Iron supply and demand in the upper ocean

Iron is hypothesized to be a limiting micronutrient for ocean primary produ ction. This paper presents an analysis of the iron budget in the upper ocea n. The global distribution of annual iron assimilation by phytoplankton was estimated from distributions of satellite-derived oceanic primary producti on and measured (Fe:C)(cellular) ratios. The distributions of iron supply b y upwelling/mixing and aeolian deposition were obtained by applying (Fe:NO3 )(dissolved) ratios to the nitrate supply and by assuming the soluble fract ion 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 es timated to be 12x10(9) mol Fe yr(-1). Atmospheric deposition of total Fe is estimated to be 96x10(9) mol Fe yr(-1) in the open ocean, with the soluble Fe fraction ranging between 1 and 10% (or 1-10x10(9) mol Fe yr(-1)). By co mparison, the upwelling/entrainment supply of dissolved Fe to the upper oce an is small, similar to 0.7x 10(9) mol Fe yr(-1). Uncertainties in the aeol ian flux and assimilation may be as large as a factor of 5-10 but remain di fficult to quantify, as information is limited about the form and transform ation of iron from the soil to phytoplankton incorporation. An iron stress index, relating the (Fe:N) demand to the (Fe:N) supply, confirms the produc tion in the high-nitrate low-chlorophyll regions is indeed limited by iron availability.