INTERACTION OF IRON AND MAJOR NUTRIENTS CONTROLS PHYTOPLANKTON GROWTHAND SPECIES COMPOSITION IN THE TROPICAL NORTH PACIFIC-OCEAN

A 6-d nutrient enrichment experiment was performed in the tropical Nor th Pacific Ocean at 9-degrees-N, 147-degrees-W with ultraclean techniq ues. Changes in phytoplankton biomass, C and N assimilation rates, gro wth rates, and species composition were monitored with HPLC pigment an alyses and flow cytometry techniques, as well as C-14 fixation into pa rticulate C, pigments, and protein. Prochlorophyte specific growth rat es (from divinyl Chl a labeling) increased from an initial value of 0. 15 d-1 to 0.96 d-1 following macronutrient addition (N, P, and Si). Di atoms, however, were unable to grow without added Fe. Diatom populatio ns were severely colimited by Fe and macronutrients but achieved a spe cific growth rate of 2.5 d-1 following Fe and macronutrient additions. Results implied that grazing rates (g) on prochlorophytes were stimul ated in approximate balance with prochlorophyte growth (mu) after 6 d (g: mu = 0.85), but that grazing processes were not as efficient (g: m u = 0.40) at controlling the diatom standing stock. Our results sugges t that grazing processes may be the most important factor regulating p rocaryotic biomass, but Fe limitation is the proximate control of diat om biomass and hence may limit the utilization of macronutrients in th e equatorial Pacific Ocean.