Study on relation of distribution of benthos and hypoxia in Yangtze River Estuary and adjacent sea

The benthos at 40 sites in the Yangtze River Estuary and adjacent sea area(the whole study area was divided into three main regions,the Yangtze River Estuary,Hangzhou Bay and Zhoushan sea area) were investigated by the taxonomy and calculating the composition,abundance,biomass and diversity indices in summer 2005.86 species of benthic animals were collected.The average benthic biomass in the whole study area was 9.55 g/m2,with the three regions' average were,respectively,12.45 g/m2 in Zhoushan sea area,8.52 g/m2 in the Yangtze River Estuary and only 0.53 g/m2 in Hangzhou Bay.As for abundance of benthos,the total average of inhabiting density was 86 inds/ m2.Thereamong,the highest density(138 inds / m2) is in the Yangtze River Estuary and the densities in Zhoushan sea area and Hangzhou Bay are 33 inds / m2 and 4 inds / m2,respectively.When it comes to the item of diversity indices,the benthic diversity index of Zhoushan sea area was 1.28,but both the Yangtze River Estuary and Hangzhou Bay's diversity index were less than 1,especially Hangzhou Bay(0.33).Comparing with the data of last 10 years,these figure showed that the desertification of benthic environment in the study area was still severe,all benthic species were absent in about one third at study sites in the Yangtze River Estuary and Hangzhou Bay.Fortunately,the trend of deterioration was contained slightly.The sites with the occurrence of hypoxia have extraordinary high benthic biomass and inhabiting densities.