Influences of initial plankton biomass and mixed-layer depths on the outcome of iron-fertilization experiments

Several in situ iron-enrichment experiments have been conducted, where the response of the phytoplankton community differed. We use a marine ecosystem model to investigate the effect of iron on phytoplankton in response to different initial plankton conditions and mixed-layer depths (MLDs). Sensitivity analysis of the model results to the MLDs reveals that the modeled response to the same iron enhancement treatment differed dramatically according to the different MLDs. The magnitude of the iron-induced biogeochemical responses in the surface water, such as maximum chlorophyll, is inversely correlated with MLD, as observed. The significant decrease in maximum surface chlorophyll with MLD results from the difference in diatom concentration in the mixed layer, which is determined by vertical mixing. The modeled column-integrated chlorophyll, on the other hand, is the highest with intermediate MLD cases, suggesting difference in iron-induced biogeochemical responses between volume and area considerations. The iron-induced diatom bloom is severely restricted below the compensation depth due to both light limitation and grazing pressure, irrespective of the MLD. Sensitivity of the model to initial mesozooplankton (as grazers on diatoms) biomass shows that column-integrated biomass, net community production and export production are strongly controlled by the initial mesozooplankton biomass. Higher initial mesozooplankton biomass yields high grazing pressure on diatoms, which results in less accumulation of diatom biomass and may account for notably lower surface chlorophyll during SEEDS (Subarctic Pacific Iron Experiment for Ecosystem Dynamics Study) II than during SEEDS. The initial diatom biomass is also important to the outcome of iron enrichment but is not as crucial as the MLD and the initial mesozooplankton biomass. This modeling study suggests that not only MLD but also the initial biomass of diatoms and its principle grazers are crucial factors in the response of the phytoplankton community to iron enrichments, and should be considered in designing future iron-enrichment experiments.