Simulated biogeochemical responses to iron enrichments in three high nutrient, low chlorophyll (HNLC) regions

To fill temporal gaps in iron-enrichment experimental data and gain further understanding of marine ecosystem responses to iron enrichments, we apply a fifteen-compartment ecosystem model to three iron-enrichment sites, namely SEEDS (the Subarctic Pacific Iron Experiment for Ecosystem Dynamics Study; 48.5°N, 165°E) in the western North Pacific, SOIREE (the Southern Ocean Iron RElease Experiment; 61°S, 140°E) in the Southern Ocean, and IronExII (the second mesoscale iron enrichment experiment; 3.5°S, 104°W) in the Equatorial Pacific. The ecological effects of iron in the model are represented by changing two photosynthetic parameters during the iron-enrichment period. The model results successfully reproduce the observed biogeochemical responses inside and outside the iron patch at each site, such as rapid increases in plankton biomass and biological productivity, and decreases in surface nutrients and pCO2, inside the patch. However, the modeled timing and magnitude of changes differ among the sites because of differences in both physical environments and plankton species. After the iron enrichment, the diatom productivity is strongly controlled by light at SOIREE and by silicate at IronExII and SEEDS. Light limitation due to self-shading by the phytoplankton is significant during the bloom at all sites. Sensitivity analysis of the model results to duration of the iron enrichment reveals that long-term multiple infusions over more than a week would not be effective at SEEDS because of strong silicate limitation on diatom growth. Sensitivity of the model to water temperature shows that export production is higher at lower temperatures, because of slower recycling of particulate organic carbon. Therefore, the e-ratio (the ratio of export production to primary production) is inversely correlated with temperature, and the relationship can be described with a linear function. Through this study, we conclude that ecosystem modeling is a powerful tool to help design future iron-enrichment experiments and observational plans.