Mapping phytoplankton iron utilization: Insights into Southern Ocean supply mechanisms

The emerging field of ocean iron biogeochemistry has prompted interest in the identification and quantification of Fe supply mechanisms. However, less attention has been given to estimating biological Fe utilization, and using the magnitude of Fe utilization to enhance our understanding of modes of supply. Here, we combine regionally validated data sets (1997–2007) on remotely sensed net primary production (NPP) with the iron:carbon (Fe:C) molar ratios for resident phytoplankton to produce Southern Ocean maps of Fe utilization. This approach exploits the resolution of remotely sensed data to investigate the spatial patterns, areal extent and interannual variability of Fe utilization, and relates it to published temporal and spatial trends for Fe supply mechanisms. We estimate that Southern Ocean Fe utilization averaged ∼3.3 ± 0.3 × 10 8 mol Fe a −1 . This utilization varied little between years (7.8–9.6 μ mol Fe m −2 a −1 ), was greatest for subpolar waters, particularly in the Atlantic (up to 53.0 μ mol Fe m −2 a −1 ), and was lowest for the polar waters of the Indian sector. Application of maps corresponding to the location and areal extent of Fe supply regions (e.g., dust deposition) revealed that Fe utilization was highest in waters supplied by Patagonian dust, and to a lesser extent, where sediment resuspension (i.e. <500 m depth) probably supplies the majority of the Fe. The Atlantic sector has regions where multiple supply mechanisms are evident, resulting in perennially high productivity. This approach provides a better assessment of the relative importance, realm of influence, and areal extent of different Fe supply mechanisms to Southern Ocean waters.