Physical and biological effects on the carbonate system during summer in the Northern Argentine Continental Shelf (Southwestern Atlantic)

The Argentine shelf and its shelf-break (Southwestern Atlantic Ocean) are known for their high biological productivity, and as an important CO2 sink region. However, many aspects of the carbonate system dynamics in the area, especially those related to the biological activity, deserve further study. Here we investigated the mechanisms affecting the carbonate system distributions, using in situ physical, chemical and biological observations collected along a section (COSTAL-AR) on the Northern Argentine Continental Shelf during two summer cruises in 2019. Our main goal was to evaluate the role of the microbial communities on the modulation of the carbonate system in the area. For that, we characterized (i) the distribution of the thermohaline properties, chlorophyll a, dissolved oxygen, carbonate system (pH, total alkalinity, dissolved inorganic carbon and high resolution underway CO2 fugacity, fCO2), dissolved inorganic nutrients, and (ii) the microbial communities (bacterioplankton, phytoplankton, and protozooplankton). Our results show that the COSTAL-AR section was likely an important CO2 sink and presented high seawater fCO2 spatial variability in both middle (272–430 μatm) and early (211–365 μatm) summer conditions. Phytoplankton played a key role in modulating the CO2 uptake and carbonate system spatial variability during summer, especially in the middle and outer shelf. The main contribution to CO2 fixation was given by small cells, since the microbial community was dominated by autotrophic picoplankton (<2 μm; e.g. Synechococcus sp. and coccal picophytoeukaryotes). Moreover, the influence of the Shelf-break front in ruling both the seawater fCO2 distribution and biological processes was evident. These findings provide new insights on the connection between the biology and the carbonate system in this sparsely sampled area of the Southwestern Atlantic Ocean.