The environmental significance and low-oxygen indicator potential of benthic foraminiferal assemblages in surface sediments off the Changjiang River (Yangtze River) Estuary, China

86 surface sediment samples and a core (CDZS38) from the Changjiang River Estuary (CRE) and its adjacent area were collected to investigate the environmental significance of foraminifera, and determine a microfaunal index for coastal hypoxia to be used for paleoenvironmental studies. Benthic foraminifera and their relationship with environmental parameters including grain size composition and total organic carbon in surface sediment samples were studied. The benthic foraminiferal assemblages were dominated by littoral species in the study area. R-mode cluster analysis identified three clusters based on the percentage of benthic foraminifera. Cluster 1 covers the transitional area between the Radial Sand Ridges of the South Yellow Sea and the CRE. The dominant foraminiferal assemblage is Ammonia beccarii vars.–Ammonia compressiuscula–Quinqueloculina spp., which is a good environmental indicator of an oligotrophic near-shore condition with a strong hydrodynamic force. Cluster 2 covers the inner-shelf off the CRE. The dominant foraminiferal assemblage of Cluster 2 is Epistominella naraensis–Bolivina cochei–Bulimina marginata, which can be used to represent a eutrophic, hypoxic inner-shelf environment. Cluster 3 covers the Hangzhou Bay, which is a typical macro-tidal river mouth. The dominant foraminiferal assemblage of Cluster 3 is A. beccarii vars.–E. naraensis–Ammonia convexidorsa, which is indicative of an environment that is strongly disturbed by tidal currents and influenced by substances from the Changjiang River. The occurrence of E. naraensis, B. marginata and B. cochei is always associated with the hypoxic region. With the correlation analysis with environmental parameters, these species can be considered low-oxygen foraminiferal assemblages (LOFA). We generated a low-oxygen foraminiferal index (the ratio of LOFA to the sum of LOFA and Euryhaline species) to explore the development of hypoxia at the inner-shelf off the CRE, and applied this index to the short core CDZS38. The result indicated that high LOFI values found for CDZS38 is consistent with the recorded trends in hypoxia and harmful algae bloom (HAB) for this area over the past 59 years, and reflect an increase in the hypoxic conditions over this time period.