Variations in abundance, diversity, photo-physiology and estimated productivity of micro-phytoplankton with depth at the Saya de Malha Bank, Mascarene Plateau

The variations in micro-phytoplankton abundance, diversity, photo-physiology, chlorophyll a (Chla) concentration and estimated productivity were assessed at depth ranges of 0-4, 5-10, and 11-29 m with 100, 28 and 11% of irradiance, respectively, in Saya de Malha waters. The total micro-phytoplankton abundance (TMPA) differed significantly (P < 0.001) with depth ranges, and between day and night samples. Out of the 34 genera identified, 27 showed a decrease in abundance of over 40% with depth. Chaetoceros, Coscinodiscus, Navicula, Nitzschia and Ceratium were most dominant. The Shannon-Wiener (H’) diversity index did not differ among depth ranges and between samples collected during day and night, but diatoms were more diverse than dinoflagellates. The effective quantum yield (ΦPSII) and the light-use efficiency factor (α) tended to decrease, while the maximum relative electron transport rate (rETRmax), the photo-inhibitory factor (β) and the maximum non-photochemical quenching (NPQmax) varied insignificantly from morning to afternoon sampling points at all depth ranges studied. The estimated productivity, rETRmax and Chla concentration decreased with depth ranges. The higher diversity of diatoms, better photosynthetic performance in the morning hours and higher near-surface estimated productivity provide new insights into micro-phytoplankton dynamics and productivity in Saya de Malha waters.