Distribution Of Benthos And Its Relation With Sediment And Hypoxia In The Changjiang Estuary

Benthic species composition, distribution and amount variety are closely related to the environment. In the Changjiang Estuary, the river brings enormous fresh water, sediment, nutrient, organic pollution and heavy metal. These factors have significant impact on the ecosystem of the estuary and alongshore sea area. And anthropic interruptions like large engineering project in the estuary also affect the ecosystem, considerably. So the environmental protection of the Changjiang Estuary and its adjacent sea area has become an urgent task, which needs serious study. Knowing the benthic species composition, distribution and biomass could be the first step and fundament for further study and protection of the benthic environment.This study observed and sampled the Changjiang Estuary and its adjacent sea area (divided to three sub area: area I, area II and area III), analyzed the benthic biomass, species composition by taxonomy and calculating and analyzed other indices such as dissolved oxygen concentration in the bottom water, heavy metal and sulphide concentrations in the surface sediment. And the results show below.86 species of benthic animals were collected. The average benthic biomass of the whole study area was 9.55 g/m~2, with the three regions' average were respectively 12.45 g/m~2 in area III, 8.52 g/m~2 in area I and only 0.53 g/m~2 in area II. As for abundance of benthos, the total average of inhabiting density was 86 inds/ m~2.Thereamong, the area I had the highest density ( 138 inds / m~2) and the densities of area III and area II are 33 inds / m~2 and 4 inds / m~2 respectively. When it comes to the item of diversity indices, the benthic diversity index of area III was 1.28, but both area I and area II 's diversity index were less than 1, especially area II (0.33). Compared with the data of last 10 years, these figure indicated that the desertification of benthic environment in the study area was still severe, all benthic species were absent in about one third of the study sites in area I and area II. Fortunately, the trend of deterioration was contained slightly.In the study sea area, where have the relatively high benthic biomass and inhabiting density, are also where the dissolved oxygen concentration were relatively low, if not the lowest. This indicates that the seasonal hypoxia in the Changjiang Estuary did not, like some researchers claimed, destroy the benthic ecosystem. But it affects the benthic ecosystem indeed. In these sites, some species, which could endure the low oxygen concentration (hypoxia), became the dominant species, such as Heteromastus , Capitella and Asychis, while some species like Gammarus is absent in the hypoxia area, and can only be found at the sites have relatively high oxygen concentration.When considering the sediment, all the sediment sampled belongs to the first class sediment according to the state criteria. The distribution of the benthos did not show clear impact by the concentration of heavy metal in the sediment. However, the element Hg showed stronger toxicity to the benthos than other heavy metals that were observed. The hypoxia area had higher concentration of sulphide, because hypoxia could facilitate sulphide precipitation and conservation.