagrass species, appears to be the most efficient in carbon storage due to a unique structure: the matte. Consequently, in accordance with the Paris Agreement (COP 21) which states that "Parties must take measures to conserve and, where appropriate, enhance the sinks and reservoirs of greenhouse gases", the Corsica regional authority has introduced a specific strategy to follow these recommendations. Three main steps have been identified and a large amount of data are already available, while further studies are in progress: (i) The Inventory of the Blue Carbon Ecosystem, based on a precise map of coastal ecosystems, confirms the very extensive range of the P. oceanica (53 736 ha; >60% of the bottoms between 0 and-40 m) and Cymodocea nodosa (nearly 2 000 ha) meadows. (ii) The Monitoring of the Blue Carbon Ecosystem, with the setting up of the Posidonia Monitoring Network (39 sites), highlights the direct impact of human pressure (sewage outfall, aquaculture, trawling), and also a general regression of the lower limit in relation with sea level rise. (iii) The Blue Carbon balance (fixation, sequestration and stocking) is in progress at a NATURA 2000 site, corresponding to 38% of the surface area covered by P. oceanica meadows in Corsica. Preliminary results indicate that the fixation is estimated at 1.38 Mg C ha-1 year-1 , while the sequestration could reach 0.7 Mg C ha-1 year-1. In addition, preliminary results from seismic investigations underline the high matte thickness (2.1 m in average). The integration of all these data will provide a basis for drawing up a reference state of coastal carbon sinks, but the major effort should be devoted to their conservation.
Seagrass meadows, recognized for their fundamental ecological, sedimentary and economic roles, also play an important role in climate regulation due to their ability to fix and sequester Blue Carbon (COP 21). In the Mediterranean Sea, the matte, a specific structure built by the seagrass species Posidonia oceanica (L.) Delile, is of particular interest because of its carbon storage capacity over thousands of years. This structure contributes strongly to the mitigation of climate change and constitutes unique carbon sinks, which require quantification in order to provide a basis for assessment of this major ecosystem service. In order to estimate the organic carbon (Corg) stocks sequestered in the Posidonia oceanica meadows, an assessment of the thickness of the matte was undertaken using seismic reflection data acquired during two oceanographic surveys: CoralCorse (2013) and PosidCorse (2015). This experimental approach, carried out at the Natura 2000 site ‘Grand Herbier de la Côte Orientale’ (Corsica, NW Mediterranean), involved the collection of more than 3 000 km of seismic profiles with three devices using distinct acquisition frequencies.The data interpretation performed with the Kingdom© software provided a basis for the identification and delineation of the structures present in situ. The calibration of the data, performed by comparing measurements of the heights of matte walls measured by scuba diving and on the seismic profiles, reveals the speed of the acoustic wave in the matte, ranging between 696 and 2 159 m.s-1. The ordinary kriging mapping shows an average matte thickness of 2.1 m at the study site as well as remarkable matte thickness (up to 8 m thick) at the mouths of the main coastal rivers, and also locally in the deep part near the underwater deltas (-30 to -40 m). Stocks of Corg sequestered in the matte of the Natura 2000 site, estimated from the average matte thickness, the area occupied by the meadow (23 503 ha) and the Corg composition data from the scientific literature, are estimated at about 17-32 million tons (that is to say between 62 and 117 million tons of equivalent CO2). Nevertheless, coring and matte characterization will help to determine the source of observed differences in speed of the acoustic wave, refine the quantification of Corg stocks and determine the temporal dynamics of this carbon in the matte.
This study synthetizes the terrestrial environmental history of the Cavallo Island, the largest Corsican satellite island (120 ha), located in the Strait of Bonifacio.
Ecosystems dynamics and terrestrial biodiversity have been deciphered for 7,000 years thanks to the paleoecological analysis of fossil pollen and beetle remains. These results underligne the profound changes in the structure and specific composition of terrestrial communities of vascular plants and insects. These changes are related to the relative sea level rise and to anthropogenic impacts.
Over time, Cavallo Island represented a hunting territory for prehistoric men from the Neolithic/Chalcolithic period, then a famous site of granite exploitation by the Romans, a refuge and a haunt for many pirates or corsairs, an “insular enclosure” facilitating the free pastures of cattle brought in “nautical transhumance” by the Bonifaciens. In the early 1970, it became an “island of billionaires” and a territory of lawlessness with a sulfurous reputation. This ultimate episode caused the most impacts on the natural environment: Ecosystems were deeply altered by an uncontrolled urbanization that has had profound consequences on terrestrial biodiversity (destruction and fragmentation of natural habitats; biological invasions; filling, pollution and eutrophication of wetlands).
Keywords: Environmental changes, Mediterranean island, human impacts, insularity, palaeoecology, relative sea-level rise.
L’histoire de la végétation en Corse est bien connue, particulièrement dans les espaces montagnards. Cependant, des zones d’ombre persistent concernant par exemple le rôle du climat ou des activités anthropiques dans certaines modifications du couvert végétal ou encore la réponse des écosystèmes littoraux à la transgression marine holocène.L’objectif de cette thèse est d’identifier les trajectoires plurimillénaires des écosystèmes de Corse à basse et moyenne altitude sous la triple influence des forçages climatique, eustatique et anthropique à travers l’étude des assemblages d’insectes fossiles (coléoptères en particulier) et des données paléobotaniques (pollen, macrorestes végétaux) disponibles ou nouvellement acquises. Cette approche multidisciplinaire a été appliquée sur trois séquences sédimentaires prélevées à l’étang du Greco (1 m) sur l’île Cavallo, au marais de Cannuta (2 m) et à la tourbière de Bagliettu (212 m).Les résultats obtenus révèlent des changements majeurs dans la structuration du couvert végétal. Des signes d’ouverture du paysage apparaissent sur Cavallo et à Cannuta dès 5500-5000 cal. BP. Ils coïncident avec des indices d’activités des sociétés insulaires, lesquelles s’intensifient après 3000 cal. BP et plus particulièrement depuis 1000 ans à Bagliettu. De surcroît, notre étude suggère que la sensibilité et la réponse des zones humides littorales face à la remontée relative du niveau marin dépend du contexte géomorphologique. Sur l’île Cavallo où le relief est faible, la transgression marine induit une augmentation de la salinité dans l’étang du Greco vers 3700 cal. BP, qui à son tour provoque une chute irréversible de la diversité des coléoptères aquatiques et hygrophiles. Au marais de Cannuta, où l’érosion des versants a favorisé la progradation de la plaine alluviale, notre étude montre une diminution de l’influence marine après 5000 cal. BP et une diversification de cette même entomofaune à partir de 1200 cal. BP. À Bagliettu, c’est l’importance des changements hydro-morphologiques des rivières dans la dynamique des écosystèmes tourbeux attenants et de leur entomofaune que nous avons pu démontrer.Ainsi, cette approche rétrospective fournit des informations originales concernant la mise en place et l’évolution sur le long terme de la biodiversité des sites étudiés, utiles pour leur conservation.
Count rates, representing the rate of 14 C decay, are the basic data obtained in a 14 C laboratory. The conversion of this information into an age or geochemical parameters appears a simple matter at first. However, the path between counting and suitable 14 C data reporting (table 1) causes headaches to many. Minor deflections in pathway, depending on personal interpretations, are possible and give end results that are not always useful for inter-laboratory comparisons. This discussion is an attempt to identify some of these problems and to recommend certain procedures by which reporting ambiguities can be avoided.
Starting from multidisciplinary punctual results, the authors offer a synthesis of lithologie seismic analysis, sedimentology, palynology and radiometrical measurements from sites in sea and at the border of the Golfe du Lion. The dated samples (more than 120) and the variety of the studied sites has permitted a true methodological approach. The authors also propose a curve of the rising of the Holocene sea that takes into account two major facts: the geomorphologie localisation of the site and the characteristics of the sedimentary environment (sedimentary facies and paleobotanic environment). They conclude to: 1) a generalised rapid rise of the transgression since 14,000 years BP, with an acceleration from 8000 to 6000 years BP; 2) intervals of relative stability from 12 000 to 8 000 years BP; 3) a high sea-level at +2 m at around 4500 years BP.
The actual estimates of carbon stocks beneath seagrass meadows worldwide are derived from few data, resulting in a tendency to generalize global carbon stocks from a very limited number of seagrass habitats. We surveyed Posidonia oceanica and Posidonia sinuosa meadows along depth-induced gradients of light availability to assess the variability in their sedimentary organic carbon (Corg) stocks and accretion rates. This study showed a 4-fold decrease in Corg stocks from 2-4 m to 6-8 m depth P. sinuosa meadows (averaging 7.0 and 1.8 kg m-2, respectively; top meter of sediment) and a 14- to 16-fold decrease from shallow (2 m) to deep (32 m) P. oceanica meadows (200 and 19 kg m-2 average, respectively; top 2.7 m of sediment). The average Corg accretion rates in shallow P. sinuosa meadows were higher (10.5 g m-2 y-1) than in deeper meadows (2.1 g m-2 y-1). The reduction of sedimentary Corg stocks and accretion rates along depth-related gradients of light reduction suggest that irradiance, controlling plant productivity, meadow density and sediment accretion rates, is a key environmental factor affecting Corg storage potential of seagrasses. The results obtained highlighted the exceptional carbon storage capacity of P. oceanica meadows at Balearic Islands (Spain), containing the highest areal Corg stocks of all seagrasses (estimated in up to 691-770 kg m-2 in 8-13 m-thick deposits). Seagrass communities are experiencing worldwide decline, and reduced irradiance (following e.g. eutrophication or sediment regime alterations) will lead to photo-acclimation responses (i.e. reduced plant productivity and shoot density), which may impact the carbon sequestration capacity of seagrasses.