Biodiversity and distribution of benthic dinoflagellates in tropical Zhongsha Islands, South China Sea

Benthic dinoflagellates have attracted increasing attention in recent years because of their toxicity and ability to form extensive harmful algal blooms. Ostreopsis producing palytoxin and its analogs, Gambierdiscus producing ciguatera toxins, and Prorocentrum producing okadaic acid and dinophysis toxins, have been concerned as serious human poisonings. We explored the benthic dinoflagellate biodiversity and distribution characteristics of a series of tropical reefs in 20–40-m water depth in wet season in the Zhongsha Islands in South China Sea using morphological, phylogenetic, and cell counting methods. Results show that benthic dinoflagellates in the islands are rich in biodiversity and 15 species from genera Amphidinium, Coolia, Ostreopsis, and Prorocentrum were identified: Amphidinium carterae, A. magnum, A. massartii, A. operculatum, Coolia canariensis, C. malayensis, C. palmyrensis, C. tropicalis, Ostreopsis cf. ovata, Prorocentrum concavum, P. cf. sculptile, P. emarginatum, P. hoffmannianum, P. lima, and P. rhathymum. Among them, A. magnum is reported for the first time in Chinese waters. The abundance of benthic dinoflagellates was relatively low at 88–4 345 cells/100 cm2 on sediment and 10–91 cells/g on macroalgae. Prorocentrum and Amphidinium were the dominant and subdominant genera, respectively. It is speculated that the low abundance of benthic dinoflagellates is closely related to the scarcity of macroalgae and stronger water motion at the depth >15 m in Zhongsha Islands. This study expanded the study in biodiversity of benthic dinoflagellates in Chinese waters, and revealed the distribution characteristics of harmful benthic microalgae in reef habitats.