Stratification Control of Phytoplankton Bloom in Circum-Antarctic Coastal Polynyas: Data Analysis and Modeling

Coastal Antarctica is marked by dozens of polynyas that foster high primary production, which is crucial for many living species as well as carbon uptake in the Southern Ocean. Although all the polynyas have elongated ice-free periods, they are distinct from each other in terms of physical dynamics and biological productivity, and thus exhibit different capability in supporting local ecosystem and different vulnerability to a changing climate. Systematic studies of the underlying physical controls on biological variability are scarcely few. Here we analyze the control of the upper-ocean stratification on the diverse response of phytoplankton bloom phenology (e.g., timing, speciation and magnitude) in circum-Antarctic polynyas. First, we compiled the existing Antarctic polynya hydrographic observations (e.g., CTD casts, seal-derived measurement), in combination with in-situ and satellite measurements of chlorophyll, to categorize the spatial heterogeneity of polynyas. Second, we utilized NPZD-type ecosystem model to explore the influence pathways of stratification on phytoplankton phenology. Our results suggested that stratification modulates phytoplankton bloom climax timing in a one-dimensional fashion through adjusting the light level the phytoplankton expose to, and it also affects bloom speciation and magnitude through regulating iron supply. Finally, we synthesized the data and model analyses to construct a diagram which can serve as a baseline for future polynya studies. The diagram separates two distinct groups of polynyas to the first order using water column stratification, and the additional variability is attributed to the sea-ice adjusted incident light. The results also shed light on the explanation of temporal variability of phytoplankton phenology in some polynyas.