IRON AND NITROGEN NUTRITION OF EQUATORIAL PACIFIC PLANKTON

In the equatorial Pacific Ocean between 9-degrees-N and 3-degrees-S, experiments were conducted to test the effects of iron addition on phytoplankton nitrogen metabolism, community structure and growth. Addition of 1 nM Fe to seawater samples increased the final concentration of chlorophyll a and particulate organic carbon and nitrogen in incubation bottles relative to the controls. The initial net rates of production were rapid, averaging 0.64 doublings day-1, and were unaffected by Fe enrichment. Ammonium was the major inorganic nitrogen source for the indigenous phytoplankton community, and little nitrate was taken up despite its high ambient concentration. Biomass-specific (N-specific) NO3- and NO2- uptake rates were enhanced two-fold following Fe addition, but NH4+ uptake rates were unaffected. In response to Fe enrichment, the phytoplankton community switched from using primarily NH4+-nitrogen (regenerated production) for growth to NO3--nitrogen (new production). This change in nitrogenous nutrition was accompanied by a shift in phytoplankton community composition. High ambient NH4+ concentrations (0.58-mu-M) at 3-degrees-S were sufficient to saturate uptake rates, but did not inhibit the enhancement of NO3- or NO2- utilization by Fe. Short-term Fe-59 uptake rates of phytoplankton in the equatorial Pacific were similar to those of Fe-deficient phytoplankton cultures and exceeded by 40 times those of phytoplankton in Fe-rich coastal waters. These results indicate that in equatorial Pacific waters Fe is in short supply and limits the rate of NO3- and NO2- utilization, but does not severely limit the growth rate of the indigenous phytoplankton community.