Seasonal occurrence and offshore accumulation of phycotoxins in a subtropical semi-enclosed bay: Insights from a year-round survey in Qinzhou Bay, South China Sea.

Phycotoxins produced by harmful microalgae pose increasing threats to coastal ecosystems, yet their occurrence patterns and environmental drivers remain poorly understood in semi-enclosed bays. In this study, a year-round survey was conducted in Qinzhou Bay, South China Sea from November 2021 to November 2022 to investigate the prevalence, phytoplankton sources, and environmental drivers of phycotoxins. Multiple toxin groups, including paralytic shellfish toxins (PSTs), lipophilic marine algal toxins (LMATs), and amnesic shellfish toxins (ASTs), were detected throughout the investigation, with gonyautoxins (GTX) analogues, pectenotoxin-2 (PTX2), homo-yessotoxin (hYTX), and domoic acid (DA) as dominant components. Toxin occurrence exhibited pronounced seasonality, with elevated diversity and concentrations in early summer (May-June), while offshore waters represented a relative accumulation region of phycotoxins. Integrated morphological and molecular analyses identified Alexandrium spp. as key PST producers, Pseudo-nitzschia spp. as dominant DA producers, while Dinophysis and Protoceratium reticulatum, Gonyaulax spinifera, and Lingulodinium polyedra were identified as potential sources of LMATs. Environmental analyses revealed contrasting ecological controls among toxin groups: PSTs and LMATs were strongly associated with warm, saline summer conditions and dinoflagellate proliferation, whereas DA displayed distinct environmental relationships linked to diatom dynamics. It's suggested that seasonal phytoplankton succession and environmental variability jointly regulate phycotoxin occurrence. These findings reveal clear seasonal windows and accumulation regions of phycotoxins in Qinzhou Bay. The results provide a scientific basis for harmful algal blooms monitoring, early-warning systems, and risk management in aquaculture-dominated coastal ecosystems of the Beibu Gulf.