Carbonate system response to iron enrichment

In May of 1995, the second iron experiment (IronEx II) was carried out in the Equatorial Pacific Ocean (beginning at 3.5°S and 104.5°W) with the purpose of examining the effect of iron enrichment in high nutrient, low chlorophyll (HNLC) waters. During the cruise, the carbonate parameters TA (total alkalinity), TCO2 (total inorganic carbon), pH and pCO2 (partial pressure of CO2) were determined from water samples both inside and outside the iron-fertilized patch. The carbonate parameters, along with hydrographic properties, were found to be uniform in the surface waters of the study area before the addition of iron. Values were found to be pH=7.93±0.02, TA=2311±5 μmol kg−1, TCO2=2051±4 μmol kg−1, and pCO2=538±12 μatm. The response of the carbonate system to an initial 2 nM iron infusion and two subsequent 1 nM reinfusions to the surface water was measured several times a day during the study. The maximum changes of the carbonate parameters within the patch relative to ‘outsideʹ measurements were −27 μmol kg−1 in TCO2, −73 μatm in pCO2, and +0.058 in pH on days 8 and 9 of the experiment. The TA did not change within the experimental error of the measurements ±3 μmol kg−1). In addition to the daily measurements, three transects were made across the patch (at 63, 78, and 122 h post-addition) that show a spatial biochemical response to the influence of iron and correlations between nutrients and the various carbonate parameters. A diel study of the patch showed daytime decreases of 27 μatm in pCO2 and 16 μmol kg−1 in TCO2. The changes in carbon to nutrients during the experiment give ΔNO3/ΔPO4=14.3±0.7, ΔC/ΔNO3=6.2±0.2, ΔC/ΔPO4=90±5, ΔC/ΔO2=−0.66±0.07, ΔC/ΔSiO2=5.05±0.3. Corrections for the flux of gases across the air–sea interface during the experiment affect the oxygen changes, giving ΔC/ΔO2=−0.72. The slightly lower ratios than those predicted from the Redfield model have been attributed to the predominant production of diatoms that have high concentrations of lipids. These results suggest the following stoichiometry, 90 CO2+104 H2O+14 HNO3+H3PO4+18 SiO2→(CH2O)76(CH2)14(NH3)14(H3PO4)(SiO2)18+125 O2for the production of phytoplankton in the iron-enriched patch.