Continental-shelf sediment as a primary source of iron for coastal phytoplankton

The availability of iron, an essential nutrient, controls rates of phytopla nkton primary productivity in the open-ocean, upwelling ecosystems of the e quatorial Pacific(1,2). Upwelling injects large amounts of macronutrients i nto the euphotic zone of eastern boundary currents, such as the California Current System (CCS), where iron can become the limiting factor on producti vity(3,4). Iron addition to samples from some areas of the CCS has been sho wn to increase rates of biomass production(5,6), but the processes that con trol iron availability in these systems remain poorly understood. Here we r eport measurements of dissolvable iron (that is, dissolved plus leachable i ron at pH 3) in transects across the CCS in March of 1997 and 1998. We foun d high concentrations of iron in 1997 during strong upwelling conditions. D uring the 1998 El Nino, the concentration of dissolvable iron in surface wa ters was low, even though that year was marked by high river flow and low o ffshore salinity. These results indicate that the primary source of iron in the CCS is resuspension of particles in the benthic boundary layer, follow ed by upwelling of this iron-rich water, rather than direct riverine input. This source of iron must be an I essential but variable component of the h igh productivity found in upwelling ecosystems.