Iron supply and demand in the upper ocean

Iron is hypothesized to be a limiting micronutrient for ocean primary production. This paper presents an analysis of the iron budget in the upper ocean. The global distribution of annual iron assimilation by phytoplankton was estimated from distributions of satellite‐derived oceanic primary production and measured (Fe:C) cellular ratios. The distributions of iron supply by upwelling/mixing and aeolian deposition were obtained by applying (Fe:NO 3 ) dissolved ratios to the nitrate supply and by assuming the soluble fraction of mineral aerosols. A lower bound on the rate of iron recycling in the photic zone was estimated as the difference between iron assimilation and supply. Global iron assimilation by phytoplankton for the open ocean was estimated to be 12 × 10 9 mol Fe yr −1 . Atmospheric deposition of total Fe is estimated to be 96×10 9 mol Fe yr −1 in the open ocean, with the soluble Fe fraction ranging between 1 and 10% (or 1‐10 ×10 9 mol Fe yr −1 ). By comparison, the upwelling/entrainment supply of dissolved Fe to the upper ocean is small, ∼0.7×10 9 mol Fe yr −1 . Uncertainties in the aeolian flux and assimilation may be as large as a factor of 5‐10 but remain difficult to quantify, as information is limited about the form and transformation of iron from the soil to phytoplankton incorporation. An iron stress index, relating the (Fe:N) demand to the (Fe :N) supply, confirms the production in the high‐nitrate low‐chlorophyll regions is indeed limited by iron availability.