The Southern Ocean, generally defined as the waters occurring south of 60°S and encircling Antarctica, plays a disproportionate role in control of atmospheric carbon by contributing ∼40% of the total oceanic uptake of anthropogenic carbon dioxide (CO 2) emissions (Frölicher et al., 2015; Khatiwala et al., 2009; Takahashi et al., 2009, 2012). In particular, the Antarctic continental shelf areas show substantial carbon uptake capacity due to efficient biological pump through primary production (Arrigo et al., 2008) during the summertime when sea ice retreat allows solar irradiance penetration into the upper ocean. However, the reason for the low efficiency of carbon export into the deep ocean despite the high productivity in the upper Abstract The Amundsen Sea continental shelf (ACS) water ecosystem is expected to undergo changes since increasing melt rate of glacier and decreasing sea ice extent by global warming would lead to the mitigation of iron and light limitation. We investigated how diatoms and Phaeocystis, two dominant taxa, and primary production in the ACS water would respond to variations in iron and light availabilities by using a 1-D pelagic ecosystem model. In the model, we added sea ice effects that reduce light penetration and optimized model parameters for diatoms and Phaeocystis. The results from our model showed good agreement with 20-year observations of Chl-a as well as the biomass proportion of diatoms and Phaeocystis and nutrient distributions during the growing season. Our model experimental results suggest that the current moderate iron and high light conditions favor the growth of Phaeocystis over diatoms. Moreover, as iron increases, the organic carbon exudation by phytoplankton increases more rapidly than net primary production (NPP), leading to a decline in phytoplankton biomass. On the other hand, irradiance plays a role in controlling NPP in terms of photoinhibition which is reduced by increasing iron. Increases in both iron and irradiance lead to an advance in the timing of the bloom peak (surface Chl-a maximum) due to increases in phytoplankton carbon loss and photoinhibition. Our results imply that the dominance of Phaeocystis can continue and that the carbon uptake capacity of the ACS in the summer seasons might increase given that iron availability will increase with future climate change. Plain Language Summary We firmly believe that this research based on an optimum biogeochemical model developed for the Amundsen Sea continental shelf water is of sufficiently broad interest for the public as well as the scientific community, since it advances the study of phytoplankton ecology and carbon uptake in the Antarctic coastal area. A complex coupled ocean-ecosystem model was developed to investigate dynamics of the two dominant species (diatoms and Phaeocystis) in the ACS water, one of the most rapidly warming regions on Earth with extremely high primary productivity. These two groups are particularly important as they could significantly contribute to the carbon export into the dee...

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