The Madagascar Bloom: A serendipitous study

Abstract

[1] The late austral summer (February-April) phytoplankton bloom that occurs east of Madagascar exhibits significant interannual variability and at its largest extent covers~1% of the world's ocean surface area. The bloom raises many intriguing questions about how it begins, is sustained, propagates to the east, exports carbon, and ends. It has been observed and studied using satellite ocean color observations, but the lack of in situ data makes it difficult to address these questions. Here we describe observations that were made serendipitously on a cruise in February 2005. These show clearly for the first time the simultaneous existence of a deep chlorophyll maximum at~70-110 m depths (seen in SeaSoar fluorimeter data) and a surface chlorophyll signature [seen in Sea-viewing Wide Field-of-view Sensor (SeaWiFS) satellite ocean color data]. The observations also show the modulation of the biological signature at the surface by the eddy field but not of the deep chlorophyll maximum. Trichodesmium dominates the bloom nearer to Madagascar, while the diatom Rhizosolenia clevei (and its symbiont Richelia intracellularis) dominates further from the island. The surface bloom seen in the SeaWiFS data is confined to the shallow (~30 m) mixed layer. It is hypothesized that the interannual variability in bloom intensity may be due to variations in coastal upwelling and thus the supply of iron, which is a micronutrient that can limit diazotroph growth.

Could the Madagascar bloom be fertilized by Madagascan iron?

“…Longhurst , , Figure 2]. It is recognized that the mesoscale eddy field plays a key role in the development and propagation of the bloom [ Longhurst , ; Srokosz et al ., ; Srokosz and Quartly , ]; therefore, it is important that the model is able to reproduce a realistic eddy field to the east of Madagascar. Marzocchi et al .…”

“…As noted by Srokosz and Quartly [, and references therein], other factors that affect the bloom are the mixed layer temperatures and depths needed to provide optimum conditions for the growth of the nitrogen fixing phytoplankton that form the bloom. Ward et al .…”

“…Ward et al . [], in discussing diazotroph biogeography, note the preference of some species for warm temperatures and stratified, high‐light environments, as found in this area in late austral summer [ Srokosz and Quartly , ]. In addition, the South Indian Ocean Counter‐Current (SICC) may play a role in the advection of iron, though its exact nature is disputed (see below) [ Palastanga et al ., ; Siedler et al ., ; Menezes et al ., ].…”

“…In the same region to the east of Madagascar, there is also an annual weak austral spring bloom [see, for example, Uz , , Figure 2], which is followed by the development of a deep chlorophyll maximum [ Srokosz and Quartly , ]. This austral spring bloom and the subsequent development of a deep chlorophyll maximum are reproduced in biogeochemical models without an iron component, whereas the late austral summer bloom studied here is not (see discussion by Srokosz and Quartly [] and references therein).…”

Mesoscale eddy variability in the southern extension of the East Madagascar Current: Seasonal cycle, energy conversion terms, and eddy mean properties

“…For the marine ecosystems, the dynamics of the eddies in the southwest Indian Ocean have been described as the main mechanisms responsible for the variability of biological properties, such as nutrients and chlorophyll‐a (indicative of phytoplankton community) [ Quartly and Srokosz , ; Tew‐Kai and Marsac , ; Omta et al ., ; Roshin et al ., ; Srokosz and Quartly , ]. Through surface divergence, eddies can upwell deep nutrient‐rich waters important for the biological primary productivity [ Robinson , ; McGillicuddy et al ., ; Chelton et al ., ; Lathuilière et al ., ].…”