a b s t r a c t Modeling and data from the JGOFS EqPac program suggested that the eastern equatorial Pacific upwelling ecosystem includes a quasi-chemostat culture system dominated by diatoms and limited by Si(OH) 4 due to a low ratio of Si(OH) 4 to NO 3 in the upwelling source water, the Equatorial Undercurrent. Diatoms were hypothesized to be the major users of NO 3 in this system and the amount assimilated limited by the low amount of Si(OH) 4 available. As a consequence NO 3 is left in the surface waters along with unused CO 2 . Two cruises to the eastern equatorial Pacific (EB04 and EB05) were made to test the existing hypothesis of Si(OH) 4 limitation, and study the roles of source concentrations of Si(OH) 4 and Fe, and nutrient uptake kinetics for comparison with model predictions. Fractionated nitrogen uptake measurements showed that diatoms at times take up the major portion of the NO 3 . Picoplankton and some phytoplankton in the 4 5-mm size group carried out primarily regenerated production, i.e. NH 4 uptake in a grazing dominated system. Equatorial diatoms followed uptake kinetics for Si(OH) 4 and NO 3 uptake as observed in laboratory investigations of diatoms under Si(OH) 4 and Fe limitations. Si(OH) 4 uptake responded to additions of Si(OH) 4 on a time scale of hours in uptake kinetic experiments while NO 3 uptake was unaffected by added NO 3 . The uptake of Si(OH) 4 varied in a narrow range on a Michaelis-Menten hyperbola of Si(OH) 4 uptake vs. Si(OH) 4 concentration, with a maximal Si(OH) 4 uptake rate, V 0 maxSi set to a relatively low value by some factor(s) other than Fe on a longer time scale, i.e., days in shipboard enclosures. Simply enclosing water collected from the mid euphotic zone and incubating for some days on deck at 50% surface irradiance increased
