Net community production in the marginal ice zone and its importance for the variability of the oceanic pCO2 in the Southern Ocean south of Australia

Abstract

Abstract In the marginal ice zone (MIZ) of the Southern Ocean south of Australia at 140°E, the net community production (NCP) above the remnant temperature-minimum (Tmin) layer was evaluated in the austral summer of 1994/95 on the basis of the deficit in normalized total inorganic carbon (NTCO2) by reference to the NTCO2 in the remnant Tmin layer (2182 μmol kg−1 at S=34). The NCP integrated from the preceding late winter ranged from 4 to 16 gC m−2 in December 1994 and from 6 to 30 gC m−2 in January 1995, showing a tendency to increase with time and distance from the ice edge. The biological TCO2 drawdown and temperature-rise after the retreat of sea ice were the dominant factors controlling the spatial and temporal variability of CO2 partial pressure in surface seawater (pCO2sw) in the seasonal ice zone including the MIZ. The water beneath the sea ice is considered to be significantly CO2-supersaturated in winter, but the effect of biological CO2 uptake in summer on average was larger than that of the temperature-rise after the retreat of the sea ice. Hence, the pCO2sw showed a tendency to decrease with time and distance from the ice edge in the MIZ. The community uptake ratio of ΔSi/ΔC/ΔN/ΔP estimated from the deficits in silicic acid, TCO2, nitrate+nitrite and phosphate above the Tmin layer was 39/54/8.7/1 in the MIZ. The community uptake ratios of ΔSi/ΔP (=39) and ΔSi/ΔN (=4.5) were even higher than the uptake ratios by the diatom-dominated phytoplankton assemblage in the field incubation experiments under iron-deficit conditions (ΔSi/ΔP=26, ΔSi/ΔN=1.9–2.1; Takeda, Nature 393 (1998) 774). These results suggest that phosphorus and nitrogen, as well as carbon, in biogenic particulate matter have undergone significant preferential remineralization compared with silicate through heterotrophic activities in the surface layer.