Species composition and diversity of Euphausiacea in the East China Sea

Based on the data of four seasonal oceanographic censuses in the East China Sea(23°30'~33°00'N,118°30'~128°00'E) in 1997~2000,the species composition and the diversity of Euphausiacea were discussed as well as their relations with environmental variables.Results showed that there were totally 23 different species of Euphausiacea,in which 16 occurred in spring and autumn respectively; 15 were present in summer and only 10 were observed in winter.According to the calculated alternation fraction(R),the species composition showed a clear seasonal alternation with the changes ofseasons.Moreover,the environmental variables had different impacts on the distribution of Euphausiacea in different seasons.The distribution in summer was not significantly related to water temperature and salinity.However,the surface salinity was a major determinant of the distribution in spring.In autumn,both surface and bottom temperatures were influencing factors.The distribution in winter depended on salinity at the surface and 10 m depth as well as the temperature at 10 m depth.Regarding to the seasonal variation of species composition,the variations in spring,summer and autumn were not so significant as those in winter.Except in summer,the species number changed with synchronous water temperature and salinity,as a result of the presence of warm currents in the East China Sea and the habitability of the dominant species.Since Euphausiacea tend to agglomerate,the distribution of different species was uneven,which was the major reason for the low diversity of Euphausiacea in the East China Sea.