The impact of the South-East Madagascar Bloom on the oceanic CO<sub>2</sub> sink

Abstract

Abstract. We described new sea surface CO2 observations in the south-western 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 per month 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 per month compared to a CO2 source of +0.12 (±0.10) TgC per month 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 a relatively deep mixed-layer depth during the preceding winter (July–September 2019) that would supply macro- and/or micro-nutrients such 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 in 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 the N2 fixation process and iron supplies) for a better understanding of the origin and termination of this still “mysterious” sporadic bloom and its impact on ocean carbon uptake in the future.

The Agulhas Current Transports Signals of Local and Remote Indian Ocean Nitrogen Cycling

“…Our conclusion that N 2 fixation occurs locally in the greater Agulhas region is consistent with previous observations. First, diazotrophs have been observed during the late‐summer bloom that occurs irregularly to the southeast of Madagascar (Poulton et al., 2009), as well as within and south of the Mozambique Channel (Huggett & Kyewalyanga, 2017; Karlusich et al., 2021), with surface N 2 fixation rates of 2–18 nM.d −1 measured southeast of Madagascar during the most recent (2020) late‐summer bloom (Metzl et al., 2022). Second, one third of the thermocline nitrate at the north‐eastern edge of the greater Agulhas region (∼23°S, 70°E; Figure 1) has previously been estimated to be newly fixed based on subsurface nutrient N:P ratios (Harms et al., 2019).…”

“…Therefore, N 2 fixation must be significant elsewhere in the Indian Ocean (Deutsch et al, 2007;Grand, Measures, Hatta, Morton, et al, 2015;Gruber & Sarmiento, 1997;Wang et al, 2019), yet there are almost no observations of this process, particularly from the southern basin (cf. Harms et al, 2019;Karlusich et al, 2021;Metzl et al, 2022;Poulton et al, 2009).…”

Air‐Sea Fluxes of CO2 in the Indian Ocean Between 1985 and 2018: A Synthesis Based on Observation‐Based Surface CO2, Hindcast and Atmospheric Inversion Models

“…However, this would not dramatically impact the mean CO 2 fluxes assuming that over 1985-2018 ocean pCO 2 increase is close to the atmospheric growth rate as observed in some parts of the Indian Ocean (e.g. Metzl, 2009;Lauvset et al, 2015;Metzl et al, 2022;Lo Monaco et al, 2021).…”

“…Despite the importance of the Indian Ocean in the global carbon cycle, this region is poorly studied with reference to the biogeochemical cycling of carbon compared to the other two major basins. The seasonal cycle of pCO 2 and carbon fluxes was studied only in the Arabian Sea (Chakraborty et al, 2021;De Verneil et al, 2022;George et al, 1994;Goyet et al, 1998;Sarma et al, 1998Sarma et al, , 2003Sarma et al, , 2013, the BoB (Chakraborty et al, 2021;Sarma et al, 2012Sarma et al, , 2015Sarma et al, , 2020Sarma, Krishna, et al, 2021) and the south-western Indian ocean (Metzl et al, 1998) whereas the long-term variability was only recently studied in the southwestern Indian Ocean region (Metzl et al, 2022) as the other regions in the Indian Ocean was either sampled once or twice during last few decades (Sarma et al, 2013;Takahashi et al, 2009). The studies carried out in the aegis of the Joint Global Flux Study (JGOFS) and the Bay of Bengal Process Studies (BoBPS) suggested that the seasonal amplitude of pCO 2 goes beyond 200 μatm in the Arabian Sea (George et al, 1994;Goyet et al, 1998;Sarma et al, 1998Sarma et al, , 2003 and BoB (Kumar et al, 1996;Sarma et al, 2012Sarma et al, , 2015Sarma et al, , 2019.…”

“…The subducted water masses are advected to the northern Indian Ocean (Miyama et al, 2003;Schott et al, 2002), carrying nutrients and anthropogenic CO 2 (Sabine et al, 1999). A perennial sink of atmospheric CO 2 was reported in the SIO (Metzl, 2009;Metzl et al, 1991Metzl et al, , 1995Metzl et al, , 1998Metzl et al, , 2022Poisson et al, 1993).…”