Ecological Studies On Plankton In Northern Beibu Gulf

The species composition, abundance, biomass, dominate species, diversity and heterogeneity of plankton in northern Beibu Gulf was studied based on the samples of Beibu Gulf collected during2006～2007. And impacts of seasonal variations of environmental factors on plankton community were discussed. The main results are as follows:1. Ecological study on phytoplankton community in northern Beibu Gulf(1) A total of273phytoplankton species were identified (included variety and form), which belonging to5phyla,79Taxa. Bacillariophyta and Pyrrophyta were the main phytoplankton groups in the survey area. The species number of zooplankton, varied seasonally, was highest in summer, followed by autumn, winter and spring. The cosmopolitan group and the subtrophic-inshore group were dominant in this area. Nutrient was the main factors on phytoplankton composition.(2) The average of abundance was(2207.3±6124.9)×104cells/m3, varied in the order of summer, autumn, winter and spring. The average concentration of chlorophyll a was (1.9±1.3) mg/m3, varied in the order of autumn, winter, summer and spring. The horizontal distribution of abundance and chlorophyll a was quite different. Salinity and nutrient were the main factors on phytoplankton abundance.(3) Thalassionema nitzschioides dominated in four seasons. Skeletonema sp. and Phaeocystis sp. bloomed respectively in summer and autumn in the inshore of Guangxi with a potential of red tide. The limiting factor of Skeletonema sp. was silicate, while the limiting factor of Phaeocystis sp. was phosphate. The dominant species changed dramatically during four seasons.(4) According to cluster and ordination methods, three phytoplankton communities could be distinguished as inshore community, the mouth of Qiongzhou Strait community and offshore community. The nutrient structure and phytoplankton community were influenced by runoff, the coastal water of western Guangdong and open sea water.2. Ecological study on zooplankton community in northern Beibu Gulf(1) A total of464zooplankton taxa were identified (including pelagic larvae), which belonging to7phyla,19Taxa. Hydromedusae and Copepoda were dominant taxa. The species number of zooplankton, varied seasonally, was highest in summer, followed by autumn, winter and spring. The wide-temperature-salinity group and the inshore low-salinity group which were dominant in this area formed a uniquely northern Beibu Gulf zooplankton community composition structure with few estuarine group and oceanic group. Topographic feature, currents and water masses, phytoplankton were the main factors on zooplankton composition.(2) The average of abundance was (183.5±142.9) ind/m3, while the average biomass was (139.5±116.8) ind/m3. There was a gradual increase in the abundance from inshore to offshore in spring and autumn, while the abundance was high inshore in summer and winter. The abundance was the highest in autumn, followed by summer, winter and spring. Biomass horizontal distribution was similar with abundance, but varied differently during four seasons, in the order of spring, autumn, winter and summer. The correlation between abundance and environmental factors was significant. Depth, temperature and chlorophyll a were the main factors.(3) Flaccisagitta enflata, Subeucalanus subcrassus, Macrura larvae and Ophiopluteus larvae dominated in northern Beibu Gulf especially. The dominant species changed dramatically during four seasons. The ecological adaptation of dominant species was wide.(4) According to cluster and ordination methods, three zooplankton communities could be distinguished as inshore community, Leizhou Peninsula community and offshore community. All these communities were associated with terrestrial runoff, the coastal water of western Guangdong and open sea water. The diversity, dominant species and the best indicator species of each community varied seasonally.3. Coupling of nutrient, phytoplankton and zooplankton(1) The characteristics of Skeletonema sp. bloom region was described as follows, nutrients concentration was high, species number of phytoplankton was low, while abundance of both phytoplankton and zooplankton were high. The Skeletonema sp. bloom was disappeared in the critical region and the dominant species was changed to Thalassionema nitzschioides. Silicate had limited effect on Skeletonema sp.. The high abundance region of Lucifer hanseni and Skeletonema sp. has had a certain degree of overlap.(2) The bloom region of Phaeocystis sp. was overlapped with the low biomass region of zooplankton. The concentration of nitrogen and phosphate was lower than the threshold of phytoplankton. Phosphate was highest absorbed, followed by nitrogen and silicate. Phosphate had limited effect on Phaeocystis sp..