Study on the outbreak characteristics and mechanisms of estuarine and offshore harmful algal blooms in the East China Sea

Harmful algal blooms (HABs) occur frequently in the coastal waters of the East China Sea and exhibit pronounced spatiotemporal heterogeneity under complex hydrodynamic conditions. However, systematic comparative studies on the driving mechanisms of HABs under different hydrodynamic regimes remain limited. Based on in-situ observations from 2015 to 2020 and a high-resolution coupled physical–ecological model, this study comparatively investigated HAB developmental characteristics and environmental driving mechanisms in two typical coastal regions of the Zhejiang coast: the near-estuary Shengsi area, dominated by Changjiang Diluted Water, and the far-estuary Cangnan area, primarily controlled by coastal current systems. Statistical analyses, Mann–Kendall trend tests, and principal component analysis were applied to identify the dominant environmental controls associated with HAB outbreaks. HABs in Shengsi occurred sporadically from May to September and were mainly characterized by short duration (≤7 days) and small-to-medium spatial scales. In contrast, HAB outbreaks in Cangnan were strongly concentrated in May–June (91% in simulations and 84% in observations), with a higher frequency of large-scale events (duration >20 days; area >400 km²). Mechanistic analyses indicated that HAB dynamics in the nutrient-poor but hydrodynamically stable Cangnan waters were primarily regulated by hydrodynamic processes. An initial increase in current velocity (~0.067 m/s) promoted the transport and accumulation of nutrients (nitrate +0.8 mmol/L; phosphate +0.06 mmol/L), whereas the subsequent rapid weakening of currents, observed in 79.8% of HAB events, enhanced water-column stability and favored algal aggregation. The turning point of current velocity generally preceded the chlorophyll-a peak by approximately one day. Principal component analysis further confirmed the dominant role of physical regulation in Cangnan, where the first principal component explained 56.44% of the total variance. In contrast, Shengsi exhibited sufficient nutrient supply but relatively unstable hydrodynamic conditions due to interactions among multiple water masses. Except for seawater temperature, most environmental variables showed no statistically directed changes before and after HAB outbreaks, with increase–decrease probabilities close to 50%. The first principal component explained only 28.94% of the total variance, indicating a highly multifactor-driven HAB system. This study reveals substantial differences in HAB formation mechanisms between near-estuary and far-estuary systems in the East China Sea. HAB outbreaks in Cangnan are strongly associated with hydrodynamic stability, whereas HABs in Shengsi are jointly regulated by multiple environmental factors. These findings improve the understanding of HAB dynamics under complex coastal hydrodynamic backgrounds and provide a scientific basis for region-specific HAB forecasting and early-warning strategies along the Zhejiang coast.