Environmental Drivers and Trophic Transfer of Domoic Acid in a Eutrophic Subtropical Estuary: Linking Toxigenic <i>Pseudonitzschia</i> Dynamics to Ecosystem Risks

Domoic acid (DA), a neurotoxin produced by certain diatoms of <i>Pseudonitzschia</i>, poses significant risks to marine ecosystems and human health, yet its dynamics in subtropical eutrophic estuaries remain poorly understood. This study investigates DA production and trophic transfer in the Pearl River Estuary, combining chemotaxonomy, morphological identification, ITS1 metabarcoding, and HPLC-MS/MS analysis. We revealed strong seasonal and spatial heterogeneity in <i>Pseudonitzschia</i> assemblages, identifying <i>Pseudonitzschia cuspidata</i> Clade III as a dominant DA producer with an estimated <i>in situ</i> cellular quota of 0.1-0.8 pg cell^-1 for the community. DA was ubiquitously detected across trophic levels, with summer maxima in phytoplankton to zooplankton, crustaceans, and mollusks, exceeding safety thresholds with 24.1 mg kg^-1 in scallops. Baseline DA contamination persisted year-round, with this chronic risk amplified by increased summer diatom biomass. Crucially, DA production was governed by optimal salinity and temperature and linked to nutrient stoichiometry rather than absolute nutrient concentration; chronic high nutrient levels showed a negative correlation with DA production. These environmental drivers also influenced DA transfer efficiency, with summer conditions amplifying contamination despite sub-bloom cell densities. These findings reveal underestimated risks in subtropical estuaries, providing a critical framework for monitoring and managing DA contamination under climate variability.