Interannual variation in air‐sea CO2 flux in the Ross Sea, Antarctica: A model analysis

Abstract

Phytoplankton dynamics, primary production, and air‐sea CO2 exchange in the Ross Sea over a 6‐year period (1997–2003) were investigated using a combination of satellite remote sensing and numerical ecosystem modeling. Sea ice cover in the Ross Sea was highly variable during this time, with 3 years exhibiting a normal springtime retreat of the annual sea ice and 3 years exhibiting very heavy ice cover, particularly 2002–2003. Annual net primary production (NPP) during normal ice years ranged from 141 to 180 g C m−2 yr−1, dropping to 27.3–96.2 g C m−2 yr−1 during heavy ice years. The prymnesiophyte Phaeocystis antarctica was the phytoplankton taxa responsible for the bulk of the annual NPP, although diatoms increased in relative importance during heavy ice years. Reductions in surface water pCO2 due to phytoplankton CO2 fixation during spring and summer and the return of the annual sea ice in the autumn and winter (thus restricting gas exchange) made the Ross Sea a net sink for atmospheric CO2. The magnitude of the annual net flux of CO2 (FCO2) into the Ross Sea varied >twentyfold between years, from −0.07 mol C m−2 in 2002–2003 to −1.55 mol C m−2 in 1999–2000 (negative values denote flux from air to sea). In some regions, annual FCO2 was as high as −2.9 mol C m−2, with daily rates approaching −62 mmol m−2 d−1. These results rank the Ross Sea as one of the stronger ocean sinks for atmospheric CO2 and demonstrate the sensitivity of FCO2 in polar waters to changes in sea ice cover.