Abstract: Experimental enclosures were set up in prawn (Litopenaeus vannamei) culture pond to study the community composition and its dynamics of phytoplankton at low salinity levels (2‰-6‰). A total of 86 phytoplankton species were identified, belonging to 7 phyla and 58 genera. There were 50 species of Chlorophyta (581% in species abundance), 18 species of Cyanophyta (209%), 9 species of Bacillariophyta (105%), 4 species of Euglenophyta (47%), and 3 species of Cryptophyta (35%). Dictyosphaerium pulchellum, Ankistrodesmus angustus, Scenedesmu sp., Microcystis aeruginosa, Cylindrospermopsis raciborskii, Cyclotella meneghinian and Chroomonas acuta were the dominant species. In the early period of phytoplankton cultivation, the majority of the dominant species was of Chlorophyta, and its subrogation was rapid; while in the mid and late periods, Cyanophyta was in predominance, and M. aeruginosa was the dominant species in 60% of the enclosures. The average dominance of dominant species was 596%. The phytoplankton had an average density of 297×107 cells·L-1, and its Shannon-Wiener diversity index was 218. In the enclosures applied with organic manure, phytoplankton species were abundant, diversity and density were higher, and the community was stable, being favorable for prawn growth. Applying large amount of chemical fertilizers was liable to cause harmful M. aeruginosa bloom.

Key words: Rock plant, Pogonatherum paniceum, Growth character, Biomass, Root-shoot ratio