Meiofauna distributions at the oxygen minimum zone in Changjiang (Yangtze) River Estuary waters

A quantitative study on meiofauna was carried out along a transect throughout the Changjiang Estuary's oxygen minimum zone (OMZ) in the East China Sea.There exist two distinct station groups in the OMZ:the fine-grained hypoxic area and the more fine-grained anoxic area.Meiofauna abundance ranged from (101.5±31.0) ind./cm² (hypoxic area) to (369.9±123.9) ind./cm² (anoxic area) along the transect.Free-living marine nematodes were the numerically dominant meiofauna at every station.The anoxic area differed significantly in meiofauna abundance and vertical distribution pattern from the hypoxic area.Within the anoxic area, nematodes abundance increased and amounted to over 90% of the total meiofauna; about 50% of nematodes were found in the 2~5 and 5~8 cm layers.At hypoxic stations, about over 85% were restricted to the top 2 cm.Benthic copepod abundance and dominance decreased consistently with the oxygen gradient.The pattern of meiofauna biomass was similar to that of abundance.The correlation analysis of the meiofauna numbers and environmental parameters indicated that abundance and biomass of total meiofauna and nematodes had significant or highly significant (P<0.05 or P<0.01) correlations with Chl a and Pha a, but no significant (P>0.05) correlations with bottom-water oxygen (BWO₂).On the other hand, there was a highly significant positive correlation between benthic copepod abundance and bottom-water oxygen (P<0.05).The investigation on the nematode community structure indicated that two different nematode communities existed in hypoxic and anoxic areas.In certain way, the shift in nematode species composition distinguishes the anoxia to hypoxia.Nematode diversity was generally lower within the anoxic stations than the hypoxic ones, but the difference was not significant.Indices of the nematode community structure varied in relation to the bottom-water oxygen together with bottom-water temperature, Chl a concentration and median grain size of sediment.