Biological response to iron fertilization in the eastern equatorial Pacific (IronEx II). III. Dynamics of phytoplankton growth and microzooplankton grazing

Phytoplankton growth and microzooplankton grazing were investigated during the IronEx II mesoscale enrichment experiment using the seawater dilution t echnique combined with group-specific pigment markers. Growth rate estimate s for the phytoplankton community increased greater than or equal to 2-fold , from 0.6 d(-1) in the ambient environment to 1.2-1.6 d(-1) in the iron-en hanced bloom. Grazing lagged growth, allowing phytoplankton biomass to accu mulate at a high rate (similar to 0.8 d(-1)) initially. However, grazing mo rtality ultimately increased 3- to 4-fold to 1.2-1.4 d(-1), largely balanci ng growth by Day 6 of the experiment. Increased rates were broadly distribu ted among phytoplankton taxa, but they differed in timing. Whereas picophyt oplankton showed more of a steady balance between growth and grazing, incre asing grazing pressure on diatoms followed a 3-5-fold increase in larger (> 20 mu m) heterotrophic dinoflagellates and ciliates, which grew in response to enhanced diatom biomass. In the ambient environment, phytoplankton prod uction was 15 to 20 mu g C l(-1) d(-1), with diatoms accounting for 17 % of growth and 7 % of grazing losses. Total phytoplankton production increased to 150-200 mu g C l(-1) d(-1) at the peak of the patch bloom, where 79 % o f growth and 55 % of microzooplankton grazing involved diatoms. Phytoplankt on grazing mortality was significantly correlated with grazer biovolume, an d high carbon-specific grazing estimates at the bloom peak indicated growth rates similar to 1.0 d(-1) for the heterotrophic community and up to 1.4 d (-1) for >20 mu m heterotrophs. During several days when high phytoplankton biomass was relatively constant in the patch bloom, the pennate diatom dom inated the community and remained physiologically healthy and growing at a fast rate, even though nutrient conditions were suboptimal. Efficient cropp ing of diatoms by large protistan grazers and rapid remineralization of iro n and biogenic silica were required to maintain this dynamic balance. Contr ary to expectations, the carbon export ratio did not increase with the iron -induced diatom bloom. Thus, mesoscale iron-enrichment of high-nutrient, lo w chlorophyll waters in the eastern equatorial Pacific clearly demonstrated that phytoplankton growth rates and standing stocks were iron-limited. How ever, the food web also demonstrated a remarkable resiliency to environment al perturbation by establishing a new balance in which the essential featur es of a microzooplankton-dominated, low export system were maintained.