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International audience; The loss of natural habitats is generally associated with a loss of biodiversity that will cause a change in the functioning of ecosystems and consequently in their capacity to provide ecosystem services. As ecosystem engineers, mangrove crabs fulfil key ecological roles in the structure of the mangrove ecosystem through burrowing and in the functioning through nutrient cycling and bioturbation. They are also of societal and economic interest that can be used by the populations. In the West Indian Ocean, there are gaps in knowledge on the identification and distribution of mangrove crab species. This challenges our understanding of the consequences of mangrove erosion on the associated biodiversity. For example, between 1950 and 2016, the mangroves of Mayotte have been affected by a moderate regression of 6%. This loss is likely to impact the distribution of crabs and consequently the structure and functioning of the ecosystem. The objective of this study is to understand the impact of mangrove erosion on the distribution of crabs through an interdisciplinary approach combining methodologies in ecology, human geography and geomorphology.The combination of these methodologies allows access to data at several temporal and spatial scales. The characterisation of erosion rates since 1950 and consequences on sedimentary movements at large temporal and spatial scales will be provided by geomorphology data. The human geography data provides access to a medium temporal scale on the characterisation of the evolution of the distribution of crabs at the scale of the bay over the last decades. At a finer scale, by replacing time by space, the ecology data will ensure a comparison of two zones representing two different instants of the mangrove erosion trajectory.This study focuses on the mangroves of Dapani and Mzouazia, located on the southern coast of Mayotte Island. We focus on the central and sea mangroves because have been in major decline since 1950. The Mzouazia mangrove covers 3 ha, consisting of Sonneratia alba in seaward fringe, followed by a fringe of Rhizophora mucronata. The Dapani mangrove covers 10 ha, is composed of a Sonneratia alba in the seaward fringe, followed by a large Rhizophora mucronata stand in the central mangrove.The methodology used in geomorphology consisted of a diachronic analysis of vertical aerial photographs between 1950 and 2020 in order to characterise the spatial evolution of the mangroves studied over 70 years. Aerial photographs taken by unmanned aerial vehicles were also acquired in order to characterise the surface changes of mangroves in the short-term between 2020 and 2022. Topographic surveys were carried out between 2020 and 2022, in order to characterise the morphology of the coastal profile and its evolution. We also have a series of topographic profiles distributed irregularly between 2005 and 2019. In human geography, we conducted semi-structured interviews with people in villages bordering the mangroves of Mzouazia and Dapani in order to better understand their local ecological knowledge regarding crabs. We targeted people over 15 years old who have been frequenting or regularly visiting the mangrove for several years or people having activities near the mangrove. We asked people about the vernacular names of the crabs present in the mangrove of their village, a recognition criteria, their location, their habitat and about the evolution of their distribution area, quantity and size.The objective of the ecology protocol was to compare the distribution of crab species across the shore, along an eroding and non-eroding mangrove section. For this purpose, two 100 m long transects were deployed per mangrove, starting from the beach ridge, and oriented perpendicular to the coastline. The Point-Centered Quarter Method was used to place 10 sampling points haphazardly along the transects where we measured the following parameters: abundance of crab species, identity of dominant mangrove trees, tree circumference, and grain size. The height on the foreshore is provided by a digital elevation model. Data have been acquired in Mzouazia and those in Dapani will be acquired in the coming weeks.The analysis of aerial photographs shows that these two mangroves have experienced a regression of -64% for Dapani and -15% for Mzouazia since 1950. On the eroding section, the morphological measurements show an accumulation of sand which reflected a significant rise of 0.55 m in the beach ridge from February 2005 to March 2022. The profiles also show a retreat of almost -10m of sediment between 2013 and 2022, reflecting the decrease in height on the foreshore. The sediment analyses performed along the transects show that muddy sediments prevail in the non-eroded sections whereas sandy sediments dominate eroded sections. This can be explained by the presence of trees, which attenuate the wave energy and favour the deposition of fine sediments, whereas in the eroded area, fine sediments are transported away and coarse sediments accumulate on the beach ridge.Preliminary results of the 19 semi-structured interviews conducted in Mzouazia show that the inhabitants are more familiar with Cardisoma carnifex and Scylla serrata, which live at the land and sea ends of the mangrove. They do not seem, however, to perceive the effects of marine erosion on the distribution of crabs. Yet, crab community distributions show a higher species abundance and richness in the non-eroded mangrove sections than in the eroded mangrove sections. Comparison with the data provided by the 18 semi-structured interviews conducted in Dapani and the ongoing study of crab distributions will allow generalising these results. The relative importance of tree presence, grain size, and elevation on the foreshore for crab community composition will also be discussed and confronted to the species used and known by inhabitants.These data provide new information on the distribution of mangrove crabs and on the factors affecting their distributions. These data may raise questions about potential changes in mangrove ecosystem functioning, and the consequences for the ecosystem services provided by eroding mangroves. As such, they will provide predictive data on the distribution of mangrove crabs that will be of interest to stakeholders and scientists. Analysis of the data will identify if environmental memory is being lost with the erosion of the mangrove.

International audience; The seasonal beach rotation visible in Sakouli beach reflects : the two distinct hydrodynamic conditions that occur in Mayotte and a residual longshore sediment transport favored by a low bay indentation. However, the island subsidence of 10 to 20 cm since 2018 may have an impact over this equilibrium. Indeed, this could lead to higher wave energy over the beach. So, study must be continued and extended to several other beaches on the island.

International audience; Mayotte Island in the Mozambique Channel holds 735 hectares of mangroves, occupying 29% of the coastline. In 2011, the island became a French overseas department meaning adopting the same legal and social system used in France. This change in status has consequences for the management of the mangrove ecosystems there. The National Forestry Authority has established itself in Mayotte and is drafting a management plan of the island's mangroves for the next 10 years.In this context, we conducted a campaign of interviews with stakeholders influencing the preservation and management of the island's mangroves in order to better understand their knowledge, representations and opinions. Overall, we carried out 25 semi-structured interviews with 12 from the central and local government, 5 from associations, 5 from the tourist sector and 3 from experts. Here, we focus on the knowledge of stakeholders and analyse it by two experts in the geomorphology and the ecology of the mangroves of Mayotte. The knowledge assessed from open-ended questions relates to the biodiversity and the specificity of Mayotte’s mangroves relative to mangroves in other territories; the nature and relative importance of impacts was evaluated from classifying and commenting a set of photographs; knowledge of the health status was assessed with a cognitive map exercise.Results show disparity in stakeholder’s knowledge depending on the topic. For instance, regarding the biodiversity, only some stakeholders think this ecosystem is particularly rich. Regarding the impacts on mangroves, all stakeholders agree that embankments, roads across mangroves and rubbish are very impacting but few are aware of the consequences of other impacts mentioned. This is the case with paths crossing the mangrove seaward: the majority of stakeholders didn’t know that this has been shown to contribute to the erosion of mangroves. These results are useful to know what kind of knowledge should be transmitted to stakeholders.

International audience

International audience; Cette note dresse un portrait de la gestion du risque cyclonique en Polynésie française. Nous rappelons les particularités de la prévision météorologique au centre du Pacifique sud en insistant sur les nombreuses contraintes techniques qui obèrent une appréhension hautement qualitative du phénomène cyclonique. A travers le cas du cyclone tropical Oli ayant traversé le territoire en février 2010, nous abordons la gestion concrète de cet événement par les autorités, qu’elles soient territoriales ou étatiques. Enfin, nous décrivons la réaction post-crise à deux niveaux : celui de l’administration et celui des particuliers qui procèdent de logiques parfois contraires.

International audience; This study evaluated the ability of the SPOT5 and Formosat-2 satellite based-sensors, which have different band wavelengths and different spatial resolutions (10 m, 8m) , to extract waterlines of a macrotidal beach in northern France. The study area (12 km²) is characterized by a large suite of sedimentary facies and morphological features (beach with multiple intertidal bars, sandflat, coastal foredune….) relevant in evaluating remote sensing monitoring methods. Extracting waterlines under various water-level conditions can provide topographic information on the intertidal area because the waterlines can be regarded as contours. Satellite sensors can extract waterlines across a wide area, and this waterline detection method can serve as an alternative to digital elevation models (DEM) created by an aircraft-based light-detection and photogrammetric methods. By regarding beach waterlines as shorelines, data from satellite sensors can be used to detect shorelines and supplement the conventional aerial photograph method of shoreline change analyses or in situ measurements. Ability to extract waterlines was quantified by calculating errors of extracted waterline positions relative to the true positions, which were determined by measuring topography along some transects using a differential GPS and a total station. 252│Amélioration de la modélisation numérique de terrain © Revue Télédétection, 2012, vol. 11, n o 1 Finally this work focuses on the use of SPOT and Formosat-2 images and their comparison with topographic in situ data. The results serve as general guidelines for using satellite-derived data to estimate intertidal topography. Waterline extraction based on satellite sensor data should prove particularly useful in remote areas difficult to access.; Cette étude évalue la capacité des capteurs satellites multispectraux SPOT5 et Formosat-2 d'une résolution spatiale respective de 10 m et 8 m, pour la mesure indirecte de la topographie des estrans d'une plage macrotidale dans le nord de la France. Le secteur d'étude (12 km²) caractérisé par une grande diversité morphologique (plage à barres interti-dales, sandflat, dunes) et sédimentaire (dépôts vaseux en milieu ouvert) est de ce fait approprié pour l'utilisation de la télédétection. La méthode utilisée consiste à (1) extraire indirectement la topographie des zones intertidales en utilisant des séries d'images satellites acquises à des pas de temps fins mais à des hauteurs de marée différentes et à (2) compa-rer les mesures extraites et celles réalisées in situ le long de transects topographiques et en semis de points réguliers (Modèle Numérique de Terrain). La quantification des marges d'erreurs entre les deux approches démontre l'intérêt de cette méthode pour compléter des mesures conventionnelles in situ ou aéroportées et qui peut se montrer particulière-ment intéressantes sur des zones intertidales éloignées voire difficiles d'accès.

Since 2003, a monitoring network of the dynamics of sandy beaches has been in operation in three tropical sites : Martinique (French West Indies), Tahiti and Moorea (French Polynesia) and Mayotte (Mozambique Channel). The objective of this network is to measure the impact of extreme weather-related marine events (storms, cyclones, strong swells, surges...) on beaches with varied structural settings and sedimentary environments. The methodology consists in the installation of observatories on the dynamics of sandy beaches in different morphological and sedimentary contexts. A "state zero ” is established from a combination of satellite imagery, low altitude digjial imagery and topographic and hydrodynamic measurements. These measurements are reiterated after an extreme event in order to apprehend the importance of morphological change, impact dampening and recovey. The monitoring is then continued over regular intervals with the aim of determining rates cf morphological and sediment budget recovery until equilibrium is restored. This experimental network design will subsequently be extended to other tropical beaches located on the propagation paths of cyclones., ALERT : un réseau de surveillance et de compréhension de la dynamique des plages tropicales sous l'influence d'événements météo-marins paroxystiques. Depuis 2003, un réseau de surveillance de la dynamique des plages est opérationnel sur trois chantiers : en Martinique, Tahiti et Moorea (Polynésie Française) et Mayotte (archipel des Comores). L’objectif de ce réseau est de mesurer l’impact des événements météo-marins paroxysmaux (tempêtes, cyclones, fortes houles, surcotes...) sur les plages, dans des environnements et des contextes sédimentaires variés. La méthodologie consiste en l’installation d’un observatoire de la dynamique des plages sableuses dans différents contextes morphologiques et sédimentaires. Un «état zéro » est établi à partir de la compilation d’images satellitales à haute résolution, d’images numériques basse altitude et de mesures topographiques, sédimentaires et hydrodynamiques. Ces mesures peuvent être réitérées à la suite d’un événement de forte intensité afin de caractériseri’importance des changement morphologiques (impact), la capacité de résistance du milieu (encaissement) et sa faculté à se reconstituer (résilience). Le réseau peut donc devenir un bon outil pour appréhender les processus et les rythmes de résiliences par un suivi régulier des mesures. Ce réseau expérimental devra à terme être étendu à d’autres plages tropicales, en particulier sur les couloirs de propagation des ouragans., Dolique Franck,Jeanson Matthieu. ALERT : A monitoring survey network to understand the dynamics of tropical beaches under marine-weather events. In: Natures tropicales : enjeux actuels et perspectives. XIIes Journées de Géographie Tropicale du Comité National Français de Géographie - 16-18 novembre 2008. Bordeaux : Presses Universitaires de Bordeaux, 2011. pp. 457-464. (Espaces tropicaux, 20)

Mayotte Island, located in the Mozambique Channel, western Indian Ocean, is characterised by a vast, highly diversified reef-lagoon complex comprising significant mangrove development. Several field experiments involving hydrodynamic measurements, topographic surveys, and observations were coupled with the analysis of aerial photographs and the collation of regional meteorological data in order to gain a better understanding of the evolution of the reef-lagoon complex and of the morphodynamic interactions between the mangroves and the coral reefs. The results highlight a remarkably variable mangrove system subject to progression or stability in the north and east of the island, but exhibiting a clearly regressive pattern along the southern and western shores. The hydrodynamic data acquired during the field experiments clearly bring out the spatial and temporal variations in wave and mean current patterns involved in these differences. These data also throw light on the short-term morphodynamics of the small pocket beaches associated with some of these mangroves. The degree of exposure to waves, and reef structure, notably in terms of width and elevation relative to the tidal frame, have a determining influence on the afore-mentioned variations in mangrove dynamics along the shores of the island, and in the greater vulnerability of the mangrove shores of the south and west of the island, especially in the face of strong impinging development pressures. The extermination of these mangrove systems leads to a reorganisation of the sediment stocks and the emergence of new shoreline morphodynamic patterns. These changes have necessitated the construction of an operational observatory aimed at monitoring the coastal dynamics. This tool is based on a network sourced by the measurements and field observations integrated into a GIS. Within a local framework of strong socio-economic and demographic pressures, and a more global context of environmental change, this observatory should lead to a better understanding and prediction of the morphodynamics of the shores of Mayotte, while providing data indispensable to stakeholders involved in decision-making in the face of the major and rapid environmental and socio-economic changes liable to affect the fragile reef-mangrove coastal systems; L'environnement côtier de l'île de Mayotte (nord du canal du Mozambique, océan Indien) se caractérise par l'existence d'un vaste complexe récifo-lagonaire au sein duquel se développe un littoral particulièrement diversifié et marqué par la présence de nombreuses mangroves. Plusieurs campagnes de mesures hydrodynamiques et d'observations morphologiques ont été effectuées afin d'apporter des éléments de compréhension sur l'évolution et le fonctionnement morphodynamique de ce type de littoral mixte à mangroves et récifs coralliens. Les observations de terrain conjuguées à une analyse diachronique de photographies aériennes ont permis de montrer le caractère très variable de la dynamique des mangroves avec une progression ou une stabilité au nord et à l'est de l'île, et inversement, une évolution nettement régressive au sud et à l'ouest. Les mesures hydrodynamiques ont donné lieu à une analyse des paramètres de houle et de courant en se focalisant sur leurs variabilités spatiales et temporelles. Les résultats obtenus montrent l'importance de l'architecture récifale sur les processus hydrosédimentaires et le développement des formes littorales. L'évolution et la dynamique des mangroves de Mayotte apparaissent ainsi fortement influencées par les caractéristiques morphologiques des structures récifales amont qui engendrent une plus forte vulnérabilité des mangroves du sud et de l'ouest face à une déstabilisation naturelle ou anthropique. La disparition des formations végétales des mangroves du sud et de l'ouest de l'île entraine une réorganisation des répartitions sédimentaires et la mise en place de nouveaux systèmes de fonctionnement morphodynamique. L'ampleur des phénomènes constatés a montré la nécessité de mettre en place un observatoire opérationnel de surveillance de la dynamique côtière. Cet outil, basé sur un réseau de mesures et d'observations de terrain et sur la réalisation d'un Système d'Information Géographique (SIG), a pour but de favoriser le suivi et l'évaluation de l'état du littoral. Dans un contexte local de fort développement et dans celui, plus global, de mutations environnementales, cet outil de surveillance et d'analyse permet une meilleure compréhension de la morphodynamique des côtes mahoraises et apporte une aide à la décision et à la gouvernance afin d'orienter les politiques de gestion de ce milieu fragile, à forts enjeux et en constante évolution.