FY 2011

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

Dugdale, R., F. Chai, R. Feely, C. Measures, A. Parker, and F. Wilkerson

Deep-Sea Res. II, 58(3-4), 477–492, doi: 10.1016/j.dsr2.2010.08.008 (2011)

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 > 5-μm 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, V′_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 V′_maxSi in accordance with V′_maxSi being a function of incident irradiance. Fe additions to the enclosures also increased V′_maxSi but not to the same extent as only enclosing the water and incubating on deck. The values of V′_maxSi and V′Si showed no relation to ambient Fe concentrations. The study was carried out in a region relatively rich in Fe, from 140°W 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 V′_maxSi 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 118°W reached a low level that may have resulted in the reduction of the V′_maxSi. 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.