The CO2 system in a Redfield context during an iron enrichment experiment in the Southern Ocean

In November 2000, a second iron enrichment experiment (EisenEx) was carried out in the Southern Ocean. Iron was added on the 8th of November in the centre of an eddy at 21°E, 48°S. During the cruise, the carbonate parameters dissolved inorganic carbon (DIC), fugacity of CO2 (fCO2) and pH on the hydrogen ion scale (pHT) were determined from water samples from both inside and outside the iron fertilized patch. Before the start of the experiment, the surface properties of the eddy were quite uniform with respect to the carbonate system and representative of the High Nutrient Low Chlorophyll (HNLC) regions in the Southern Ocean. The response of the carbon dioxide system to the initial ≈4 nM iron (Fe) infusion and to two subsequent reinfusions at 15 m depth was measured every day during the study. The changes in the carbon dioxide system and major nutrients were strongly influenced by the meteorological conditions with a rapid succession of calm, often sunny spells and storm force winds during the 21 days of experiment. Twenty days after the first Fe-infusion, the maximum changes of the carbonate parameters in surface waters of the patch relative to outside patch were −15 μmol kg−1 in DIC, −23 μatm in fCO2, +0.033 units in pHT, −1.61 μM in nitrate and −0.16 μM in phosphate in a mixed layer of 80 m depth. In addition to the daily measurements, several transects were made across the patch that showed a response of the carbonate system to the influence of iron, concomitant with a response in nutrients and chlorophyll. The relative changes in dissolved inorganic carbon to nutrient concentrations inside the patch during the experiment give N/P=12, C/P=82, C/N=5.9, C/Si=2.9 and N/Si=0.5. The effect of the influx of atmospheric CO2 on the DIC inventory was small with values between 0.05 and 0.10 μmol kg−1 day−1, and did not significantly affect these ratios. Although the observed change in DIC in the Fe-enriched surface waters was lower than in the previous Fe-enrichment experiments, the equivalent biological C-uptake of 1.08×109 mol C across the patch after 20 days was significant due to the large horizontal dispersion of the patch. The ratio of biological carbon uptake to Fe added (Cbiological uptake/Feadded) was 2.5×104 mol mol−1.