Iron in the Sargasso Sea: Implications for the processes controlling dissolved Fe distribution in the ocean

In the surface water of the winter Sargasso Sea, “dissolved” Fe concentrations (defined as iron that passes through a filter with an 0.4 micrometer diameter pore size) decrease with increasing latitude from 0.8 nM at 26°N to 0.2 nM at 31°N. It does not appear that this pattern can be explained solely by spatial variations of eolian Fe deposition. Vertical mixing, phytoplankton growth and particle scavenging all appear to play important roles in controlling dissolved Fe distributions in the surface water. Concentrations of dissolved Fe in the subsurface waters increase with increasing depth from ∼0.2 nM at ∼200 m to ∼0.6 nM at depths below 1000 m, reflecting an imbalance between Fe removal by particle scavenging and Fe regeneration from microbial decomposition of organic matter sinking from the euphotic zone. Simple model calculations suggest that scavenging loss of Fe and ventilation of low preformed Fe high P water from source region may create deep ocean Fe:P concentration ratios (5 × 10 −4 for the Atlantic and 1.2 × 10 −4 for the Pacific) that are much lower than the ratios of Fe to P regenerated from the organic matter sinking from the euphotic zone (24–74 × 10 −4 for the Atlantic and 11–32 × 10 −4 for the Pacific). We hypothesize that this low Fe:P ratio may be the driving force for Fe limitation of algal growth observed in high nutrient low chlorophyll oceans and that regional changes of Fe:C ratios in organic matter sinking from the euphotic zone may play an important role in regulating dissolved Fe concentrations in the deep waters of the world ocean.