Fig. 1

Distribution of sampling sites in the Yangtze River Estuary. Sampling region S1 represents sampling sites 1–3 from freshwater upstream, sampling region S2 represents sampling sites 4–7 in the south tributary transition area, sampling region S3 represents sampling sites 8–11 in the area between the south port and north port, and sampling region S4 represents sampling sites 12–14 in the north branch of the estuary area