In the 1990’s, two concepts were linked to provide a new focus on iron biogeochemistry within the ocean-atmosphere system. The first recognised that iron could limit phytoplankton productivity in the ocean. Photosynthesis is usually controlled by the delivery of nutrients by upwelling from the deep waters to the photic zone. However in some regions of the oceans the delivery of nutrients exceeds demand and planktonic photosynthesis is limited by some other factor (Fig. 5.1). We now know that iron is the limiting factor in these regions, which are termed High Nutrient Low Chlorophyll (HNLC). One such area of...

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