Variation and distribution characteristics of phytoplankton in ecology-monitoring area of Hangzhouwan Bay from 2005 to 2008

Hangzhouwan Bay is a typical strong-tide bay in bell-mouth shape located to the northeast of Zhejiang Province,south of Shanghai and west of the East China Sea.It is also an ecological barrier in guarding against and mitigating marine disasters.Phytoplankton is the base of marine ecosystem.The study of its distribution,biodiversity and variation trend is of great importance for the assessment of ecosystem health and maintenance of the ecological safety and sustainable development of the ecosystem in Hangzhouwan Bay.On the basis of the data of phytoplankton collected in this ecology-monitoring area in April and August,2008,combined with the data from 2005 to 2007,the species composition,abundance distribution,diversity and their inter-annual variation are analyzed.The results showed that of the 61 identified species of phytoplankton that belong to 29 genera of 5 phyla,Bacillariophyta was the richest with 49 species from 21 genera,accounting for 80.3 % of the total species number,and the next was Pyrrophyta with 7 species from 4 genera accounting for 11.5 %;Cyanophyta 2 species from 2 genera accounting for 3.3 %;Chlorophyta 2 species from 1 genus accounting for 3.3 % and Chrysophyta 1 species from 1 genus accounting for 1.6 %.The dominant species of the phytoplankton in April were Skeletonema costatum,Coscinodiscus jonesianus,Biddulphia sinensis,Coscinodiscus oculus-iridis and Nitzschia lorenziana.But in August they were S.costatum,C.jonesianus,and Chaetoceros abnormis.S.costatum and C.jonesianus were not only the dominant species of phytoplankton in this area,but also the dominant species in the bays along the coastal areas of the Yellow Sea and the East China Sea.If the surveyed area was divided into southern and northern parts with the line between sampling stations S13 and S36 as the boundary,the phytoplankton abundance in April was higher in the southern part than in the northern part and contrary in August,which showed an evident seasonal variation of the phytoplankton abundance distribution.The biodiversity were low in this area with the diversity indexes of 1.31 and 1.58 in April and August respectively.The evenness indexes in April and August were low too,which were 0.324 and 0.413 respectively.Comparison with historical data(from 2005 to 2008) showed that the phytoplankton community composition was not stable,the dominance of single dominant phytoplankton was becoming increasingly evident and the biodiversity index presented a declining tendency in Hangzhouwan Bay.