Summary
The distribution of carbon dioxide partial pressure (pCO2) in surface seawaters South of Tasmania has been compared for two very different years; summer 1996/1997 and summer 2002/3. Field data were obtained in the summer of 2002/3 during cruises onboard the S.S. Astrolabe as part of an Australian-French biogeochemical observing system program, and on one cruise onboard the R.V. Marion-Dufresne (OISO program, France) in the frame of the long-term observational ORE/CARAUS project (INSU/IPSL). The 1996/7 data were obtained on the R.V. Marion-Dufresne. The surface waters of the sub-Antarctic region showed intense CO2 uptake in both February 1997 and 2003, driven by strong biological activity over the summer period (net CO2 fluxes of -13.6 mmol.m-2.d-1 and –12.3 mmol.m-2.d-1 respectively). The summer CO2 fluxes exhibit a pattern already noticed south of Tasmania. In the seasonal sea-ice zone, south of about 62°S, the CO2 exchange is highly variable and appears related to the extent and evolution of the sea ice. The largest interannual change in the CO2 system between February 1997 and 2003 occurred in the Permanent Open Ocean Zone (POOZ), sandwiched between the seaice zone and the sub-Antarctic waters to the North. The estimated fluxes in the POOZ, were - 0.3 mmol.m-2.d-1 for February 1997, and –20.6 mmol.m-2.d-1 for February 2003. The large CO2 uptake in February 2003 was associated with an increase of phytoplankton biomass and was unexpected for the high-nutrient low-chlorophyll (HNLC) waters of the POOZ. Environmental variables examined to investigate the mechanisms responsible for such a large interannual change in the air-sea flux suggest the input of iron from dust storms generated over the Australian continent in 2002/3 may have contributed to enhanced biomass and CO2 uptake.