Large-scale toxic bloom of Dinophysis acuminata complex following a major flood disaster in southern Brazil.

Dinophysis species are the main producers of diarrhetic shellfish toxins (DSTs) in the Southwestern Atlantic, where recurrent blooms frequently lead to shellfish harvesting bans. In 2024, an extreme rainfall event over Rio Grande do Sul State, southern Brazil, caused one of the most severe floods ever recorded in the country, and generated an anomalous freshwater discharge through the Patos Lagoon estuary. The resulting low-salinity plume spread northward along the southern Brazilian coast, reaching aquaculture areas hundreds of kilometers away from the estuary. Here, we integrate hydrological data, satellite imagery, phytoplankton monitoring, shellfish toxin records, long-term regional datasets, and laboratory grazing experiments to characterize a large-scale toxic bloom of Dinophysis acuminata complex that developed after the flood event. Surface salinity decreased by 6-7 psu below historical values, while D. acuminata complex reached up to 6.8 × 10⁴ cells L⁻¹ further north in Santa Catarina State by mid-July 2024. The bloom caused harvesting bans in 17 of the 19 main bivalve farming areas in the state, with 61% of mussel and 43% of oyster samples exceeding the regulatory limit for DSTs, reaching concentrations as high as 4140 and 820 µg DST kg⁻¹, respectively. Comparison with long-term monitoring data from Uruguay and Brazil (Santa Catarina and Paraná states) showed that massive D. acuminata complex blooms in the region are episodic, seasonally structured, and often associated with anomalous meteoceanographic conditions. Field observations, satellite imagery and laboratory experiments further indicated that heterotrophic protists, particularly Noctiluca scintillans, may be likely contributors to bloom decline by grazing and act as transient vectors of okadaic acid within the planktonic food web. These findings highlight the vulnerability of coastal aquaculture areas to increasingly frequent and extreme hydrological disturbances, and reinforce the need for integrated analytical approaches combining microscopy, toxin analyses, remote sensing, and long-term ecological monitoring.