Exploring the roles of iron and irradiance in dynamics of diatoms and Phaeocystis in the Amundsen Sea continental shelf water

The Amundsen Sea continental shelf (ACS) water ecosystem is expected to undergo changes since the increasing melt rate of glaciers 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.