Interaction of iron and major nutrients controls phytoplankton growth and species composition in the tropical North Pacific Ocean

Abstract

A 6‐d nutrient enrichment experiment was performed in the tropical North Pacific Ocean at 9°N, 147°W with ultraclean techniques. Changes in phytoplankton biomass, C and N assimilation rates, growth rates, and species composition were monitored with HPLC pigment analyses and flow cytometry techniques, as well as 14C fixation into particulate C, pigments, and protein. Prochlorophyte specific growth rates (from divinyl Chl a labeling) increased from an initial value of 0.15 d−1 to 0.96 d− following macronutrient addition (N, P, and Si). Diatoms, however, were unable to grow without added Fe. Diatom populations were severely colimited by Fe and macronutrients but achieved a specific growth rate of 2.5 d−1 following Fe and macronutrient additions. Results implied that grazing rates (g) on prochlorophytes were stimulated in approximate balance with prochlorophyte growth (µ) after 6 d (g : µ = 0.85), but that grazing processes were not as efficient (g : µ = 0.40) at controlling the diatom standing stock. Our results suggest that grazing processes may be the most important factor regulating procaryotic biomass, but Fe limitation is the proximate control of diatom biomass and hence may limit the utilization of macronutrients in the equatorial Pacific Ocean.