High-frequency observations of toxigenic microalgae and phycotoxins reveal contrasting HAB dynamics in the Beagle...

Harmful algal blooms (HABs) pose a recurrent threat to sub-Antarctic coastal ecosystems such as the Beagle Channel. Here, we used a 15-month high-frequency dataset (January 2021-April 2022) to assess the spatiotemporal dynamics of toxigenic microalgae and associated phycotoxins across multiple compartments, including plankton, bivalves, and passive samplers, together with concurrent oceanographic and meteorological conditions. Alexandrium catenella was the dominant toxic species, producing paralytic shellfish toxins (PST) during two consecutive austral summers with contrasting intensity. The 2021 event was moderate and spatially restricted, with A. catenella abundances reaching 4.2 × 10² cells L⁻¹, whereas the 2022 bloom was severe and widespread, with abundances up to 5.0 × 10⁴ cells L⁻¹. PST levels in plankton and mussels exceeded 2021 values by more than two orders of magnitude, reaching up to ∼3.7 × 10⁵ ng NT⁻¹ and ∼1.9 × 10⁵ µg STXeq kg⁻¹, respectively. Exploratory generalized additive models (GAMs) indicated that A. catenella variability was associated with water temperature, wind speed, ciliate abundance, predominantly positive values of the Southern Annular Mode, and seasonality, suggesting that bloom development reflected by the interaction of these environmental and biological controls rather than by a single controlling factor. During the 2022 event, planktonic PST levels varied markedly among nearby stations, whereas mussels reached similarly high toxicities, consistent with the time-integrated nature of shellfish toxin accumulation, local retention, and hydrodynamic connectivity within the same basin. Additional toxin groups, including DTX1, PTX2, SPX1, 20-Me-G, and domoic acid, were also detected, indicating the presence of multiple potentially toxigenic taxa and suggesting that future HAB risk in the region may extend beyond the historically dominant A. catenella-PST paradigm. These findings highlight the Beagle Channel as a cold sub-Antarctic HAB hotspot and support the need for integrated monitoring frameworks combining species-specific cell counts, targeted toxin analyses, shellfish toxicity records, passive sampling for dissolved lipophilic toxins, and environmental observations.