Dilmahamod A. F. auteur aut IRD Penven Pierrick auteur aut IRD Aguiar-Gonzalez B. auteur aut IRD Reason C. J. C. auteur aut IRD Hermes J. C. auteur aut IRD text journalArticle eng text/pdf born digital access The South-East Madagascar Bloom occurs in an oligotrophic region of the southwest Indian Ocean. Phase locked to austral summer, this sporadic feature exhibits substantial temporal and spatial variability. Several studies, with different hypotheses, have focused on the initiation mechanism triggering the bloom, but none has been as yet clearly substantiated. With 19years of ocean color data set available as well as in situ measurements (Argo profiles), the time is ripe to review this feature. The bloom is characterized in a novel manner, and a new bloom index is suggested, yielding 11 bloom years, including 3 major bloom years (1999, 2006, and 2008). Spatially, the bloom varies from a mean structure (22-32 degrees S; 50-70 degrees E) both zonally and meridionally. A colocation analysis of Argo profiles and chlorophyll-a data revealed a bloom occurrence in a shallow-stratified layer, with low-salinity water in the surface layers. Additionally, a quantitative assessment of the previous hypotheses is performed and bloom occurrence is found to coincide with La Nina events and reduced upwelling intensity south of Madagascar. A stronger South-East Madagascar Current during La Nina may support a detachment of the current from the coasts, dampening the upwelling south of Madagascar, and feeding low-salinity waters into the Madagascar Basin, hence increasing stratification. Along with abundance of light, these provide the right conditions for a nitrogen-fixing cyanobacterial phytoplankton bloom onset. Plain Language Summary The South-East Madagascar Bloom is one of the largest bloom in the world. It can play a major role in the fishing industry, as well as capturing carbon dioxide from the atmosphere. Hence, it needs to be better understood. In previous studies, several hypotheses have been put forward to explain the bloom but none have as yet been clearly substantiated. This study shows that the bloom occurs in a surface layer of low salinity, which tends to favor a specific type of phytoplankton, namely, cyanobacteria. The climate phenomenon, La Nina, also seems to co-occur with the bloom, 10 out of 11 events. The input of nutrient-rich waters to the surface south of Madagascar is weakened during the bloom, and this goes against what was previously thought. The present study gives a possible new reasoning as to why the bloom occurs. The current southeast of Madagascar detaches from the coast and brings low-salinity and nutrient-rich waters into the bloom region where enhanced photosynthesis occurs, hence causing the bloom. specialized South-East Madagascar Bloom South-East Madagascar Current La Nina cyanobacterial bloom OCEAN INDIEN MADAGASCAR MADAGASCAR SUD EST COURANT 032 036 124 3 1717-1735 2019 2169-9275 https://www.documentation.ird.fr/hor/fdi:010075624 10.1029/2018jc014582 2169-9275 [F B010075624] https://www.documentation.ird.fr/hor/fdi:010075624 https://www.documentation.ird.fr/intranet/publi/2019/05/010075624.pdf Accès réservé (Intranet de l'IRD) IRD - Base Horizon / Pleins textes 2019-06-04 2025-02-24 fdi:010075624 fre