RESPONSE OF EQUATORIAL PACIFIC PHYTOPLANKTON TO SUBNANOMOLAR FE ENRICHMENT

In the equatorial Pacific Ocean along 160 degrees W, surface-water samples with natural plankton communities were placed in incubation bottles to which sybnanomolar 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 Chi a concentration of large-(> 10 mu m) and medium-size (3-10 mu m) 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 mu m) 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.