Tidal modulation constrains phytoplankton bloom resilience in a eutrophic estuary

Abstract

Phytoplankton blooms in tidally energetic estuaries are typically short‐lived, yet the mechanisms driving their rapid onset and abrupt collapse remain poorly understood. Nutrient enrichment alone cannot account for these rapid boom‐bust cycles, pointing to the potential regulatory role of periodic forcings such as tidal cycles in governing bloom dynamics. Here, we used a high‐resolution three‐dimensional physical‐biogeochemical coupled model, validated with field observations, to investigate a transient bloom in Xiamen Bay, a eutrophic subtropical embayment. Bloom initiation was favored by neap‐tide stratification and optimal temperature‐light conditions, but subsequent spring tides induced rapid decline through turbulent disruption, vertical export into light‐limited layers, and offshore advective loss. Diagnostic budget analyses revealed that although biological production remained positive, it was consistently outweighed by physical export, underscoring the primacy of biomass retention over nutrient supply in sustaining blooms. Despite persistent nitrogen excess, bloom resilience was further constrained by phosphorus depletion, light dilution, and episodic wind forcing. Spatial heterogeneity further shaped bloom trajectories, with semi‐enclosed sub‐basins supporting local growth while open areas were dominated by transport processes. Together, these results support a retention‐controlled framework for bloom regulation in macrotidal estuaries, in which the spring‐neap cycle acts as a hierarchical temporal regulator governing the balance between production and loss. This shift from nutrient‐centric to retention‐mediated controls helps explain why eutrophic estuaries often host short‐lived blooms and provides a generalized mechanistic basis for interpreting bloom variability in tidally energetic coastal environments.