Phytoplankton Bloom Changes During Three Regime Shifts Derived From Sea Ice Concentration in the Ross Sea

Sea Ice Concentration (SIC) plays an important role in the Antarctic polynyas, especially where biological interactions through changes in chlorophyll-a concentration (CHL) phenology are concerned. To specifically understand how much SIC change affects CHL in the Ross Sea polynya (RSP), the regime shift index (RSI) was applied to the SIC anomaly, as per Rudionov (2004), https://doi.org/10.1029/2004gl019448. This resulted in three principal temporal regimes: Regime 1 (R1), 1997–2004; Regime 2 (R2), 2004–2016; and Regime 3 (R3), 2016–2019. Based on reconstructed daily CHL observations from remote sensing and application of a random forest algorithm with 10 physical predictors, our analysis revealed that (a) earlier sea ice melt onset corresponds to an earlier CHL peak across all regimes; (b) SIC changes explain approximately 10%–12.5% of CHL variation in the RSP: a first-ever estimation of SIC influence in the RSP; and (c) although CHL is significantly influenced by SIC changes, the relative contributions of the 10 geophysical predictors vary across the three regimes, with photosynthetically active radiation consistently emerging as the dominant driver. This result may explain why CHL has been observed to decrease despite the unprecedented decrease in Antarctic SIC since 2016.