Abstract
We described new sea surface CO2 observations in the southwestern Indian Ocean obtained in January 2020 when a strong bloom event occurred south-east of Madagascar and extended eastward in the oligotrophic Indian Ocean subtropical domain. Compared to previous years (1991–2019) we observed very low fCO2 and dissolved inorganic carbon concentrations (CT) in austral summer 2020, indicative of a biologically driven process. In the bloom the anomaly of fCO2 and CT reached respectively −33 µatm and −42 µmol.kg-1 whereas no change is observed for alkalinity (AT). In January 2020 we estimated a local maximum of air-sea CO2 flux at 27° S of −6.9 mmol.m-2.d-1 (ocean sink) and −4.3 mmol.m-2.d-1 when averaging the flux in the band 26–30° S. In the domain 25–30° S/50–60° E we estimated that the bloom led to a regional carbon uptake of about −1 TgC.month-1 in January 2020 whereas this region was previously recognized as an ocean CO2 source or near equilibrium during this season. Using a neural network approach that reconstructs the monthly fCO2 fields we estimated that when the bloom was at peak in December 2019 the CO2 sink reached −3.1 (±1.0) mmol.m-2.d-1 in the band 25–30° S, i.e. the model captured the impact of the bloom. Integrated in the domain restricted to 25–30° S/50–60° E the region was a CO2 sink in December 2019 of −0.8 TgC.month-1 compared to a CO2 source of +0.12 (± 0.10) TgC.month-1 in December when averaged over the period 1996–2018. Consequently in 2019 this region was a stronger CO2 annual sink of −8.8 TgC.yr-1 compared to −7.0 (±0.5) TgC.yr-1 averaged over 1996–2018. In austral summer 2019/2020, the bloom was likely controlled by relatively deep mixed-layer depth during preceding winter (July–September 2019) that would supply macro and/or micro-nutrients as iron to the surface layer to promote the bloom that started in November 2019 in two large rings in the Madagascar Basin. Based on measurements in January 2020, we observed relatively high N2 fixation rates (up to 18 nmol N.L-1.d-1) suggesting that diazotrophs could play a role on the bloom in the nutrient depleted waters. The bloom event in austral summer 2020, along with the new carbonate system observations, represents a benchmark case for complex biogeochemical model sensitivity studies (including N2-fixation process and iron supplies) for a better understanding on the origin and termination of this still <q>mysterious</q> sporadic bloom and its impact on ocean carbon uptake in the future.
Highlights
In the south-western subtropical Indian Ocean a phytoplankton bloom, called the South-EastMadagascar Bloom (SEMB) occurs sporadically during austral summer (December-March, Figure 1)
4.1 Sea surface fugacity of CO 2 (fCO2), CT and AT distributions in the South-EastMadagascar Bloom (SEMB) in January 2020 In January 2020, the SEMB occupied a large region in the Southern section of the Mozambique
The new observations in the South-Western Indian Ocean presented here showed that the fCO 2 and CT
Summary
In the south-western subtropical Indian Ocean a phytoplankton bloom, called the South-EastMadagascar Bloom (SEMB) occurs sporadically during austral summer (December-March, Figure 1). Following the first study by Longhurst (2001), the frequency, extension, levels of Chl-a concentration and processes that would control the SEMB and its variability have been investigated in several studies (Srokosz et al, 2004; Uz, 2007; Wilson and Qiu 2008; Poulton et al 2009; Raj et al 2010; Huhn et al2012; Srokosz and Quartly 2013). Most of these studies were based on Chl-a derived from remote sensing and altimetry. Modelling studies suggested an eastward propagation of the SEMB through advection or eddy transport originating from the south-east coast of Madagascar (Lévy et al 2007; Srokosz et al 2015; Dilmahamod, et al 2020) but a precise explanation of the internal (e.g. local upwelling, Ekman pumping, meso-scale dynamics) or external processes (e.g. iron from rivers, coastal zones or sediments) at the origin of this “mysterious” bloom is still missing
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