Functional redundancy and structural flexibility as contrasting plankton strategies under macrotidal pulses.

Macrotidal estuaries are hydrodynamic pulse systems that continuously reconfigure coastal environments. While seasonal dynamics are well documented, the short-term mechanisms enabling plankton to persist in these hyperdynamic habitats remain poorly understood. We investigated how planktonic communities at a tropical Amazonian Ramsar site respond to high-frequency tidal oscillations. Integrating diel sampling with random forest and SHAP modeling revealed that extreme tidal energy acts as a severe ecological filter, driving a temporal decoupling between nutrient pulses during ebb flows and phytoplankton biomass peaks during flood tides. Under this physical stress, communities employ divergent resilience strategies. Phytoplankton rely on functional redundancy, and maintain a stable functional architecture despite high taxonomic turnover driven by tidal advection. Conversely, zooplankton exhibit structural flexibility and synchronously reorganize their taxonomic composition and functional traits to exploit transient trophic resources. Predictive modeling demonstrated that functional diversity captures fine-scale mechanistic responses overlooked by taxonomic metrics, confirming a hierarchical bottom-up control. Specifically, hydrodynamics modulate the availability of specific nutrients, such as dissolved nitrogen and silicate, for producers, which subsequently affects the resource quality, thereby structuring consumer traits. These findings establish that the interplay between producer redundancy and consumer flexibility acts as a fundamental persistence mechanism. Ultimately, this study highlights that high-frequency functional traits provide sensitive early indicators of ecosystem shifts, offering a novel predictive framework essential for assessing resilience and guiding biodiversity conservation in highly dynamic coastal environments.