Differences and potential driving mechanisms of marine bloom patterns between hemispheres

Marine phytoplankton blooms are expanding globally, yet the hemispheric divergence in their phenology and the quantitative role of anthropogenic-climatic interactions remain poorly resolved. By integrating high-resolution satellite observations with multi-source environmental datasets, we reveal a distinct hemispheric asymmetry: coastal blooms in the Northern Hemisphere exhibit a robust bimodal seasonality driven by thermal lag and stratification-mixing cycles, whereas the Southern Hemisphere shows a predominantly unimodal pattern regulated by light availability and oceanic forcing. Crucially, while climate variables (sea surface temperature and wind speed) set the baseline for bloom dynamics, our nonlinear interaction analysis reveals that anthropogenic stressors dramatically amplify these natural drivers. The synergy between human activities (e.g., nutrient loading from cropland and population centers) and climatic factors increases the explanatory power for bloom size and duration by over 250% compared to individual factors. These findings highlight a critical mechanism where human pressures exacerbate climate-driven bloom risks, providing a basis for targeted coastal management in the Anthropocene.