Coastal Seawater Pollutants in the Coral Reef Lagoon of a Small Tropical Island in Development: The Mayotte Example (N Mozambique Channel, SW Indian Ocean)

Abstract

Mayotte Island, a small volcanic island, is surrounded by the largest coral reef lagoon of the Indian Ocean (almost 1,500 km2), with all coral reefs types represented: ribbon barrier reefs, lagoonal and fringing reefs, and also coastal mangroves. Since 1975, Mayotte has shown an incredible development of its population (today about 180,000) and of its economy, with increases in consumer goods, fresh water use, construction of concrete houses and buildings, roads, truck and car parks, harbor and airport activity, gas and oil consumption, etc. At the same time, land vegetation coverage has decreased (there are still burnings of areas for agriculture and charcoal making) as land erosion has increased in some areas. As a consequence of this development of towns, villages, roads, harbor and airport traffic, which is mainly along the coastal areas, today an increase in the turbidity and the pollutants in the surface waters of the coast has been observed. Pollution was analyzed by monitoring the midlittoral oyster (Saccostrea cucullata): some trace metals, as Zn, Cd, Hg, and Cr; some polyaromatic hydrocarbons, such as naphthalene, pyrene, and fluoranthene; and some polychlorobiphenyls (PCB), even some that are particularly toxic, were found. Associated with this degradation of the coastal water quality and human impacts in general, in the decades since 1997, a decrease in the vitality of the fringing reef communities that surround the islands (predominant coral coverage ratio <5%) has been observed. Therefore, the human impacts are stronger than the natural stresses, such as the ENSO and/or coral bleaching events, and the crown-of-thorns starfish Acanthaster infestations. Muddy flats now overlay inner areas of the fringing reef flats. Linked with this, coastal fish populations have decreased.