CARBON AND NUTRIENT CYCLING DURING THE SOUTHERN OCEAN IRON ENRICHMENT EXPERIMENTS

The flux of carbon dioxide across the air-sea interface of Antarctic waters are thought to exert a major control on atmospheric CO2 and climate. Yet this flux is intricately linked to the processes controlling production, sinking and remineralization of particulate organic carbon in the form of phytoplankton and phytoplankton derived debris. The stoichiometry of both uptake and remineralization is a function of community structure and macro- and micronutrient availability. From a macronutrient perspective, the Southern Ocean is distinguished by two major biogeochemical provinces. The waters north of the Antarctic Polar Front Zone (APFZ) are characterized by low silicate concentrations (< 5 mM), while the surface ocean to the south is characterized by high silicate concentrations (>60 mM). Both regions contain high and unlimiting quantities of nitrate (~25 mM). The gradient in silicate is thought to exert a differential control on community structure and thus carbon flux from the surface waters. This gradient may influence carbon export from these waters by limiting the growth of ballasted phytoplankton. Throughout the entire region, iron plays a key role in the micronutrient nutrition of phytoplankton. In this study, iron additions were performed both north and south of the APFZ in high and low Si waters. Large phytoplankton blooms, were induced in both locations as a direct result of iron enrichment. Primary production exceeded the values observed for both regions during the US JGOFS Southern Ocean program and significant depletions in carbon dioxide were observed. Although evidence of Si limitation of diatom growth was present, the iron addition was nearly equivalent north and south of the Polar Front. Although the increase in particulate organic carbon tracked the increase in particulate organic nitrogen in well-behaved Redfieldian stoichiometry, the water column bloom signature indicated preferential remineralization of nitrogen. Such a shallow recycling of nitrogen could lead to enhanced carbon flux, greater than would have been anticipated from past studies. These results support the role of iron in controlling carbon uptake and export from both high and low silicate regions.