IRON DISTRIBUTIONS IN THE EQUATORIAL PACIFIC - IMPLICATIONS FOR NEW PRODUCTION

Several recent studies have shown that phytoplankton growth rate and p roduction at 0 degrees, 140 degrees W is physiologically limited by ir on. Therefore, changes in iron supply to the euphotic zone will result in variations in phytoplankton growth. We show that the flux of iron to this region is dominated by upwelling of the iron-rich Equatorial U ndercurrent waters. Variations in the depth and strength of upwelling and changes in iron concentrations at the base of the euphotic zone wi ll account for variations in primary and new production in this region . We determined dissolved and particulate iron profiles for the upper water column of the eastern equatorial Pacific including a vertical se ction from 9 degrees N to 3 degrees S along 140 degrees W. One of the more prominent features of the section was a peak in dissolved and par ticulate iron associated with the Equatorial Undercurrent. The possibl e lithogenic origin of this iron is substantiated by the vertical sect ion of particulate aluminum and manganese, which is consistent with a shallow hydrothermal source in the western equatorial Pacific. A simpl e one-dimensional model was used to calculate iron fluxes into the eup hotic zone at the equator. Upwelling rates and dissolved iron concentr ations were coupled to estimate the upwelling iron flux at 120 m (0.1% light level). Diffusive and atmospheric inputs of iron were also cons idered but were less significant than the upwelling flux. Iron-based p otential new production was estimated to be 10-82 mmol C m(-2) d(-1) w ith C:Fe ratios of 100,000-500,000:1. In a similar manner, nitrate-bas ed potential new production was 99-106 mmol C m(-2) d(-1). This demons trates that iron supply limits new production to only 9-83% of the nit rate-based potential.