Response of equatorial Pacific phytoplankton to subnanomolar Fe enrichment

In the equatorial Pacific Ocean along 160 ° W, surface-water samples with natural plankton communities were placed in incubation bottles to which subnanomolar levels of Fe were added under ultraclean conditions. Addition of 0.1–0.8 nM Fe to seawater samples containing high NO3 increased stocks of Chl a and POC, NO3 consumption and net growth rate of phytoplankton in incubation bottles relative to the controls. A large increase in the Chl a concentration of large- (10 μm) and medium-size (3–10 μm) fractions was observed in the Fe-enriched samples. POC concentrations doubled even with 0.1 nM Fe at the equator. The net growth rates of large- and medium-size phytoplankton increased systematically with added Fe concentration. The dissolved Fe concentration in the incubation bottles, which was determined on board by flow injection analysis, decreased significantly during the first 3 days of incubation. However, 50–90% of the added Fe remained in the dissolved fraction (< 0.2 μm) at the end of the experiments. These results indicate that changes in subnanomolar Fe levels affect the equatorial phytoplankton communities by promoting the growth of large phytoplankton.