Species-specific responses of zooplankton to intermittent seawater intrusion in a re-opened estuarine system

This study evaluated zooplankton community dynamics under an intermittent seawater intrusion regime following the partial re-opening of the Nakdong River Estuary Barrage, South Korea from 2022 to 2024. Environmental variables and zooplankton distributions were monitored across vertical water layers (surface, middle, and bottom) at five upstream and three downstream sites relative to the barrage. Linear Mixed-Effects model (LMM) and Canonical Correspondence Analysis (CCA) revealed that salinity was the primary driver of broad-scale differences in community composition between upstream and downstream regions. However, its influence was largely confined to the study site adjacent to the estuarine barrage, where water temperature, pH, and spatiotemporal variables had stronger effects on community composition. Piecewise Structural Equation Modeling (PiecewiseSEM) and Generalized Additive Models (GAMs) further demonstrated species-specific responses associated with seawater intrusion volume; specifically, intrusion was negatively associated with the abundance of the rotifer Polyarthra vulgaris , whereas it was positively associated with the occurrence of the cladoceran Daphnia galeata . These findings suggest that under intermittent seawater pulses, the ecological impact of salinity is largely indirect and mediated by hydrodynamic dynamics rather than direct salinity stress. Consequently, relying solely on community-level and diversity indices may obscure subtle ecological changes, highlighting the necessity of establishing a species-level biological monitoring strategy. To reconcile ecological restoration with the protection of upstream freshwater resources, this study proposes an integrated estuarine management framework featuring adaptive gate-operation guidelines based on empirical benchmarks (approximately 500,000 m 3 of seawater inflow maintaining a 7-day bottom-layer salinity pulse within 1 km upstream) alongside event-aligned monitoring strategies.