Abstract: Phytoplankton community and nutrient concentration in the northern Beibu Gulf in April and August 2011 were investigated. In this research, 312 phytoplankton taxa were identified, which belonged to four phyla and 78 genera, with Bacillariophyta being the main group. The dominant species were Thalassiosira subtilis, Leptocylindrus danicus, Bacillaria paradoxa, and Pseudonitzschia pungens in spring, and Thalassiosira sp. in summer. Phytoplankton community in northern Beibu Gulf could be divided into two natural groups in spring (spr1 & spr2) and three in summer (sum1, sum2 & sum3). N/P ratio was in a relatively high level in the study area, and most of the waters were phosphorus limitated which resulted from diatom blossom. Chaetoceros debilis was the dominant species of group spr1 and had significant correlation with nutrient in spring, except for ammonium. Bacillaria paradoxa was the dominant species of group sum3 and had significant correlation with phosphate in summer, but, it was uncorrelated with any environmental factors as the dominant species of group spr2 in spring. Phosphate was transported to northern Beibu Gulf through the Qiongzhou Strait corridor water, and silicate was input by land runoff.

Key words: northern Beibu Gulf, phytoplankton, nutrient, community structure