Asymmetric bloom lengthening drives high-latitude ocean chlorophyll accumulation imbalance

Terrestrial ecosystems maintain a stable time-allocation budget between growth and senescence, but whether this applies to the ocean remains unclear. Using fused satellite LiDAR and ocean-color data (2007–2022), we show that marine phenology is variable and asymmetric. In high latitudes ( > 50°N), the bloom growth-to-decline ratio (RGD) is declining by ~5.3% per decade. While the chlorophyll-increase phase has lengthened by 2.3–5.5 days, the post-peak chlorophyll-decline phase has extended disproportionately by 5.7–6.2 days. Environmental sensitivity analyses reveal distinct hemispheric patterns in the statistical associations between phenological variability and multiple co-varying physical and biogeochemical factors, consistent with contrasting nutrient regimes between the Northern and Southern high-latitude oceans. We further show that these shifts are associated with spatially heterogeneous changes in marine chlorophyll accumulation patterns. Our results identify bloom asymmetry as an important dimension of marine phenological change, providing an observational baseline for evaluating future links between chlorophyll phenology, ecosystem timing, and polar biogeochemical processes under climate change. This study shows that unequal changes in bloom duration between hemispheres contribute to contrasting high-latitude ocean chlorophyll trends, linking asymmetric phytoplankton phenology to marine ecosystem changes under climate warming.