Mangrove Evolution in Island, Indian Ocean: A 60-year Synopsis Based on Aerial Photographs

Matthieu Jeanson, Edward J. Anthony, Franck Dolique & Caroline Cremades

Wetlands Official Scholarly Journal of the Society of Wetland Scientists

ISSN 0277-5212

Wetlands DOI 10.1007/s13157-014-0512-7

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Wetlands DOI 10.1007/s13157-014-0512-7

ARTICLE

Mangrove Evolution in Mayotte Island, Indian Ocean: A 60-year Synopsis Based on Aerial Photographs

Matthieu Jeanson & Edward J. Anthony & Franck Dolique & Caroline Cremades

Received: 7 November 2013 /Accepted: 15 January 2014 # Society of Wetland Scientists 2014

Abstract The Island of Mayotte, in the Indian Ocean, is Keywords Mangroves . Aerial photography . Coastal characterized by a highly diversified shoreline comprising processes . Coastal development pressures . Mayotte . numerous mangrove zones set within a vast coral reef and Indian Ocean lagoon complex. An analysis of aerial photographs covering the period from 1950 to 2011 shows marked variability in the evolution of these mangroves and clear spatial differences in Introduction the dynamics of these communities over the 60-year period of analysis. The northern part of the island shows stability or a Mangrove communities fulfill an important role in the eco- slight increase in mangrove area whereas the southern and logical and physical dynamics of coastal systems, especially western shores of the island have been characterized by a clear near coralline environments. Their importance in ecosystem- regression in mangroves. The total surface area of mangroves based management of coasts has been highlighted by several on Mayotte (7.03 km2 in 2011) has diminished overall by authors, not only in terms of their extremely diversified about 5 %, a relatively moderate figure relative to the world habitat function (Nagelkerken et al. 2008), but also regard- context of increasing diminution of mangrove area. The pattern ing carbon storage (Donato et al. 2011) and coastal protec- of evolution of mangroves in Mayotte is explained jointly by tion against waves (Barbier et al. 2008; Anthony and Gratiot development pressures on the coast and spatial variability in 2012), mangrove forests reducing casualties and damage to mangrove recovery determined by hydrological conditions and property in the course of high-energy events such as tsu- exposure to waves on this reef-fringed island. Mayotte Island namis and cyclones (Alongi 2008; Yanagisawa et al. 2009; acquired the status of a French administrative department in Zhang et al. 2012). The trapping of sediments by mangroves 2011 subject to both French and European Community legis- is also essential, for instance, for adjacent coralline systems, lation on the environment. In consequence, mangroves are several studies having shown a link between the quantity, now much better protected from future urbanization. rate of sedimentation and residence time of fine-grained sediment input and the degradation of coral reefs (Fabricius 2005; Victor et al. 2006;Golbuuetal.2008). In consequence, gaining insight on the distribution, areal extent M. Jeanson (*) and conservation of mangroves at regional and local scales Laboratoire de Géomorphologie et Environnement Littoral, is of fundamental importance for the functioning, protection, EPHE, UMR 8586 Prodig, Dinard, e-mail: [email protected] biodiversity and resilience of tropical coastal environments, especially coralline, and is of direct relevance to the quality E. J. Anthony and quantity of resources available to the local populations Aix-Marseille Université, Institut Universitaire de France, (Mazda et al. 2002; Munby et al. 2004;Mansonetal.2005; CEREGE UMR 34, Aix en Provence, France Nagelkerken 2009). F. Dolique In spite of these numerous benefits, mangroves are Université Antilles-Guyane, Campus de Martinique, BP 7207, among the ecosystems under the greatest threat from human 97275 Schoelcher Cedex, France activities, and are rapidly decreasing on many of the world’s C. Cremades coasts. Over the last 30 years, 20 to 35 % of the mangroves UICN - Comité Français, Route nationale, 97670 Coconi, Mayotte of the world have disappeared, mainly to the benefit of Author's personal copy

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10°S Comoro Archipelago Indian Ocean

Mayotte

Mozambique Madagascar Mozambique Channel Mauritius 20°S

Reunion

Bandraboua 40°E 50°E Dzoumogné Port Miangani Kangani Longoni

Majicavo Koropa

Majicavo Grande Lamir 12°45' Terre Badamiers Kawéni Soulou Petite Zidakani Mgonbani Baobab Terre Tzoundzou Passamaïnti Ironé Bé

Chiconi Dembéni 12°50'

Hajangoua Iloni Hajangoua Sud Bénara Mount (660 m)

Hagnoundrou

Bandrélé Bouéni Bay 12°55'

Mzouazia

Mounyambani

Altitude (m) Settlements in 2011 600 Kani Kéli 500 400 Dapani Mangroves in 2011 300 13°S Mronabéja 200 100 Coral reefs 0

0 5 10 km

45°E 45°05' 45'10' 45°15' 45°20'

Fig. 1 Mayotte Island, Indian Ocean, showing locations of mangroves agriculture, aquaculture and spread of urban zones, i.e., a rare archive of aerial photographs dating back to 1950. This rate of decrease of 1 to 2 % per annum (FAO 2007;Giri report is a contribution to the mangrove inventory at the et al. 2011). The aim of this study is to determine and global scale. In the face of mangrove degradation on Ma- characterize the evolution of the surface area of mangroves yotte Island, it also highlights the need for implementation, in Mayotte (Fig. 1), in the Indian Ocean, using a relatively by the regional and national authorities, of a policy of Author's personal copy

Wetlands sustainable and integrated coastal management, Mayotte lies on the track of tropical storms and cyclones. These occur being a French department and subject to European Com- exclusively during the rainy season, and essentially at the start munity legislation. and end of the season. Mayotte is, however, relatively shel- tered by Madagascar. The cyclone return period is about 15– 20 years (mean wind speeds exceeding 117 km/h). Tides Mayotte Island affecting Mayotte are semi-diurnal and the mean spring tidal range is about 3.2 m. Mayotte is situated approximately 13° S and 45° E about The socio-economic context of Mayotte is one of acceler- 300 km northwest of Madagascar in the north of the Mozam- ated development and strong demographic growth. Together bique Channel and about 450 km away from the African with the islands of Anjouan, Mohéli and Comore, Mayotte is continent (Fig. 1). The island acquired the status of a French part of the Comoros Archipelago. In 2012, the population of Department, identical to that of French metropolitan depart- the island was 212,600 (density of 568 inhabitants/km2,i.e., ments, in 2011. Mayotte has an area of 374 km2, and is over five times that of metropolitan France), a significant composed of two main volcanic islands, Grande Terre and increase compared to the 1958 figure of 23,000 inhabitants Petite Terre, culminating at a peak elevation of 660 m at (INSEE 2012). This strong population growth is due to a high Mount Bénara. Mayotte also comprises about 30 islets of birth rate (41.2‰) and sustained immigration from the other volcanic or coral reef origin spread out in a lagoon behind a poorer islands of the Comoros Archipelago. The population ring of remarkable coral reef barriers, with a circumference of forecast for 2017 ranges from 260,000 to 320,000. One of the 157 km and an additional 40 km of submerged reefs where consequences of this strong demographic pressure is an in- this ring is perforated. Mayotte lagoon is 3–15 km wide, up to crease in terrigenous sediment input into the lagoon resulting 80 m deep, and, with an area of nearly 1500 km2, is one of the from combined increased deforestation of the steep inland largest reef lagoons in the Indian Ocean, The volcanic and slopes, poor agricultural practices and major infrastructural tropical context of Mayotte and the vast reef-lagoon system works (Wickel and Thomassin 2005). Several studies have have resulted in remarkable coastal geomorphic diversity. The shown that terrigenous sedimentation has increased consider- total shoreline length of Mayotte is 265 km, and is an intricate ably to the detriment of carbonate sedimentation in the lagoon alternation of cliffs separating variably indented pocket over the last three decades (Masse et al. 1989; Thomassin et al. beaches of sand and sandy mud, the sheltered low-energy 2011). These studies further show that these increased terrig- backshores of many of which are colonized by mangroves enous inputs are leading to degradation of the fringing coral (Fig. 1). reefs and the lagoon environment of Mayotte. Similarly, Mayotte experiences a humid tropical climate comprising observations on the evolution of mangroves since the late two seasons, a hot rainy monsoon season, and a cooler, dry 1990s highlight erosion of many stands with pronounced trade wind season. Mean annual rainfall is about 1500 mm. losses of surface area (Lebigre 1997; Laulan et al. 2006). Pressure gradients between the Intertropical Convergence The creation of the Sea Park of Mayotte in 2010 and increas- Zone and anticyclones in the Indian Ocean generate, respec- ing awareness of the patrimonial and the reef-purifying role of tively, hot and humid monsoon winds from a north to north- mangroves (Herteman et al. 2011)showthatitisimportantto west sector during the southern hemisphere summer (Novem- understand the past evolution and dynamics of these coastal ber to April) and cool, dry trade winds from south to southeast environments. during the southern hemisphere winter (May to October). The latter form the dominant winds. These wind conditions are directly reflected in the hydrodynamic regime of the reef- Methods lagoon system, which is characterized by marked contrasts in the seasonal wave climate. Waves are from the north during Image Analysis the southern hemisphere summer and from the south during winter. Voluntary Observing Ship (VOS) wave observations The methodology employed consisted in a comparison of (sector: 11–15°S, 43–47°E) over the period 1960–2000 and mangrove distribution on sets of vertical aerial photographs AVISO data from spatial altimetry show that the offshore from a remarkably long archive between 1950 and 2011 wave climate of Mayotte is dominated by moderate waves (Table 1), probably among the longest for mangrove systems with characteristic periods of 5 to 6 s that denote a regional in the world. Aerial photographs are a robust and efficient fetch. Monsoon winds generate moderate NW to NE waves, technique for comparing mangroves and determining local with a tendency towards relatively calm conditions at the end patterns of evolution especially when small communities are of the monsoon season. The highest and most frequent waves being analysed (Kuenzer et al. 2011), which is the case of the are from S to SE (60 %), in response to the highly regular and Mayotte mangroves. Moreover, aerial photographs present sustained trade winds from May to September. Mayotte also two other advantages. They have the highest spatial Author's personal copy

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Ta b l e 1 Characteristics of a 60-year time series (1950–2011) of vertical aerial photographs used to identify changes in mangroves in Mayotte

Identification of mission Dates of photography Type of photograph Scale Size of pixel (m) Colour (C)/Black & White (BW)

2011 FD 976 Couleur 25 20/10 to 03/11/2011 orthophotographs − 0.20 C 2008 FD 976C 68 17 to 18/10/2008 orthophotographs − 0.50 C 2003 MAY004 C 25000 10 to 25/06/2003 orthophotographs 1 : 25 000 0.50 C 1997 MAY 003C 25000 02 to 10/06/1997 orthophotographs 1 : 25 000 0.50 BW 1989 MAY 1 P 20000 13/06/1989 geo-referenced 1 : 20 000 0.50 BW 1969 COMORES 8 P 20000 14/07 and 09/08/1969 geo-referenced 1 : 20 000 0.50 BW 1950 MAD 20 P 40000 03/08/1950 geo-referenced 1 : 40 000 1.00 BW resolution, as far as remote sensing images are concerned exercise that relies on a good degree of operator subjectivity (Dahdouh-Guebas and Koedam 2008), although the most in analysis and interpretation, although this shortcoming recent optical satellite imagery, such as Pléiades, is beginning should tend to become less marked with long operator expe- to attain resolutions close to those of current aerial photogra- rience. The problem also depends on which limits are being phy. Aerial photographs are the only sources of remote sens- mapped. Unlike sandy coasts where shoreline position may ing data available to reconstruct mangrove dynamics over not be easily identified, the seaward limits of mangroves are several decades, well in advance of satellite imagery commonly quite easily identified. Their landward limits are (Morgan et al. 2010). For our study, the last four, and most more difficult to identify from aerial photographs, especially recent, series of aerial photographs (Table 1)are where mangroves grade into non-mangrove forests. In order orthophographs entirely assembled and processed by the to attenuate this problem, we conducted several ground-truth French Institut Géographique National (IGN). These series observations between February 2005 and March 2012 to are of particularly high precision, with a ground resolution attempt to match observed limits with those identified on of 50 cm, and even 20 cm for the 2011 series. Regarding the aerial photographs. However, for the period prior to 1997, oldest series, the photographs used are IGN images digitized we used variations in image tone, texture and contrast between at a high resolution of 1000 dpi. These photographs were then mangroves and other vegetation types, following, for instance, corrected and georeferenced using ERDAS Er-Mapper® soft- Benfield et al. (2005) and Jupiter et al. (2007). Moreover, the ware following a second order polynomial model based on landward fringes of mangroves in Mayotte were, in many ground control points on the 2008 IGN orthophotographs. An cases, bordered by roads or by natural features such as average of 25 control points, homogeneously distributed as backshore cliffs and beach ridges enabling good visual delim- much as possible in the vicinity of the analysed mangrove itation. Ideally, the delimitation process should be reiterated stands, were selected on each image. All images were several times by different operators in order to estimate as georeferenced in RGM04, the local reference system of Ma- objectively as possible the digitization error, which is, on yotte. Following these operations, all the images were incor- average, 5 pixels (i.e., 1.0 m for 2011, 5.0 m for 1950 and porated in a GIS using MapInfo® (Pitney Bowes Software), 2.5 m for the intermediate years). In this study, for practical from which the manual numerical delimitation of mangrove and logistical reasons, delimitation was conducted by a single limits was carried out. Stands of mangroves of less than 2 ha operator. The total uncertainty margin is given by the square (i.e., 0.02 km2) were not included in the analysis. root of the squared sum of the various sources of uncertainty, i.e., a value between 1.0 and 5.8 m. Converting this to man- Uncertainty Analysis grove surface area yielded an uncertainty margin that ranged from 2 % in 2011 to 8 % in 1950. There are several sources of uncertainty in delimiting man- groves and changes in area of mangrove stands over time. These include image resolution, georectification and digitiza- Results tion of the mangrove fringe. The error arising from pixel size varies from 0.2 to 1.0 m (Table 1). The uncertainty calculated Table 2 summarizes the surface area evolution of the total of by the software Er-Mapper in the course of the georeferencing 28 mangrove stands exceeding 2 ha and changes between of the various images (RMS error) varies from 0.6 to 2.8 m. In various aerial photograph time slices over the 60-year period addition to these technical errors of imaging and of study. The changes have been particularly marked since georeferencing is the uncertainty related to the determination 1969. The overall evolution of the total mangrove area of the of the limits of mangrove stands, which can be a tricky island between the two end dates of analysis is shown in Author's personal copy

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Ta b l e 2 Changes in surface area of mangroves in Mayotte from 1950 to 2011. The uncertainty margin ranges from 2 % in 2011 to 8 % in 1950

Mangroves Surface area (ha) Total change

1950 1969 1989 1997 2003 2008 2011 ha %

Bandraboua 5.64 5.55 5.80 5.88 5.88 6.03 6.10 0.46 8.15 Dzoumogné 106.63 106.75 106.95 106.96 106.95 106.81 106.81 0.18 0.17 Longoni 22.33 22.48 22.63 22.71 22.71 22.74 22.75 0.42 1.88 Port 3.93 3.97 4.04 4.00 4.00 0.00 0.00 −3.93 −100.00 Miangani 13.19 13.19 13.19 13.08 13.08 13.09 13.09 −0.10 −0.76 Kangani 2.23 2.58 3.87 3.94 4.08 4.12 4.12 1.89 84.75 Majicavo Koropa 11.16 11.84 11.97 11.96 12.02 12.29 12.29 1.13 10.13 Majicavo Lamir 12.00 12.52 12.54 12.54 12.54 12.54 12.54 0.54 4.50 Kawéni 53.75 53.75 53.25 50.62 47.77 46.24 46.22 −7.53 −14.01 Badamiers 0.00 0.05 2.45 8.67 24.44 33.84 35.78 35.78 Mgonbani-Baobab 17.23 17.27 16.79 11.63 10.58 10.73 10.74 −6.49 −37.67 Tzoundzou-Passamainti 53.96 nd 43.87 43.32 36.82 35.81 35.35 −18.61 −34.49 Ironi Bé 12.21 12.32 12.32 12.32 12.32 12.32 12.32 0.11 0.90 Dembéni 45.25 45.38 45.20 45.07 45.00 44.14 44.17 −1.08 −2.39 Hajangoua-Iloni 15.28 15.26 15.35 15.59 15.73 15.82 15.82 0.54 3.53 Hajangoua Sud 3.04 2.89 2.82 2.77 2.75 2.75 2.68 −0.36 −11.84 Bandrélé 38.25 38.04 39.12 37.15 37.07 36.61 36.49 −1.76 −4.60 Mounyambani4.894.885.064.814.674.654.51−0.38 −7.77 Dapani 27.61 24.03 21.09 18.58 17.56 15.98 14.18 −13.43 −48.64 Mronabéja 10.49 10.03 8.20 7.15 6.46 5.92 5.75 −4.74 −45.19 Kani Kéli 20.03 20.22 19.32 18.81 18.61 18.42 18.09 −1.94 −9.69 Mzouazia 6.30 6.27 6.02 5.83 5.48 5.06 4.86 −1.44 −22.86 Hagnoundrou nd 3.36 2.95 2.56 2.52 2.48 2.48 −0.88 −26.19 Bouéni Bay 201.80 199.13 195.10 192.29 191.26 187.36 186.08 −15.72 −7.79 15.02 14.99 12.83 11.92 11.31 10.59 10.11 −4.91 −32.69 Tsingoni 24.45 24.72 25.07 26.50 24.85 24.23 23.90 −0.55 −2.25 Zidakani 4.28 4.29 4.40 4.45 4.45 4.45 4.45 0.17 3.97 Soulou 9.63 9.99 10.50 11.09 11.20 11.18 11.10 1.47 15.26 Total 740.58 645.75 722.70 712.20 712.11 706.20 702.78 −37.80 −5.10

Fig. 2. The changes show contrasting patterns among the construction of the port occurred to the detriment of 4 ha of various sectors of coast of Mayotte. mangroves lost to infill. Among the seven stands of the north coast, five show an The mangroves of the east coast and Petite-Terre Island increase and two a decrease in area. The increase shown by the exhibit highly contrasting patterns. The Badamier stand on former is quite weak but regular over the period of analysis. Petite-Terre Island did not exist in 1950 but covered 35.78 ha The mangroves of Bandraboua increased, for instance, by in 2011. Other stands, such as those of Hajangoua-Iloni 8.15 % (i.e., 0.46 ha), those of Majicao Koropa and Longoni (+3.53 %) Majicavo Lamir (+4.50 %), show mild extension respectively by 10.13 % (1.13 ha) and 1.88 % (0.42 ha), towards the lagoon. The Ironi Bé stand exhibits stability whereas those of Dzoumogné, the second most important whereas the Hajangoua-sud and Mounyambani stands have stand in Mayotte with a cover of 106.81 ha in 2011, remained regressed moderately (respectively −11.84 % and −7.77 %) quite stable (+0.18 ha). The mangroves of Kangani show a over the study period. The mangroves of Kawéni (−14.01 %), much greater gain in area (84.75 %) since 1950, but it must be Tsoundzou-Passamaïnti (−34.49 %) and Mgombani-Baobab noted that this is a relatively small mangrove patch initially (−37.67 %), located in the vicinity of Mamoudzou (58,197 covering only 2.23 ha. The mangrove stands of Miangani and inhabitants in 2012), the island capital city (Fig. 1) have of Port have been impacted since 1989 by the construction of regressed strongly, especially since the 1980s, the inland the deepwater port of Longoni and its access routes. The sections of these forests being the most affected. Author's personal copy

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Port Miangani -3.93 -0.10 Bandraboua +0.46 Kangani +1.89 Majicavo Koropa +1.13 Dzoumogné + 0.18 Longoni +0.42 Soulou Majicavo Lamir +1.47 +0.54 Evolution of Badamiers Kawéni +35.78 mangrove surface area -7.53 Zidakani + 0.17 Mgonbani- Tzoundzou- Baobab Tsingoni Passamaïnti + 35.78 ha -6.49 -0.55 -18.61

Ironé Bé +0.11 - 18.61 ha Chiconi Dembéni -4.91 Hajangoua Iloni -1.08 +0.54

Hajangoua Sud Mangroves in 2011 -0.36

Coral reefs Hagnoundrou -0.88 Bouéni Bay -15.72 Bandrélé -1.76

Mzouazia -1.44 Kani Kéli Mounyambani -1.94 -0.38 Dapani Mronabéja -13.43 -4.74

0 5 10 km

Fig. 2 Overall evolution of mangroves in Mayotte between 1950 and 2011 Author's personal copy

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All the mangrove stands of the south coast of Mayotte have stands of Mgombani-Baobab and Tzoundzou-Passamainti regressed over the period 1950–2011. The diminution in around Mamoudzou over the last 20 years. The strong moun- mangrove area commenced in the 1950s and the regression tain relief of Mayotte has implied that much of the infrastruc- rates have been on the increase since. Among these stands, tural development of the island is concentrated in the narrow those of Dapani (−13.43 ha, i.e., −48.64 % loss) and coastal fringe, especially around Mamoudzou. A similar effect Mronabéja (−4.74 ha, i.e., −45.19 %) are particularly con- concerns the mangroves of Kawéni, situated close to the cerned. The mangroves of Mzouzia show a more moderate largest industrial estate of Mayotte and the surface area of regression (−22.86 %, i.e., −1.44 ha), whereas those of Kani which has diminished progressively as the estate has grown. Kéli have retreated the least (−1.94 ha, i.e., −9.69 %). In these Human pressures are also common in the vicinity of numerous last four mangrove stands, the destruction has essentially villages and have generally affected the inland mangrove concerned the seaward fringes. fringes. Such pressures are either due to the extension of these The six mangrove stands of the west coast show, like those settlements (Bandrélé, Dembéni) or to agriculture (Dapani, of the east coast, variable trends. Two stands exhibit regular Baie de Bouéni). A relatively common source of degradation increase: Soulou (+15.26 %, i.e., 1.47 ha) and Zidakani is that caused by paths cut across the mangroves in order to (+3.97 %, i.e., +0.17 ha). The other four show more or less facilitate access to the lagoon, especially for fishermen. Aerial important regression. The Tsingoni stand exhibits the least photographs and field observations show that these access loss (−2.25 %; −0.55 ha), but has evolved from a prograding paths act as weak points in the mangrove front where the system (+2.05 ha) between 1950 and 1997 to a regressive one substrate is destabilized by waves and currents, generating since 2003. The Chiconi and Hagnoundrou stands have lost subsequent erosion and mangrove retreat, as in the mangrove respectively 32.69 % and 26.19 % of their area. Finally, the stands of Mzouazia, Kani Kéli and Bouéni Bay. largest mangrove stand in Mayotte, that of Bouéni Bay Mangrove recovery potential is low on the more exposed (186.08 ha in 2011), exhibits a moderate loss of 15.72 ha south and west coasts of the island, where all the mangrove (−7.79 %), especially on the lagoon side. stands fronting the lagoon have retreated, compared to those on the north coast which show stability or increase in area (Fig. 2). This contrast suggests that mangrove resilience is conditioned Discussion by the morphodynamic dichotomy of the island, which is hinged, in turn, on the characteristics of the reef platform Aerial photographs can be a particularly useful source of facing the lagoon and on the wave regime (Jeanson et al. information on patterns of mangrove evolution, especially 2013). With an area of about 270 km2, the reef platform of when they provide an exceptionally long record as in the case Mayotte varies considerably in width and morphology (Fig. 1). of the island of Mayotte. The analysis of mangrove patterns on The north coast is exposed to a more moderate wave energy this small, but rapidly developing, island in the Indian Ocean regime, dominated by monsoon waves, than the south and brings out marked variability in mangrove area over the last west coasts, which are more exposed to higher-energy trade- 60 years. Eleven mangrove sites show an increase in area and wind waves. The west coast is also characterized by a largely 17 others a net loss in area. The sites of loss are essentially perforated barrier reef (Fig. 1), enabling more significant wave located on the west and south coasts of the island where the energy transmission to the coast. Jeanson et al. (2013)further entire lagoonal mangrove fringe is receding, but also in the showed from an analysis of wave height patterns across the vicinity of Mamoudzou and the port of Longoni, in contrast to reef platform that the mean wave attenuation values were the north coast, which shows relatively stable to slightly slightly more important on the north coast compared to the prograding mangrove stands. These different patterns reflect south coast due to the greater width of the Northeast barrier the impact of human pressures over the last 60 years and the reef (1.2 km) compared to the South barrier reef (1 km). Where recovery capacity of mangroves hinged on their the mangrove stands have been degraded, recovery is lower on morphodynamic setting. These pressures are varied and more the more wave-exposed shores of the west and south coasts. or less intensive, and, depending on the exposure of the coast This difference in exposure to waves is illustrated by the to waves, are leading to irreversible mangrove destruction. presence of single to multiple sand bars in the lower intertidal The major source of disturbance is heavy infrastructure, zone of mangrove stands on the south shores (Fig. 3c). Man- notably roads (Fig. 3a) related to the economic growth of grove cut-down and retreat on these shores (Fig. 3b) is accom- Mayotte. The creation and successive extensions of the com- panied by erosion of the muddy-sandy substrate by waves mercial deepwater port of Longoni in the 1990s and since which rework and concentrate the sand into well defined bars, 2000 resulted in total clearing of the initial mangroves in this whereas the mud is dispersed towards the lagoon, where the sector (Fig. 2) and of a small part of the mangroves of long-term effect of muddy sedimentation is becoming increas- Miangani for access roads. The construction of roads has also ingly detrimental to the health of the fringing coral reefs been responsible for the strong reduction of the mangrove (Thomassin et al. 2011). The sand bars progressively migrate Author's personal copy

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a b

c d

0 0.5 1 km

Breaches in degraded beachrock

Dzaoudzi

19th century inlet

Labattoir e shoreward as swash bars that are built up into beach ridges by beach ridge accretion leads to burial and asphyxia of mangrove waves behind the subsisting mangrove fringe. Continuous root systems, further leading to mangrove mortality. Author's personal copy

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ƒFig. 3 Photographs showing sources of human-induced and natural There are, at present, no direct concerted mangrove regen- changes in mangroves in Mayotte. a South circular road to Mamoudzou eration initiatives in Mayotte other than a few promising with massive roadside dike constructed through cleared mangroves. b replanting attempts undertaken in the last 2 years. However, Erosion of wave-exposed mangroves in Dapani. c Wave-formed sand bar migrating onshore over mangroves in Bandrélé. d Ground photograph of administrative and legal measures aimed at regulating activi- young expanding mangrove colony of Badamiers. e Aerial photograph of ties in mangrove areas have been considerably tightened over mangroves in Badamiers showing diked road that blocked the 19th the last 4 years. The notions of management and controlled century connection between mangroves and the lagoon, and breaches in development of the coastal zone are central themes in the degraded beachrock that contribute to mangrove regeneration by enabling saltwater intrusion establishment of a Management and Sustainable Development Plan for Mayotte established in 2009 and in the creation of the Mayotte Marine Park in 2010, both of which define the main orientations and the management policy of Mayotte and its The mangroves of Badamiers on the island of Petite-Terre coral reefs and lagoon for the decades to come. These orien- appear to have undergone a cycle of destruction and recovery tations qualify the mangrove areas of Mayotte as ‘strictly also conditioned by human pressures and geomorphic condi- protected natural zones’. Mangrove protection legislation tions (Fig. 3d, e). Reported on 19th century maps, they no now requires that destruction by future urbanization and in- longer existed in 1950, completely cut down in the mid-19th frastructure development is strictly compensated by mangrove centuryinordertolimittheproliferation of mosquitoes replanting over at least the same surface as that of mangrove (Archives Départementales de Mayotte 2005). Regeneration removal. was further prevented by the construction of a road and dike in 1848 that stopped saltwater from the lagoon attaining the Acknowledgments The authors acknowledge aid from ULCO, the marsh. The remarkable recovery of this mangrove stand French Ministry of Overseas Territories, IRD of Reunion and French (35.78 ha in 2011), despite the highly urbanized context, is Guiana, and local services in Mayotte involved in the management of the related to renewed seawater intrusion following progressive coastal and lagoon systems, especially the DEAL and the Conseil Général de Mayotte. Editor-in-Chief Marinus Otte and three reviewers are degradation of beachrock outcrops (Fig. 3e) that hitherto thanked for their salient suggestions for improvement of the manuscript. formed a natural barrier between the mangroves and the lagoon. References Conclusions Alongi DM (2008) Mangrove forests: resilience, protection from tsu- namis, and responses to global climate change. Estuar Coast Shelf Orthorectified aerial photographs spanning a period of Sci 76:1–13 60yearshavebeenusedtoidentifypatternsofmangrovearea Anthony EJ, Gratiot N (2012) Coastal engineering and large-scale man- change on the island of Mayotte in the Indian Ocean, giving a grove destruction in Guyana, South America: averting an environ- particularly long time series of mangrove evolution in an mental catastrophe in the making. Ecol Eng 47:268–273 Archives Départementales de Mayotte (2005) une histoire island context of strong development pressures. The results contrariée, 1843–1866. 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