<![CDATA[Abstract. Hang NTT. 2026. Updated checklist and floristic composition of marine macroalgae and macroscopic Cyanobacteria in Phu Yen, Vietnam. Biodiversitas 27 (9): d270911. https://doi.org/10.13057/biodiv/d270911. Records of marine macroalgae and macroscopic Cyanobacteria from Phu Yen Province, south-central Vietnam, are distributed among field surveys, historical publications, and sources that use different taxonomic systems. This study consolidated those records, updated their nomenclature, and examined variation in floristic composition along the provincial coast. Five sampling campaigns conducted between June 2017 and May 2019 covered 19 sites encompassing lagoons, estuaries, rocky shores, seagrass beds, and coral-associated habitats. The records were assigned to nine survey areas and analyzed as occurrence-nondetection data using Sørensen similarity, UPGMA clustering, and the Cheney index. The campaigns documented 167 reconciled taxon treatments: 81 Rhodophyta, 59 Chlorophyta, 23 Ochrophyta, and four Cyanobacteria. After nomenclatural reconciliation, integration of the field data with published and historical records resulted in a provincial checklist of 195 reconciled taxon treatments. Of these, 63 field-supported taxa were additions relative to the accepted-name-reconciled 2020 provincial checklist. Recorded richness varied from 10 to 127 taxa among survey areas. Pairwise Sørensen similarity ranged from 0.034 to 0.762 with a descriptive arithmetic mean of 0.251. UPGMA clustering summarized the main differences in floristic composition, while the Cheney index of 6.09 indicated a predominantly tropical floristic affinity. The revised checklist preserves the source of each record and provides a verifiable reference for taxonomic review, biodiversity monitoring, and subsequent surveys in south-central Vietnam.]]>
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