Decadal-scale Dynamics and Morphological Evolution of Mangroves and Beaches in a Reef-lagoon Complex, Mayotte Island

Abstract

Jeansson, M.; Dolique, F.; Anthony, E.J., and Aubry, A., 2019. Decadal-scale dynamics and morphological evolution of mangroves and beaches in a reef-lagoon complex, Mayotte Island. In: Castelle, B. and Chaumillon, E. (eds.), Coastal Evolution under Climate Change along the Tropical Overseas and Temperate Metropolitan France. Journal of Coastal Research, Special Issue No. 88, pp. 195–208. Coconut Creek (Florida), ISSN 0749-0208.Mayotte Island is characterized by a vast coral reef-lagoon complex comprising significant mangrove development and numerous pocket beaches nested between volcanic headlands. Since 2005, field experiments involving topographic surveys, observations and hydrodynamic measurements have been coupled with the analysis of aerial photographs (1950-2016) in order to improve understanding of the morphodynamic interactions between mangroves, beaches and the coral reefs. The results, integrated in a coastal observatory of Mayotte Island, highlight a remarkably variable mangrove system subject to advance or stability in the north and east of the island but exhibiting a clearly regressive pattern along the southern and western shores. This variability largely reflects the impact of humans on mangrove stability. The hydrodynamic data acquired during the field experiments clearly bring out the spatial and temporal variations in wave patterns involved in these differences. These data also shed light on the short-term morphodynamics of small pocket beaches associated with these mangroves. Patterns of beach morphological change driven by residual wave energy following reef attenuation are strongly affected by the importance of beach embayment. These patterns affect, in turn, mangrove resilience, which is weak on the more exposed south and west coasts of the island, where all the mangrove stands fronting the lagoon have retreated, compared to the more stable stands on the north coast. These results highlight the complex time and space-varying morphodynamic interactions involved in a reef shoreline environment, and how these can also reflect the consequences of humans on mangrove stability. The results should, within the framework of the Mayotte coastal observatory, contribute to the management and conservation of the coast.