Nutrient and carbon parameters during the Southern Ocean iron experiment (SOFeX)

During the 2002 austral summer, two in situ mesoscale iron fertilization experiments were conducted in two distinct silicic acid regimes of the Southern Ocean (SOFeX). The iron was added to the low Si(OH)4 waters of the Subantarctic (<3 μmol kg−1, North Patch) and the high Si(OH)4 waters poleward of the southern boundary of the Antarctic Circumpolar Current (ACC), (∼63 μmol kg−1, South Patch). The low iron availability in these two regimes of the Southern Ocean limits biological utilization of the ambient nutrients, influences phytoplankton species composition and species-specific nutrient consumption rates. The response of the carbon dioxide system and macronutrients to initial and subsequent iron additions were monitor relative to measurements outside the iron-enriched waters. These iron fertilization experiments demonstrate that iron addition to the High Nutrient Low Chlorophyll (HNLC) regions of the Southern Ocean increases primary productivity and decreases the fugacity of carbon dioxide. Dilution of the iron fertilized sites diminished the observed geochemical signals produced as a result of phytoplankton growth, while the entrainment of silicic acid sustained the increase of diatom biomass in the low Si(OH)4 waters of the Subantarctic. Empirical observation of the elemental composition of the iron fertilized ecosystems and the observed nutrient-depletion ratios upon the addition of iron were indicative of changes in the community structure and/or changes in the unique physiological nutrient consumption rates of a species within the community. Ultimately, variations in the nutrient-depletion ratios induced by increased iron concentrations influences biogeochemical cycles in the Southern Ocean and the export of carbon. Molar ratios in the north and south iron fertilization sites, respectively, were determined to be ΔN:ΔP=14.1±0.2 and 14.2±0.4; ΔC:ΔN=9.1±1.7 and 7.2±0.6; ΔC:ΔP=106±4 and 99±4; ΔC:ΔSi=7.7±0.8 and 6.2±0.5; ΔC:ΔO2=−0.75±0.02 and −0.73±0.02. Combining these results, the following stoichiometry is obtained for the growth of phytoplankton during the SOFeX in the north and south iron enrichment sites, 106CO2+120H2O+14HNO3+H3PO4+14SiO2→ ( CH 2 O ) 92 ( CH 2 ) 14 ( NH 3 ) 14 ( H 3 PO 4 ) ( SiO 2 ) 14 +141O2 and 99CO2+113H2O+14HNO3+H3PO4+16SiO2→ ( CH 2 O ) 81 ( CH 2 ) 18 ( NH 3 ) 14 ( H 3 PO 4 ) ( SiO 2 ) 16 +136O2, respectively. Variations in the ratios of carbon to macronutrients utilized by the ecosystem under iron replete conditions are important for the interpretation of the effect of iron fluxes on atmospheric CO2 concentrations in glacial times, highlighting the integral role of Fe and the Southern Ocean regulation of atmospheric carbon dioxide.