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See detailPosidonies : Lire dans les prairies sous-marines
Abadie, Arnaud ULg; Gobert, Sylvie ULg; Pergent, Gérard

Article for general public (2017)

Les “feuilles” qui s’amoncèlent sur les plages de Méditerranée sont le souvenir échoué des luxuriantes prairies sous-marines qui tapissent les fonds. C’est un écosystème fragile dont dépend un grand ... [more ▼]

Les “feuilles” qui s’amoncèlent sur les plages de Méditerranée sont le souvenir échoué des luxuriantes prairies sous-marines qui tapissent les fonds. C’est un écosystème fragile dont dépend un grand nombre d’espèces : les chercheurs doivent apprendre à “lire” les fonds pour identifier les menaces qui pèsent sur la posidonie. [less ▲]

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See detailEtude des intermattes des herbiers à Posidonia oceanica
Abadie, Arnaud ULg

Doctoral thesis (2016)

Among Mediterranean marine ecosystems, Posidonia oceanica (L.) Delile seagrass meadows form a habitat playing an important ecological and economical role. They form heterogeneous seascapes influenced by ... [more ▼]

Among Mediterranean marine ecosystems, Posidonia oceanica (L.) Delile seagrass meadows form a habitat playing an important ecological and economical role. They form heterogeneous seascapes influenced by the impact of natural and anthropogenic phenomenon (of which the anchoring of leisure ships). These impacts lead to the formation of sand and bare mat patches called ‘intermattes’. By describing and using intermatte characteristics observed in Calvi Bay and more widely around Corsica, these PhD works precisely described intermatte and P. oceanica seascape dynamics in order to develop new methods of analysis. On the one hand, natural sandy intermattes formed by bottom currents show two types of border. Each has its own characteristics at the level of the sediments biogeochemistry. On the other hand, anthropogenic intermattes generated by anchoring (bare mat) present different sizes and shapes resulting from specific dynamics. They are not recolonized when the anchoring pressure remains high. A depletion of the substrate chemical quality occurs after the mechanical destruction. A new cartographic seascape index, the Patchiness Source Index, was created using the size and the nature of intermattes at large spatial scale. After a comparison of methods used for the study of intermattes, two new indices (Map Anchoring Index and Anchoring Index) for managers of coastal areas were developed to evaluate the impact of anchoring within P. oceanica meadows. [less ▲]

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See detailInvestigating the potential of Cymodocea nodosa (Ucria) Ascherson as a coastal carbon sink coupling marine habitat cartographies and in situ nondestructive sampling
Abadie, Arnaud ULg; Richir, Jonathan ULg; Pieraccini, Riccardo ULg et al

Poster (2016, October)

Seagrass meadows are major carbon sinks, trapping about 10% of the total CO2 sequestrated in the oceans. In the Mediterranean, a major focus has been made on the climax species Posidonia oceanica (L ... [more ▼]

Seagrass meadows are major carbon sinks, trapping about 10% of the total CO2 sequestrated in the oceans. In the Mediterranean, a major focus has been made on the climax species Posidonia oceanica (L.) Delile, while other species remained little studied. In the framework of the STARECAPMED project, we thus chose to study Cymodocea nodosa (Ucria) Ascherson, a pioneer species with a rapid turnover and an expected high stocking capacity. Furthermore, the area covered by that species has been largely underestimated. In order to fill these two knowledge gaps, we first mapped all seagrass habitats within a Mediterranean bay (Calvi, Corsica, France) using side scan images, aerial photographs and ground truths. This cartography was followed by seasonal in situ density measurements and non-destructive shoot sampling (leaf cutting). Samplings were performed at different depths (5 to 23 m depth) in 6 contrasted stations (small patchy meadows to continuous beds) in order to cover all the existing facies of the bay. Elementary contents (carbon, nitrogen and stable isotope ratios) were measured in laboratory. This first work shows that C. nodosa meadows in Calvi Bay cover an area of 0.498 km2. Carbon stocks of the leaves reached 0.25 tons in winter and 2.72 tons in summer. Their nitrogen contents showed a marked seasonality with a maximum value of 0.020 mgN.m-2 in July and a minimum value of 0.005 mgN.m-2 in March. Some modifications in the trophic conditions of the water column at several stations were put in an obvious through the N stable isotopes values, mostly during the summer period. The actual underestimation of the area covered by that species in Calvi Bay has been properly mapped thanks to side scan sonar techniques revealing, together with elementary content analysis, its importance in the carbon balance of coastal areas. [less ▲]

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See detailImpact de l’ancrage sur les herbiers de posidonie
Abadie, Arnaud ULg

Scientific conference (2016, May 11)

La posidonie (Posidonia oceanica), forme en Corse des herbiers colonisant les substrats depuis la surface jusqu’à 40 m de profondeur. Ces herbiers jouent un rôle écologique essentiel en oxygénant le ... [more ▼]

La posidonie (Posidonia oceanica), forme en Corse des herbiers colonisant les substrats depuis la surface jusqu’à 40 m de profondeur. Ces herbiers jouent un rôle écologique essentiel en oxygénant le milieu grâce à la photosynthèse et en séquestrant du CO2 au sein de leur complexe racinaire dans le sédiment (ou "matte"). Ces activités métaboliques entraînent une importante modification des conditions biogéochimiques du substrat qui favorisent le maintien de la plante. Les herbiers de posidonie se développant dans les zones côtières, ils sont impactés par les activités anthropiques (urbanisation, pollution, fermes aquacoles, ancrage) qui peuvent entraîner d’importantes destructions et des régressions de leur zone de recouvrement. L’ancrage intensif dans les herbiers à faible profondeur y génère des trouées, ou "intermattes". Les destructions engendrées par les navires de petite taille (< 10 m) peuvent, dans certaines conditions, être recolonisées par la posidonie. Cependant certaines zones sont soumises à l’ancrage intensif de navires de plaisance de grande taille (> 20m), qui est susceptible de générer des intermattes d’une étendue plus importante. Ce phénomène d’expansion ne peut s’expliquer par la seule destruction mécanique. L’arrachage de la strate foliaire de la posidonie par les ancres semble entraîner de profondes modifications physiques et chimiques dans le sédiment. Un milieu anoxique défavorable à son développement paraît se créer. Des composés toxiques pour la plante, tel que le sulfure d’hydrogène (H2S), pourraient alors s’introduire dans ses tissues entraînant une limitation de son développement, voire sa disparition menant ainsi à une amplification des dégâts mécaniques de l’ancrage. [less ▲]

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See detailEstimating carbon fluxes in a Posidonia oceanica system: Paradox of the bacterial carbon demand
Velimirov, Branko; Lejeune, Pierre; Kirschner, A. et al

in Estuarine Coastal & Shelf Science (2016), 171

A mass balance ecosystemic approach, based on bacterial carbon demands and primary production data, was used to investigate if the bacterial community (freewater bacterioplankton and benthic bacteria of ... [more ▼]

A mass balance ecosystemic approach, based on bacterial carbon demands and primary production data, was used to investigate if the bacterial community (freewater bacterioplankton and benthic bacteria of the oxygenated sediment layer) could be sustained by the main primary producers (Posidonia oceanica and its epiphytes, adjacent macroalgae and phytoplankton communities; hereafter called the P. oceanica system) of a non-eutrophic Mediterranean bay. Unexpectedly, the findings of this study differed from previous works that used benthic incubation chamber and O2 optode methods. In this study, data were grouped in two categories, corresponding to two time periods, according to the seawater temperature regime (<18 °C or >18 °C): from May to October and from November to April. Between May and October, the produced benthic macrophyte tissues could not provide the carbon required by the bacteria of the oxygenated sediment layer, showing that the balance production of the investigated bay was clearly heterotrophic (i.e. negative) during this time period. In contrast, between November and April, benthic bacteria respiration nearly equated to carbon production. When integrating the open water carbon dynamics above the meadow in the model, a negative carbon balance was still observed between May and October, while a slight carbon excess was noticed between November and April. In the light of these findings, the carbon balance being negative on an annual basis, alternative carbon sources are required for the maintenance of the bacterial carbon production. [less ▲]

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See detailFrom mechanical to chemical impact of anchoring in seagrasses: the premises of anthropogenic patch generation in Posidonia oceanica meadows
Abadie, Arnaud ULg; Lejeune, Pierre; Pergent, Gérard et al

in Marine Pollution Bulletin (2016), 109(1), 61-71

Intensive anchoring of leisure boats in seagrass meadows leads to mechanical damages. This anthropogenic impact creates bare mat patches that are not easily recolonized by the plant. Several tools are ... [more ▼]

Intensive anchoring of leisure boats in seagrass meadows leads to mechanical damages. This anthropogenic impact creates bare mat patches that are not easily recolonized by the plant. Several tools are used to study human impacts on the structure of seagrass meadows but they are not able to assess the indirect and long term implication of mechanical destruction. We chose to investigate the possible changes in the substrate chemistry given contrasted boat impacts. Our observations show that hydrogen sulfide concentrations remain high at 15 and 20 m depth (42.6 µM and 18.8 µM) several months after the highest period of anchoring during the summer. Moreover, our multidisciplinary study reveals that anchoring impacts of large boats at 15 and 20 m depth can potentially change the seascape structure. By taking into account both structural and chemical assessments, different managing strategies must be applied for coastal areas under anthropogenic pressures. [less ▲]

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See detailSTAtion of Reference and rEsearch on Change of local and global Anthropogenic Pressures on Mediterranean Ecosystems Drifts: The STARECAPMED project
Richir, Jonathan ULg; Abadie, Arnaud ULg; Binard, Marc ULg et al

Conference (2015, November 08)

The Marine and Oceanographic Research Station STARESO in the Calvi Bay, Corsica (France), is a unique tool in a preserved natural site that includes all the characteristic ecosystems of the Mediterranean ... [more ▼]

The Marine and Oceanographic Research Station STARESO in the Calvi Bay, Corsica (France), is a unique tool in a preserved natural site that includes all the characteristic ecosystems of the Mediterranean littoral. The station, established in 1970, has archived environmental data for decades. The STARECAPMED project, multidisciplinary, articulates itself around these two main features. Its objective is to understand how human activities can interact with the fundamental processes that govern the functioning of the different coastal ecosystems of a Mediterranean bay. The understanding of these interactions involves: (i) the identification of the anthropogenic pressures; (ii) the quantification of their impacts on the ecosystems; (iii) the prioritization of these impacts. STARECAPMED also aims to confirm the relevance of the use of the Calvi Bay as a reference in the study of local and global pressures and the changes they may cause on the structure and the functioning of Mediterranean coastal ecosytems. [less ▲]

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See detailSTARECAPMED (STAtion of Reference and rEsearch on Change of local and global Anthropogenic Pressures on Mediterranean Ecosystems Drifts) - Année 2014. Rapport de recherches.
Richir, Jonathan ULg; Abadie, Arnaud ULg; Binard, Marc ULg et al

Report (2015)

La prise de conscience, par le grand public, de l'impact grandissant de l'homme sur l'océan est récente. Elle se traduit par une volonté politique sincère de correction par des mesures de protection, de ... [more ▼]

La prise de conscience, par le grand public, de l'impact grandissant de l'homme sur l'océan est récente. Elle se traduit par une volonté politique sincère de correction par des mesures de protection, de gestion et de développement durable. Ces politiques, et leurs conséquences économiques et sociétales lourdes, ne peuvent être acceptées que si les décisions se fondent sur des connaissances scientifiques incontestables et montrent des résultats scientifiquement prouvés. Par ailleurs, ces décisions doivent prendre en compte des impacts qui s'opèrent à des échelles de temps et d’espace très variables, de quelques heures à plusieurs dizaines d’années et de quelques mètres à plusieurs milliers de km. En termes politiques, l'information scientifique nécessaire à la prise de décision doit pouvoir couvrir les différentes échelles depuis le niveau local et régional, jusqu'à l'échelle nationale, européenne voire globale, et cela sur le plus long terme possible. Enfin, pour être complète, l'information scientifique sur les écosystèmes marins doit pouvoir répondre à trois questions objectives : (i) quel est l'état? (ii) quelle est l'évolution? (iii) quels sont les mécanismes et processus mis en jeux? et à une question plus prospective : (vi) que peux-t-on prévoir et comment agir? Le présent rapport ne peut pas, à lui seul, refléter toute la richesse du programme STARECAPMED. En 3 ans, STARECAPMED a généré plusieurs centaines de milliers de données nouvelles, ré-exploité plusieurs centaines de publications, rapports et autres enregistrements passés et présents. Le programme a aussi généré de nombreux documents, rapports et mémoires. Enfin, des publications internationales et des thèses de doctorat sont en cours de réalisation ou abouties. Afin de rester lisible, nous avons donc choisi de présenter ce rapport 2014 sous la forme de 12 exemples parmi les travaux en cours. Ces exemples sont traités selon un schéma identique en 4 points simples : (i) La présentation du cas d’étude dans le projet global ; (ii),Les approches innovatrices développées ; (iii),La présentation de résultats marquants ; (iv),Les délivrables de STARECAPMED pour les politiques publiques. [less ▲]

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See detailCaractéristiques des patchs dans les herbiers à Posidonia oceanica. Origine naturelle versus origine anthropique
Abadie, Arnaud ULg

Scientific conference (2015, June 25)

La plante marine endémique de Méditerranée Posidonia oceanica, ou posidonie, forme en Corse des herbiers colonisant les substrats depuis la surface jusqu’à 40 m de profondeur. Ces herbiers constituent un ... [more ▼]

La plante marine endémique de Méditerranée Posidonia oceanica, ou posidonie, forme en Corse des herbiers colonisant les substrats depuis la surface jusqu’à 40 m de profondeur. Ces herbiers constituent un véritable écosystème, l’un des plus riches en espèces de Méditerranée. Ils sont notamment soumis à des phénomènes de fragmentation naturels (hydrodynamisme) et anthropiques (par exemple l’ancrage, le chalutage, la pollution) à l’origine de la formation de patchs (ou "intermattes"), créant des zones de contact entre habitats de nature disparate. Issus de phénomènes différents, les patchs auront ainsi des caractéristiques et une dynamique propre. Les présents travaux de recherche ont été réalisés en baie de Calvi à 15 m de profondeur et visent à comparer les caractéristiques de patchs naturels générés par l’hydrodynamisme, et de patchs anthropiques issus de l’ancrage intensif. Dans cette optique, un patch de chaque type a été sélectionné et cartographié par triangulation à l’aide de balises afin d’étudier précisément leur dynamique spatiale. Les patchs au sein des herbiers à P. oceanica étant en grande partie composés de sédiments, le flux de déposition des particules a été étudié à l’aide de pièges à sédiments. Les caractéristiques chimiques du sédiment ayant une importante influence sur le développement de la posidonie, l’oxygène (O2), un composé vital, et le sulfure d’hydrogène (H2S), un composé létal, ont été mesurés à différentes profondeurs dans la couche sédimentaire et comparés avec les valeurs obtenues dans l’herbier adjacent. Des données ont été récoltées en hiver et en été afin de prendre en compte la variation saisonnière des phénomènes biotiques et abiotiques. Le suivi de la dynamique spatiale des patchs montre que, concernant le patch naturel, l’érosion de l’herbier (-10,3 cm.a-1) est compensée par sa colonisation (+10,1 cm.a-1), alors qu’une très grande proportion du patch d’ancrage (64 %) est érodée. Le flux de particules dans les patchs est de façon générale très faible en été (de 1,71 à 4,45 gDW.m-².d-1) par rapport à l’hiver (de 32,17 à 91,48 gDW.m-².d-1). Le dépôt total de particule est beaucoup plus important au sein du patch d’ancrage en hiver. Les deux patchs montrent des profils de concentration en oxygène et en sulfure d’hydrogène dans le sédiment similaire en hiver et contrastés en été, avec une concentration importante en sulfure dans le patch d’ancrage (9,3 µM à 10 cm de profondeur), limitant la potentielle recolonisation par l’herbier environnant. [less ▲]

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See detailPatch types in Posidonia oceanica meadows around Corsica. How can we use them in seascape ecology?
Abadie, Arnaud ULg; Bonacorsi, Marina; Gobert, Sylvie ULg et al

Conference (2015, May)

The meadows formed by the Mediterranean seagrass Posidonia oceanica are subjected to various natural (e.g., water movement, light availability, sedimentation) and anthropogenic (e.g., anchoring, trawling ... [more ▼]

The meadows formed by the Mediterranean seagrass Posidonia oceanica are subjected to various natural (e.g., water movement, light availability, sedimentation) and anthropogenic (e.g., anchoring, trawling, fish farms, explosives) phenomena that erode them and create diverse types of patches. The assemblage of the P. oceanica matrix and these patches creates particular seascapes. On the basis of this assessment, we aimed to investigate the importance of the patch type in structuring P. oceanica seascapes and to offer new prospects in the large scale studies of seagrass meadows. Five sites encompassing large P. oceanica meadows ranging from 1.86 km² to 4.42 km² along the Corsican coast (France) were considered. Eleven patch types with different sizes, shapes and origins were identified using side scan sonar images (sonograms). Five were recognized as natural and five as anthropogenic. One can be of both origins. The resolution of the sonograms allowed to detect patches of various sizes ranging from 1 m² to 111 829 m². The relation between structural characteristics of patches and the whole seascape aspect was explored using seven landscape metrics relevant for the study of meadows patchiness (patch area, mean radius of gyration, area-weighted radius of gyration, coefficient of variation of the Euclidean nearest-neighbor distance, area-weighted perimeter-area ratio, landscape division index, number of patches). Only a small number of patch types appears to play the strongest role in the characterization of the P. oceanica seascapes. Furthermore, the use of seascape structures seems to be suitable for the development of new tools like indices for the assessment of human impacts on P. oceanica meadows. In this perspective we propose a new and simple index, the Patchiness Source Index (PaSI), to estimate the origin of the patchiness (natural or anthropogenic) for a given area. A landscape approach, as well as information on patch dynamic, should be integrated in the new indices that aim to assess the state of conservation of the whole P. oceanica ecosystem. [less ▲]

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See detailMapping Posidonia oceanica meadows through time A story of precision, evaluation and fragmentation
Abadie, Arnaud ULg; Jousseaume, Matthieu; Lejeune, Pierre et al

Poster (2015, May)

Over the last decades, the interest in mapping Posidonia oceanica beds has increased along with the improvement of the equipment’s precision of data acquisition. In Calvi Bay (Corsica, France) the meadows ... [more ▼]

Over the last decades, the interest in mapping Posidonia oceanica beds has increased along with the improvement of the equipment’s precision of data acquisition. In Calvi Bay (Corsica, France) the meadows cover an area of about 5 km² and are found at a depth ranging from 3 m to 37 m. The availability of three distinct datasets for 1997, 2002 and 2010 allowed to assess changes in the patchiness of the meadows in the bay and to investigate evolution of maps precision through a surface analysis via GIS software. Thus, three maps were elaborated combining aerial photographs and side scan sonar images. The meadows percentage of cover through time was assessed using four bathymetric sections: 0-10 m, 11-20 m, 21-30 m and 31-40 m. Differences in the patchiness of P. oceanica meadows between 1997 and 2010 appear to be moderate (less than 3 %) in the sections 0-10 m and 11-20 m and then greatly increase with depth: 24 % at 21-30 m and 39 % at 31-40 m. This amazing regression seems hardly natural and unlikely given the slight quantity of human activities that can cause damages on the P. oceanica meadows of the Calvi Bay. These results are likely to be mainly due to the improvement of precision and resolution of the aerial photographs (5 m in 1997, 0.8 m in 2002 and 0.5 m in 2010) and sonar images (5 m in 1997, 3 m in 2002 and 0.5 m in 2010). An issue of habitat determination (human vs instrumental) linked with the method adopted for mapping can also cause differences in the percentage of cover. Given the different accuracy among the three maps, the real regression and fragmentation of P. oceanica meadows could be hardly assessed. However, in several areas where the human activities are important, a clear regression or even a disappearance of the meadows has been observed. It is obvious that the last maps are more accurate than the previous ones and, thus, the former can be used for management purpose as well as for study on the patchiness; however, they still keep uncertainty no matter which method is used to create them. [less ▲]

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See detailMarine space ecology and seagrasses. Does patch type matter in Posidonia oceanica seascapes?
Abadie, Arnaud ULg; Gobert, Sylvie ULg; Bonacorsi, Marina et al

in Ecological Indicators (2015), 57

The use of landscape tools in the study of seagrass meadows (seascapes) begins to be widely spreadbut still require the establishment of several basis, i.e. a patch type classification based on ... [more ▼]

The use of landscape tools in the study of seagrass meadows (seascapes) begins to be widely spreadbut still require the establishment of several basis, i.e. a patch type classification based on numericalcharacteristics. Thanks to the complex seascapes created by the Posidonia oceanica meadows, they appearto be suitable for a study at a patch type level (class), which bring a new insight of their arrangement at thewhole seascape scale. By interpreting side scan sonar images from the Corsican coast (France) through aGIS software, it was possible to describe 11 types of patches and to evaluate their natural or anthropogenicorigin. Comparison of different landscape metrics and wave exposure (Relative wave Exposure Index,REI) at the seascape and the patch level showed that the particularity of P. oceanica seascapes are mainlycharacterized by certain types of patches often of anthropogenic origin. Furthermore the REI seems not tobe a relevant index for a study at a class scale. A bathymetrical succession of natural patches was outlinedfrom the lower to the upper limit of the meadow, with a long-term dynamic opposed to a shorter oneconcerning anthropogenic patches. In order to assess the origin (natural or induced by human activities)of the patches in P. oceanica meadows, as well as in any other seagrass, a Patchiness Source Index (PaSI),ranging from 0 to 1, was defined. [less ▲]

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See detailTowards a framework for assessment and management of cumulative human impacts on marine food webs
Giakoumi, Sylvaine; Halpern, Benjamin S.; Michel, Loïc ULg et al

in Conservation Biology (2015), 29(4), 1228-1234

Effective ecosystem-based management requires understanding ecosystem responses to multiple human threats, rather than focusing on single threats. To understand ecosystem responses to anthropogenic ... [more ▼]

Effective ecosystem-based management requires understanding ecosystem responses to multiple human threats, rather than focusing on single threats. To understand ecosystem responses to anthropogenic threats holistically, it is necessary to know how threats affect different components within ecosystems and ultimately alter ecosystem functioning. We used a case study of a Mediterranean seagrass (Posidonia oceanica) food web and expert knowledge elicitation in an application of the initial steps of a framework for assessment of cumulative human impacts on food webs. We produced a conceptual seagrass food web model, determined the main trophic relationships, identified the main threats to the food web components, and assessed the components’ vulnerability to those threats. Some threats had high (e.g., coastal infrastructure) or low impacts (e.g., agricultural runoff) on all food web components, whereas others (e.g., introduced carnivores) had very different impacts on each component. Partitioning the ecosystem into its components enabled us to identify threats previously overlooked and to reevaluate the importance of threats commonly perceived as major. By incorporating this understanding of system vulnerability with data on changes in the state of each threat (e.g., decreasing domestic pollution and increasing fishing) into a food web model, managers may be better able to estimate and predict cumulative human impacts on ecosystems and to prioritize conservation actions. [less ▲]

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See detailVulnerability assessment of ecosystem components to human stressors: the case of a seagrass ecosystem
Giakoumi, S; Possingham, H; Gobert, Sylvie ULg et al

Conference (2014, August 17)

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See detailIslands as reference stations for environmental studies: the case of Calvi Bay in Corsica
Abadie, Arnaud ULg; Gobert, Sylvie ULg; Lejeune, Pierre

Conference (2014, July 05)

Islands are subject to human activities and their impacts on land and marine ecosystems. They are also often isolated from some of the continental influences but on the other hand different kind of human ... [more ▼]

Islands are subject to human activities and their impacts on land and marine ecosystems. They are also often isolated from some of the continental influences but on the other hand different kind of human activities can be concentrated in small areas. These characteristics make possible the management of many programs that use whole islands, or some of their parts, as a reference station for environmental studies. From this perspective, the program STARE-CAPMED has begun in 2012 at STARESO, an oceanographic research station established at Calvi Bay (Corsica) since the early 70’s. It aims to create a reference station for the study of emerging local and global anthropogenic impacts on marine pristine ecosystems. Several universities are involved in this project and provide their expertise in various fields of marine sciences. This program provides a precise view of the environmental processes that occur, which are strongly linked with economic and cultural issues. [less ▲]

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See detailL’ancrage dans les herbiers à Posidonia oceanica. Conséquences chimiques de la destruction mécanique ?
Abadie, Arnaud ULg

Poster (2014, June 26)

La plante marine endémique de Méditerranée Posidonia oceanica, ou posidonie, forme en Corse des herbiers colonisant les substrats depuis la surface jusqu’à 40 m de profondeur. Ces herbiers constituent un ... [more ▼]

La plante marine endémique de Méditerranée Posidonia oceanica, ou posidonie, forme en Corse des herbiers colonisant les substrats depuis la surface jusqu’à 40 m de profondeur. Ces herbiers constituent un véritable écosystème, l’un des plus riches en espèces de Méditerranée. Ils jouent un rôle écologique essentiel en oxygénant le milieu grâce à la photosynthèse et en séquestrant du CO2 au sein de leur complexe racinaire dans le sédiment (ou "matte"). Ces activités métaboliques entraînent une importante modification des conditions biogéochimiques du substrat qui favorisent le maintien de la plante. Les herbiers ont également une grande importance économique servant de nurserie pour de nombreuses espèces exploitées par la pêche commerciale et récréative. Les herbiers de posidonie se développant dans les zones côtières, ils sont impactés par les activités anthropiques (urbanisation, pollution, fermes aquacoles, ancrage) qui peuvent entraîner d’importantes destructions et des régressions de leur zone de recouvrement. L’ancrage intensif dans les herbiers à faible profondeur y génère des trouées, ou "intermattes". Les destructions engendrées par les navires de petite taille (< 10 m) peuvent, dans certaines conditions, être recolonisées par la posidonie. Cependant, dans la plupart des zones d’ancrage intensif en Corse, les intermattes ne sont pas recolonisées et leur surface peut souvent augmenter. De plus, certaines zones sont soumises à l’ancrage intensif de navires de plaisance de grande taille (> 20m), susceptibles de générer des intermattes d’une étendue plus importante. Ce phénomène d’expansion ne peut s’expliquer par la seule destruction mécanique. L’arrachage de la strate foliaire de la posidonie par les ancres semble entraîner de profondes modifications physiques et chimiques dans le sédiment. La photosynthèse via les feuilles étant ainsi altérée, il n’y a plus d’oxygénation du sédiment via les racines de la posidonie et un milieu anoxique défavorable à son développement paraît se créer. Des composés toxiques pour la plante, tel que le sulfure d’hydrogène (H2S), pourraient alors s’introduire dans ses tissues entraînant une limitation de son développement, voire sa disparition menant ainsi à une amplification des dégâts mécaniques de l’ancrage. Des observations seront réalisées tout autour de la Corse et notamment dans la Baie de Calvi, site atelier de la Méditerranée, qui a été définie pour mener la phase d’expérimentation. Les données récoltées à l’issue de ces travaux de recherches, réalisés dans le cadre d’un doctorat en entreprise (bourse CIFRE), seront compilées afin de mettre au point de nouveaux outils d’analyse de l’impact de l’ancrage. [less ▲]

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See detailSediments oxidation by seagrasses: influence on the S cycle in Posidonia oceanica (L.) Delile intermatte dynamics
Abadie, Arnaud ULg; Lejeune, Pierre; Pergent, Gérard et al

Poster (2014, May)

The Mediterranean seagrass Posidonia oceanica forms meadows and develops a complex of rhizomes, roots and sediment which is called “matte”. P. oceanica meadows show discontinuity patterns in the form of ... [more ▼]

The Mediterranean seagrass Posidonia oceanica forms meadows and develops a complex of rhizomes, roots and sediment which is called “matte”. P. oceanica meadows show discontinuity patterns in the form of sand or dead matte (matte without living shoots) patches, called “intermattes”, which can have natural or anthropogenic origins. Mechanical processes (e. g. currents, anchoring) can initiate intermattes formation but their dynamic after creation seems to be linked with the sediment chemistry, especially with S cycle. P. oceanica plays an important role in controlling coastal belowground biogeochemistry, in particular by oxidizing sediments through the release of O2 by roots. This process allows creating more suitable condition for plant growth and colonization. The lack of H2S oxidation in SO4 2- can lead to limitation of the plant development or its regression. In order to investigate the effect of oxidation condition in sediments on intermattes dynamic and the neighboring meadow, we initiate, in December 2013, a study on six intermattes (three natural, three anthropogenic) at different depths in Calvi Bay, in Corsica (France). We hypothesize that redox potential and H2S concentration in sediments play an important role in the regression of P. oceanica meadows, particularly after a mechanical anthropic impact like anchoring. It also may be possible that two different kinds of processes are involved for each type of intermatte. Regular samplings throughout two years are planed with the aim of evaluating the seasonal variations of physicochemical parameters. [less ▲]

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See detailEvolution des herbiers à Posidonia oceanica (L.) Delile dans la baie de Calvi (Corse, France) et influence de l’ancrage dans la baie de l’Alga
Abadie, Arnaud ULg

Master's dissertation (2012)

In the framework of the STARE-CAPMED program we studied the impact of human activities on Posidonia oceanica meadows in Calvi Bay (Corsica, France). We mostly focused on the consequences of boats ... [more ▼]

In the framework of the STARE-CAPMED program we studied the impact of human activities on Posidonia oceanica meadows in Calvi Bay (Corsica, France). We mostly focused on the consequences of boats anchoring in Alga Bay. The map of the marine habitats realized during this study shows that P. oceanica meadows cover a surface of 4.94 km2 in 2010 and have declined of 26 % since 1997. Advances in the data acquisition techniques allow a better identification of the habitats and a map reliability improvement. In several places in the bay, especially near the waste waters rejection site, the lower limit of the meadow is shallower than in the past. The diminution of the meadow surface in the 21-30 m bathymetric section is more important in the Alga site than in the rest of Calvi Bay. The ecological state of the water surrounding the meadow in Alga was assessed as "good" with a PREI index of 0.685. However, big units anchoring in this area lead to the creation of dead “mattes” whose compactness are medium. In this area, the generation of dead mattes seems to favor the settlement of the invasive Chlorobionta Caulerpa racemosa var. cylindracea, of which anchoring is known to be one of the introduction vectors. [less ▲]

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