Influence of interannual sea-ice anomalies on spring phytoplankton phenological regimes in the Pacific Arctic (2003-2023)

Rapid changes in Arctic sea ice provide the context for shifts in the timing and structure of spring phytoplankton production in the Pacific Arctic. Using a 21 yr satellite record (2003-2023), this study investigated the responses of spring phytoplankton blooms to interannual sea-ice anomalies by applying a parametric Gaussian fitting to daily chlorophyll a (chl a ) timeseries. The parametric Gaussian framework allows us to classify the chl a timeseries into 4 phenological regimes based on the timing of the fitted chl a peak relative to ice retreat timing: no-bloom, under-ice (UI) bloom, marginal-ice-zone (MIZ) bloom, and open-water (OW) bloom. The 4 phenological regimes form a continuum from strongly ice-associated to purely open-water-driven production. Earlier sea-ice retreat preferentially supports OW blooms that develop after an extended cold, ice-free period, whereas later sea-ice retreat favors UI blooms that develop beneath sea ice. MIZ blooms are most common near climatological retreat timing, consistent with the classic ice-edge bloom paradigm, while no-blooms tend to occur under relatively late retreat at higher latitudes and warm surface conditions. Despite its simplicity and the limitations of satellite chl a and strict data-coverage criteria, this process-informed characterization of chl a timeseries, anchored to sea-ice retreat timing, captures meaningful structure in phytoplankton bloom phenology across the Pacific Arctic. The strong coupling between phenological regime and sea-ice regime provides a baseline for detecting and attributing future ecosystem changes as sea ice continues to thin and retreat under ongoing climate change.