Circulation and seasonal evolution of polar waters south of Australia: implications for iron fertilization of the Southern Ocean

The Southern Ocean Iron Release Experiment (SOIREE) was carried out in late summer (February 1999) south of Australia (61°S, 140°E). This region of the southern Antarctic Zone (AZ-S), between the southern branch of the Polar Front (PF) and the southern front of the Antarctic Circumpolar Current (SAACF), is characterized by weak currents and is remote from the influence of sea-ice or coastal waters. The SOIREE site exhibits high nutrient concentrations year-round (phosphate, nitrate and silicate remain above 10 μM), low chlorophyll accumulations (<0.5 μg/l), and moderate summer mixed-layer depths (50–70 m). The SOIREE iron fertilization led to a large increase in algal biomass, particularly large diatoms, and persisted into March well after normal seasonal production is complete. No increase in carbon export occurred during the SOIREE 13-day observation period. The seasonal cycles of mixed-layer development and low biomass accumulation at the SOIREE site are representative of most of the region between the PF and the SACCF, i.e. between ∼54 and ∼62°S, and to a lesser extent the Polar Frontal Zone. However, north of ∼59°S surface waters are depleted in silica by mid-summer (as occurs year-round north of the Subantarctic Front). A different response to iron fertilization is likely under these conditions, possibly the promotion of lightly silicified diatoms and non-siliceous organisms, whose ability to export carbon is uncertain. The SOIREE fertilized waters are likely to have remained at the surface in the AZ-S throughout the winter. In general, carbon sequestration by subduction of iron-enhanced biomass accumulations is unlikely south of the SAF, except in very limited regions. Moreover, intermediate water masses formed in the Southern Ocean sink with little pre-formed silicate, so that the “silica pump” is already working at close to maximal capacity. Therefore, in the absence of significant changes in community structure or algal physiology, which increase the ratio of carbon export to silicate export, increased iron supply is unlikely to increase the magnitude of carbon sequestration.