Features of the spatial distribution of phytoplankton in Nhatrang Bay of the South China Sea during the rainy season

The species composition, phytoplankton abundance, and relative yield of the variable fluorescence (Fv/Fm) were determined in the mesotrophic Nhatrang Bay in October–November of 2004. The species diversity (250 taxonomic units) and heterogeneity of the phytoplankton structure were high. With respect to the number of species and their abundance, diatoms prevailed. In selected parts of the bay, dinoflagellates dominated. The mean biomass in the water column under 1 m2 (Bt) varied from 2.3 to 64.4 mg C/m3 being 31.0 mg C/m3 on average. The values of Bt were the lowest at the stations nearest to the river mouth. Seaward, Bt increased. The values of Bt increased with depth at some stations and decreased at others. In the surface sea layers, the biomass was lower than that in the underlying waters. The values of Fv/Fm ranged from 0.10 to 0.64 (at a mean value of 0.49). The lowest values of Fv/Fm were observed in the area close to the seaport. Over the greater part of the bay, the values of Fv/Fm were higher than 0.47. Such values are indicative of the relatively high potential photosynthetic activity of the phytoplankton. The abundance and species diversity were higher than those in the dry season (March–April).