The island mass effect and biological carbon uptake for the subantarctic Crozet Archipelago

Marine productivity is often higher downstream than upstream of islands. This so-called island mass effect was tested and quantified with respect to biological carbon uptake and air–sea exchange of carbon dioxide (CO2) at the Crozet Plateau between November 2004 and January 2005 during two CROZEX cruises. The remote plateau is situated at 45.5–47.0°S 49.0–53.0°E, south of the Subantarctic Front (SAF) in the Polar Frontal Zone (PFZ). Surface waters upstream (south) of the plateau had high nutrient and low chlorophyll (HNLC) concentrations. The fugacity of carbon dioxide (fCO2) in surface water was just below the atmospheric value and oceanic CO2 uptake was small (0.2±0.1 mol m−2) throughout CROZEX. The mixed-layer concentration of dissolved inorganic carbon (DIC) decreased by 15 μmol kg−1 from November to January in these HNLC waters, indicating significant biological carbon uptake. Extensive phytoplankton blooms occurred downstream (north) of the plateau in austral spring. These reduced surface water fCO2 by 30–70 μatm and DIC by 30–60 μmol kg−1 and created an important oceanic sink for atmospheric CO2 of 0.6–0.8±0.4 mol m−2, corresponding to a total uptake of 1.3±0.8 Tg C (1 Tg=1012 g). The reduction of DIC in the upper 100 m was much larger downstream (2–3 mol m−2) than upstream (1 mol m−2) of the plateau in January, further confirming the existence of the island mass effect for the Crozet Archipelago. An additional finding is the sizeable DIC deficit in the HNLC waters upstream (south) of the plateau, suggesting that some HNLC waters of the PFZ are more productive than commonly thought. Deep mixed layers of 60–90 m may hide such sustained, modest marine productivity from detection by satellite.