A new definition of the South-East Madagascar Bloom and analysis of its variability

Type Article
Date 2019-03
Language English
Author(s) Dilmahamod Ahmad Fehmi1, 2, 3, 4, Penven Pierrick2, Aguiar gonzalez B.3, 5, Reason C. J. C.1, Hermes J. C.1, 3, 6
Affiliation(s) 1 : Department of Oceanography; University of Cape Town; Cape Town, South Africa
2 : Univ. Brest, CNRS, IRD, Ifremer; Laboratoire d'Oceanographie Physique et Spatial (LOPS), IUEM; Brest, France
3 : South African Environmental Observation Network; Egagasini Node; Private Bag X2 Roggebaai ,South Africa
4 : GEOMAR Helmholtz Centre for Ocean Research Kiel; Kiel, Germany
5 : School of Marine Science and Policy College of Earth, Ocean and Environment; University of Delaware; Newark Delaware ,USA
6 : Nelson Mandela University; Port Elizabeth ,South Africa
Source Journal Of Geophysical Research-oceans (2169-9275) (American Geophysical Union (AGU)), 2019-03 , Vol. 124 , N. 3 , P. 1717-1735
DOI 10.1029/2018JC014582
WOS© Times Cited 13
Keyword(s) South-East Madagascar Bloom, South-East Madagascar Current, La Nina, cyanobacterial bloom

The South‐East Madagascar Bloom occurs in an oligotrophic region of the South‐West 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 19‐years of ocean colour dataset 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° S‐32° S; 50° E‐∼70°E) both zonally and meridionally. A co‐location 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 Niña events and reduced upwelling intensity south of Madagascar. A stronger South‐East Madagascar Current during La Niña 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 Niña, 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 south‐east 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.

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