| Summary: | Seasonality of the Ecosystem Response to Iron Stimulation in the Southern Ocean Ernesto Molina 1,2, Thomas W. Trull 1,2,3, Mathieu Mongin 3 and Andrew R. Bowie 1,2 1 Antarctic Climate and Ecosystems CRC, Hobart 7001, Australia 2 Institute of Antarctic and Southern Ocean Studies, University of Tasmania, Hobart 7001, Australia 3 CSIRO Marine and Atmospheric Research, Hobart 7001, Australia Abstract Mesoscale iron enrichment experiments and studies of natural iron inputs, in high nutrient low chlorophyll (HNLC) waters have demonstrated that iron limitation is widespread and affects atmospheric carbon dioxide and thus global climate. Of the HNLC regions, the Southern Ocean plays the greatest role in the global carbon cycle, but the dominant controls on net algal production are still controversial. Although broad ecosystem responses among mesoscale experiments are similar in different oceanic regions, the biomass generated and carbon dioxide drawdown at the surface differ substantially. Here we apply a lower trophic level Nitrate-Phytoplankton-Zooplankton-Detritus (NPZD) ecosystem model to investigate the effect of iron on phytoplankton growth in response to different iron-enrichment simulations in the Australasian-Pacific sector of the Southern Ocean. While the ecosystem model represents a simple approximation of the complex lower trophic ecosystem of this region, simulated chlorophyll concentrations reproduce the main characteristics of the first Southern Ocean Iron Release Experiment (SOIREE), the surrounding HNLC environment, and the naturally iron fertilized waters of the Kerguelen plateau. Furthers runs with the same iron enhancement treatment differed dramatically according to the different location and time of the year of the simulated iron infusions. The magnitude of the iron-induced biogeochemical responses in surface waters, such as maximum chlorophyll, is strongly controlled by both light and grazing pressure, and shows highest values during the early spring infusions. In contrast, the highest POC export estimates are observed during the summer infusions. These findings suggest that not only light limitation but also the overwintering biomass of phytoplankton and their principle grazers are crucial factors in the response of phytoplankton community to iron enrichment.
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