The role of phytoplankton composition and microbial community metabolism in sea–air 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 subsurface waters, with ΔpCO₂ in the Weddell Sea, during four austral summers (2002–2005). 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 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, 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 community 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 primary production until the time of sampling for all stations when estimated from nutrients depletion between surface and subsurface 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.