Spatiotemporal distribution dynamics of heavy metals in water, sediment, and zoobenthos in mainstream sections of the middle and lower Changjiang River.

Water, sediment, and zoobenthos are crucial carriers and storage media for heavy metal migration and transformation. The distribution characteristics of heavy metals in water, sediment, and zoobenthos can reflect their pollution status and potential influences on the health of aquatic ecosystems. On the basis of monitoring data related to Cu, Zn, Pb, Cd, Ni, Mn, Hg, and As in mainstream water, surface sediment, and zoobenthos at eight sections-from Wuhan to Shanghai-of the Changjiang River (also known as the Yangtze River) and historical monitoring data on heavy metal distributions in different environmental media of the Changjiang River since the 1980s, this study undertook systematically analyzed the spatiotemporal distribution dynamics, pollution levels, and corresponding environmental risks related to heavy metals in water, sediment, and biota, and examined the effects of pollution source variations and water and sediment regimes on heavy metal distribution. Heavy metal concentrations in the waters were much lower than the water quality threshold of Grade III of the Chinese Surface Water Environmental Quality Standard (GB3838-2002); the concentrations of different heavy metals were irregularly distributed and varied significantly along the river mainstream; Cu, Cd, Zn, and Pb concentrations in sediment all exceeded their respective critical effect concentrations and exposed the ecosystem to pollution risks; the pollution levels of and ecosystem health risks posed by Zn and Cu in zoobenthos were high, with pollution concentrations ranked as shrimp > snails > crabs. In general, heavy metal concentrations in the three environmental mediums were ranked as sediment > zoobenthos > water. Finally, comparison with historical monitoring data revealed increasing Cd and Hg concentrations, with the average heavy metal concentration in sediment reaching its maximum value in the 2000s in the middle and lower reaches of the Changjiang River, which are subjected to the combined effect of pollutant emissions and changing water and sediment regimes.