Summer trends and drivers of sea surface fCO 2 and pH changes observed in the southern Indian Ocean over the last two decades (1998–2019)

• Published in: Biogeosciences
• Main Authors: C. Leseurre, C. Lo Monaco, G. Reverdin, N. Metzl, J. Fin, C. Mignon, L. Benito
• Format: Article in Journal/Newspaper
• Language: English
• Published: Copernicus Publications 2022
• Subjects: Ecology; QH540-549.5; Life; QH501-531; Geology; QE1-996.5; Indian; Kerguelen; Kerguelen Island; Kerguelen Islands
• Online Access: https://doi.org/10.5194/bg-19-2599-2022; https://doaj.org/article/e435d3c27c0043de886de1c5dcb9e3ec

The decadal changes in the fugacity of CO 2 ( fCO 2 ) and pH in surface waters are investigated in the southern Indian Ocean (45–57 ∘ S) using repeated summer observations, including measurements of fCO 2 , total alkalinity ( A T ) and total carbon ( C T ) collected over the period 1998–2019 in the frame of the French monitoring programme OISO ( Océan Indien Service d'Observation ). We used three datasets (underway fCO 2 , underway A T – C T and station A T – C T ) to evaluate the trends of fCO 2 and pH and their drivers, including the accumulation of anthropogenic CO 2 ( C ant ). The study region is separated into six domains based on the frontal system and biogeochemical characteristics: (i) high-nutrient low-chlorophyll (HNLC) waters in the polar front zone (PFZ) and (ii) north part and (iii) south part of HNLC waters south of the polar front (PF), as well as the highly productive zones in fertilised waters near (iv) Crozet Island and (v) north and (vi) south of Kerguelen Island. Almost everywhere, we obtained similar trends in surface fCO 2 and pH using the fCO 2 or A T – C T datasets. Over the period 1998–2019, we observed an increase in surface fCO 2 and a decrease in pH ranging from + 1.0 to + 4.0 µ atm yr −1 and from − 0.0015 to − 0.0043 yr −1 , respectively. South of the PF, the fCO 2 trend is close to the atmospheric CO 2 rise ( + 2.0 µ atm yr −1 ), and the decrease in pH is in the range of the mean trend for the global ocean (around − 0.0020 yr −1 ); these trends are driven by the warming of surface waters (up to + 0.04 ∘ C yr −1 ) and the increase in C T mainly due to the accumulation of C ant (around + 0.6 µ mol kg −1 yr −1 ). In the PFZ, our data show slower fCO 2 and pH trends (around + 1.3 µ atm yr −1 and − 0.0013 yr −1 , respectively) associated with an increase in A T (around + 0.4 µ mol kg −1 yr −1 ) that limited the impact of a more rapid accumulation of C ant north of the PF (up to + 1.1 µ mol kg −1 yr −1 ). In the fertilised waters near Crozet and Kerguelen islands, fCO 2 increased and pH ...