Marine heatwaves reorganize phytoplankton bloom phenology in the Gulf of Alaska

Abstract

Phytoplankton blooms structure primary production and food‐web dynamics in the Gulf of Alaska, yet their response to recent warming and marine heatwaves (MHWs) remains poorly described. Using a parametric Gaussian framework applied to 21 years of satellite chlorophyll a (Chl a), this study classified daily Chl a timeseries into double‐bloom, single spring‐bloom, single fall‐bloom, and no‐bloom regimes and related these to large‐scale oceanographic and atmospheric forcing. Double blooms with distinct spring and fall peaks dominated much of the domain in cold, dynamically active years characterized by negative sea surface temperature anomalies, depressed sea level, strong cross‐shelf pressure gradients, and more frequent storms associated with an intensified Aleutian Low. In contrast, warm years with elevated sea level and weaker cross‐shelf and atmospheric forcing favored single spring blooms and, in some areas, no‐bloom conditions, while single fall blooms occurred under intermediate physical states. Although no significant linear trends in the areal extent occupied by each regime were detected over 2003–2023, years influenced by major MHWs projected strongly onto the warm, weakly forced end of this physical gradient and were associated with reduced double‐bloom extent and an increased prevalence of single spring and no‐bloom regimes. These shifts imply that contraction and weakening of the seasonal production window, with likely consequences for zooplankton growth, match–mismatch dynamics for fish early life stages, and the balance between pelagic retention and export, highlighting how MHWs and ongoing climate change can suppress and reorganize phytoplankton bloom phenology in the Gulf of Alaska.