Environmental determinants of zooplankton assemblages during summer hypoxia in the Changjiang Estuary and its adjacent waters, China

Abstract

An extensive area (6800 km2) of bottom hypoxia (dissolved oxygen concentration <2.86 mg L−1), which could be ascribed to strong stratification of the water column and bottom organic carbon oxidation, was observed in the Changjiang Estuary and its adjacent waters in July–August 2006. The relationship between environmental conditions and zooplankton community structure was studied based on samples collected from a total of 72 stations. The zooplankton community was dominated by copepods, and could be divided into five groups by CLUSTER analysis. Spatial patterns in zooplankton community were structured by a combination of sampling depth, temperature, salinity and bottom dissolved oxygen. The most dominant species Centropages dorsispinatus maintained its high abundance in the hypoxic area, perhaps due to its adaptive strategies to hypoxia. While the abundance of other taxa such as the copepod Acartia pacifica showed a positive relation to bottom oxygen levels at hypoxic stations. In general, the total amount of zooplankton biomass and diversity present in the water column were high in hypoxic areas, which was probably due to the presence of large phytoplankton stock in the surface water.