Species diversity, taxon structure and distribution of the macroalgae in Con Co Special Administrative Region, Vietnam.

Abstract

This study presents the results of seaweed investigations conducted at four transects in the Con Co Special Administrative Region from 1994 to 2025. A total of 101 macroalgal species were recorded, belonging to four phyla. Among these, Cyanophyta comprised 5 species (4.9%), Rhodophyta 40 species (39.6%), Ochrophyta (Phaeophyta) 36 species (35.6%), and Chlorophyta 20 species (20.5%). We classified these species into 23 orders, 38 families, and 49 genera, indicating a mixed algal flora. Species richness across the studied transects ranged from 51 species (transect I) to 76 species (transect IV), with an average of 63.8 species per transect. The similarity coefficient among transects varied from 52.63 (between transects I and IV) to 78.14 (between transects IV and V), with an average value of 67.64, reflecting a moderate to high degree of similarity in species composition. In terms of ecological distribution, 20 species (19.8%) were found exclusively in the intertidal zone, 44 species (43.6%) occurred only in the subtidal zone, and 37 species (36.6%) were distributed across both zones. Most species were concentrated from the lower intertidal zone to the upper subtidal zone, highlighting this transitional area as a key habitat for macroalgal diversity. Taxonomic structure varied considerably among phyla. Rhodophyta exhibited the highest diversity, with 12 orders, 17 families, 23 genera, and 40 species, while Cyanophyta showed the lowest diversity. On average, each phylum included 5.75 orders, each order contained 9.5 families, and each phylum comprised approximately 12.25 genera and 25.25 species. Several genera contained only a single species, whereas others, such as Sargassum (10 species), Lobophora (9 species), and Caulerpa (5 species), showed higher species richness. Similarly, some families included only one genus, while others contained multiple genera. The distribution of the 23 recorded orders was uneven among the four phyla, with Rhodophyta accounting for the highest proportion (52.1%). Notably, several recorded species belong to economically important genera such as Porphyra, Grateloupia, and Sargassum, which have applications in food, pharmaceuticals, and cosmetics.