Response of equatorial Pacific phytoplankton to subnanomolar Fe enrichment

Abstract

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 NO 3 increased stocks of Chl a and POC, NO 3 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.