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- Understanding planktonic diatoms to inform site selection for shellfish offshore aquaculture systemsPublication . Araújo Freitas, Carla Alexandra; Goela, Priscila; Icely, John; Dias Duarte Fragoso, Bruno; de Oliveira Júnior, Luciano; Cristina, Sónia; Newton, AliceSustainable bivalve aquaculture offers a solution to rising food demand. This study aimed to help identify suitable offshore shellfish aquaculture sites in coastal upwelling regions. Here, planktonic diatom abundance, crucial for bivalve growth, is a key success factor. A successful bivalve production area was analysed to understand how diatom dynamics and upwelling influence a suitable location for shellfish offshore aquaculture. A diatom-based criterion for site selection is proposed. Surface water samples were collected over two years in southwest Iberia, near a major upwelling centre. Phytoplankton abundance and composition were assessed along with chlorophyll-a concentration and wind-stress upwelling indices. Episodes of high diatom concentrations were frequent and distributed throughout the year. Most diatom blooms were dominated by Pseudo-nitzschia, particularly the delicatissima group. Although this genus is potentially toxic, the levels of amnesic shellfish poisoning (ASP) toxin-the domoic acid (DA) in bivalves were mostly low or undetectable, even during periods of high Pseudo-nitzschia abundances. Conversely, the less abundant Pseudo-nitzschia seriata group presence often coincided with detectable DA in bivalves. These findings suggest that high Pseudo-nitzschia abundance does not hinder aquaculture and may even benefit it when the delicatissima group prevails in high numbers. Focusing on the Pseudo-nitzschia seriata group in predictive models may better predict DA levels for harvest closures, minimising economic losses and health risks. This is particularly true for locations where the seriata group is the one linked to DA production. Additionally, we identified oceanographic conditions coinciding with DA levels exceeding regulatory limit.