Spatial variation in subtidal plant communities around the Socotra Archipelago and their biogeographic affinities within the Indian Ocean

Abstract

The subtidal plant communities of the Socotra Archipelago were studied by means of quadrat sampling. Ordination and statistical analyses revealed 6 distinct clusters corresponding to the geographic location and the physico-chemical factors. The north coast of Socotra Island supports algae commonly found in the Indian Ocean, with an intermediate species richness and alpha diver- sity for the archipelago. This entity includes 2 species-poor subentities: the seagrass beds and the coral-dominated communities. The transition zone is an overlapping area between Socotra's north and south coast where the greatest similarity in community structure with the upwelling flora of the south coast is found, owing to similar environmental conditions. This zone is subject to intense cur- rent patterns favouring a pronounced diversity of red algae. The south coast features the highest number of recorded species and a lower affinity with the (sub-)tropical Indian Ocean flora, and is marked by disjunctly distributed species. The plant communities of the outer islands comprise a mixture of the other entities due to the drastically changing seasonal environmental conditions in a limited coastal area. The intermediate character of this entity, i.e. ongoing competition amongst biota without reaching a climax in the vegetation succession, is reflected in the vegetation analyses and the biogeographic comparison.