The role of microbial community composition and metabolism in air-sea ΔpCO2 variation in the Weddell Sea

The Weddell Sea is known to be a CO2 sink due to active biological and physical pumps. Here we study the relationships of phytoplankton biomass and composition and microbial community metabolism, estimated from simulated in situ incubations and from nutrient’s difference between surface and deep waters, with ∆pCO2 (∆pCO2 = pCO2 surface water - pCO2 atmosphere) in the Weddell Sea, during four austral summers (2002 to 2005). The ΔpCO2 was significantly negative throughout the Weddell Sea in 2002 (- 17.2 ± 28.1 µatm), 2003 (- 64.1 ± 31.3 µatm), 2004 (- 54.9 ± 61.8 µatm) and 2005 (- 63.8 ± 60 µatm), indicating that the Weddell Sea potentially acted as an atmospheric CO2 sink during those summers. The ΔpCO2 was significantly lower in the south than in the center or north of the Weddell Sea. This was consistent with the significantly higher Chlorophyll-a concentrations (Chl-a) observed in the south (2.3 ± 1.9 µg l-1) than in the center (1.3 ± 1.2 µg l-1) or north (1.4 ± 1.7 µg l-1). In contrast, waters were mainly undersaturated in O2, possibly due to the upwelling of oxygen poor Warm Deep Water (WDW). The negative relationship between the ΔpCO2 and the %O2 saturation suggested that planktonic metabolic activities played a role in these gases dynamics, along with the upwelling of WDW. However, these relationships could not be observed from the results of the incubation experiments, probably because of different temporal scales between gas exchanges in incubation experiments and in situ CO2 and O2 dynamics. The dynamics of CO2 and O2 were solely related to the net primary production (NCP) and to the gross primary production (GPP) when only stations with Chl-a > 1 µg l-1 were considered. A significant relationship was, however, found between ΔpCO2 and the annual primary production for all stations when estimated from nutrients depletion between surface and deep waters. Finally, the distribution of CO2 and O2 were related to the biomass of diatoms and, contrarily to other seas, to the biomass of phytoflagellates. Hence, general phytoplankton species composition did not seem to affect CO2 exchanges between the ocean and the atmosphere in the Weddell Sea. However, when considered among separate assemblages, the contribution of diatoms and phytoflagellates to CO2 dynamics in the Weddell Sea was variable. In addition, changes in specific phytoplankton composition due to global change may have an influence on organic carbon sequestration to deeper waters