The Saya de Malha Bank (SMB) is one of the largest and least studied marine banks on the Mascarene Plateau. This study aimed to examine the diversity and distribution of the main benthic habitats in the shallow waters of the SMB (23 to 50 m). The survey was carried out in May 2018 during the EAF-Nansen Indian Ocean Research Expedition using a Remotely Operated Vehicle (ROV) deployed at 15 stations. Four main benthic habitats were investigated and their relative abundance determined during the survey. The 143,110 m2 surveyed area revealed proportional benthic habitat cover of 43.6 ± 22.4, 24.5 ± 21.9, 21.2 ± 27.8, and 10.5 ± 12.6 % for seaweed, abiotic substrate, seagrasses and corals, respectively. The seaweed habitat (43.6 %) was mainly composed of Halimeda spp. It represented up to 77 % of the habitats observed at SS34 (4553 m2). Even though seaweeds are considered seasonal, their dominance at all stations creates an important habitat structure for many organisms. The seagrass habitat (21.2 %) was dominated by Thalassodendron ciliatum. This habitat covered up to 93 % of the area investigated at SS38 (5950 m2) and was found mainly on the eastern side of the bank. The live hard coral habitat (10.5 %) was the highest at SS36-2 (35% of 9819 m2) and was more homogenously spread within the shallow areas. The unstable and the stable bare bottom substrate habitat (24.7 %) characterized as abiotic habitat was mainly composed of bedrock, sand, and rubble. It dominated at SS42 where it constituted 72.5 % of the 5114 m2 investigated and was recorded at all stations. Further research is warranted to better understand the diversity and the distribution of benthic habitats within the shallow waters of the SMB, along with collection of targeted benthic organisms for identification at higher taxonomic levels, to better formulate conservation and management measures and strategies.

The Saya de Malha Bank is one of the major banks of the Mascarene Plateau in the South West Indian Ocean. It is known for its unique ecosystem, remoteness (nearest island is about 300 km away) and complex oceanographic conditions. This study presents the results of a survey conducted in May 2018 on-board the R/V Dr Fridtjof Nansen, and aims to provide a descriptive overview of the interaction of the large-scale South Equatorial Current (SEC) over the shallow Saya de Malha Bank. The analysis of the current pattern revealed a two-layered structure of the current over the shallow topography of the bank compared to the vertically rigid-structure of the SEC throughflow in its deeper region. This two-layered flow consists of a surface layer and a sub-thermocline layer. The top layer flow, carrying the lower salinity mass (Tropical Surface Water) is driven by the Ekman dynamics observed in the southern hemisphere whereas the sub-thermocline current layer is most likely governed by the tidal and internal wave dynamics generated by the topographic relief of the bank.

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