Plankton communities in the five Iles Eparses (Western Indian Ocean) considered to be pristine ecosystems

Coral reef environments are generally recognized as being the most threatened of fragile marine ecosystems. They are highly susceptible to stress and disturbance, especially to anthropogenic pressure. However, it is extremely difficult to distinguish the effects of climate change from other forcing factors, mainly because it is difficult to study ecosystems that are isolated from human pressure. The five Iles Eparses (Scattered Islands) are located in the Western Indian Ocean (WIO) and form the 5th district of the French Southern and Antarctic Lands (TAAF). They are considered to be pristine ecosystems as they are occupied only by a small group of military personnel. This study described the plankton assemblages for the first time, by determining the abundances of microbial (virus, bacteria, heterotrophic protists and phytoplankton) and metazooplankton communities in various lagoon and ocean sites around each island. The Europa lagoon has extensive, productive mangrove forests. Their particular structure and functioning, with the highest nutrient concentrations (nitrogen forms, dissolved organic carbon), explain bacterial production and growth rates that are higher than those reported for the other islands. The marine ecosystem of Tromelin, which lies outside the Mozambique Channel, had the lowest biological productivity, with the lowest nutrient concentrations and bacterial growth rates. Comparison with a sampling station in Mayotte lagoon, considered to be the reference for an anthropized island, showed that the patterns of microbial assemblages in the Iles Eparses were different from those found in Mayotte lagoon.