Transboundary pathways of Dinophysis acuta in the Iberian upwelling system using biophysical modeling.

Harmful algal blooms (HABs) pose major environmental, socio-economic, and health risks, mainly due to toxins produced by microalgae such as Dinophysis acuminata and D. acuta, responsible for diarrhetic shellfish poisoning. These blooms are particularly problematic in shellfish production areas, including the Galician Rías Baixas (NW Spain), at the northern limit of the Canary Current Upwelling System. This region, one of the world's leading mussel producers, suffers recurrent economic losses from HABs. Blooms of D. acuta typically occur from late summer to autumn, and when preceded by D. acuminata, harvesting closures are extended. To investigate the role of alongshore transport in D. acuta dynamics, we applied a biophysical modeling approach combining a high-resolution hydrodynamic model (CROCO) with a lagrangian model (ROMSPath), covering August-November of 2015, 2017, and 2019. Monitoring programs in Portugal and Spain confirmed D. acuta presence in Aveiro (Portugal) and the Rías Baixas (Spain), with cells also detected at intermediate stations in 2015 and 2019, but not in 2017. Lagrangian simulations, validated with monitoring data, revealed poleward transport in 2015 and 2019, driven by alternating wind relaxation and southerly winds that promoted a coastal current extending from the shoreline to the 50 m isobath. In contrast, persistent northerly winds in 2017 prevented the onset of this current, consistent with the absence of cells at intermediate locations. The integration of monitoring and biophysical modeling improves understanding of HAB transport pathways across transnational aquaculture areas, underscoring the need for international cooperation to mitigate their impacts on the shellfish industry.