Spatiotemporal Characteristics and Physical–Ecological Coupling Mechanisms of Spring Phytoplankton Blooms in the Bohai Sea

Spring phytoplankton bloom mechanisms in the Bohai Sea show clear spatial differences, but the physical–biological coupling in the ice-covered Liaodong Bay (LDB) remains poorly understood. Utilizing satellite observations and high-resolution reanalysis data from 2009 to 2023, this study explores the drivers of spring blooms through generalized additive models (GAMs) and the Equation of State of Seawater (EOS). The results reveal pronounced regional heterogeneity. In the southern Bohai Sea, bloom dynamics are co-regulated by a complex combination of nutrient availability and localized physical mixing. In contrast, blooms in LDB are predominantly driven by the shoaling of the mixed layer depth (MLD), a physical state intrinsically linked to winter sea-ice melt. Linear decomposition of water density via EOS quantitatively demonstrates that spring stratification in LDB is salinity-dominated (contributing ~60.7%), rather than thermally driven. The rapid influx of low-salinity meltwater forms a strong halocline that suppresses vertical mixing and physically compresses the MLD into the euphotic zone. Consistent with Sverdrup’s Critical Depth Theory, this inferred physical pathway effectively alleviates light limitation and acts as the primary trigger for the early bloom peak timing. This complete melting–freshening–stratification–light coupling chain provides a novel physical perspective on how mid-latitude marginal sea ecosystems respond to climate change, distinct from canonical polar light-limitation models.