The regulation of equatorial Pacific new production and pCO2 by silicate-limited diatoms

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 NO3 in the upwelling source water, the Equatorial Undercurrent. Diatoms were hypothesized to be the major users of NO3 in this system and the amount assimilated limited by the low amount of Si(OH)4 available. As a consequence NO3 is left in the surface waters along with unused CO2. 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 NO3. Picoplankton and some phytoplankton in the 45-mm size group carried out primarily regenerated production, i.e. NH4 uptake in a grazing dominated system. Equatorial diatoms followed uptake kinetics for Si(OH)4 and NO3 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 NO3 uptake was unaffected by added NO3. 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, VmaxSi 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 VmaxSi in accordance with VmaxSi being a function of incident irradiance. Fe additions to the enclosures also increased VmaxSi but not to the same extent as only enclosing the water and incubating on deck. The values of VmaxSi and V0Si showed no relation to ambient Fe concentrations. The study was carried out in a region relatively rich in Fe, from 1401W eastward. These results call into question conclusions that Fe and Si(OH)4 co-limit production based upon enclosure experiments amended with Fe and incubated at near surface irradiance without first considering what causes the initial large increase in VmaxSi in the on-deck control enclosures. Some evidence for an Fe effect was seen at the eastern end of the EB04 equatorial section, where Fe concentration generally declined in the eastward direction and at about 1181W reached a low level that may have resulted in the reduction of the VmaxSi. Data from the EB04 and EB05 cruises showed a close correlation between surface TCO2 and NO3 concentration as expected from the demonstrated limitation of diatom NO3 uptake by Si(OH)4, highlighting the important role of equatorial diatoms in the global carbon cycle. & 2010 Elsevier Ltd. All rights reserved.