Distribution and drivers of phytoplankton biomass along the Saya de Malha Bank in the Western Indian Ocean

The Saya de Malha bank found in the Western Indian Ocean is known as a key biodiversity area with high productivity. In-situ data, however, is characterised by paucity, therefore the Indian Ocean 2022 mission of the Monaco Explorations provided an opportunity to assess the productivity and investigate the major drivers of the phytoplankton community across this bank. The main findings on phytoplankton indicated a well-mixed water column across the bank, with some spatial differences observed in total chlorophyll-a, and the relative abundance of the pico-, nano- and microplankton. Based on the total chlorophyll-a, higher pelagic productivity was found on the shelf in the north-eastern section of the bank, decreasing towards the south. Size distribution of phytoplankton biomass indicated that the communities of the bank were dominated by pico-phytoplankton (∼46 %), and that highest relative abundance of this group was encountered in the surface waters. Conversely, the micro-phytoplankton had low relative abundance in the surface water (∼13 % at <60 m), increasing towards deeper waters (∼36 % at >60 m). Comparison of the phytoplankton biomass with Acoustic Doppler Current Profiler (ADCP) data obtained during this expedition shows a strong alignment between maximum biomass and dominant current velocities at 30 m depth across the bank, and therefore can be used to inform the likely export patterns of biomass towards the rest of the Western Indian Ocean and the contribution of the Saya de Malha bank as a carbon sink and a carbon exporter to the rest of the pelagic trophic web in the region.