Trophic transfer of heavy metals through aquatic food web in the largest mangrove reserve of China.

Understanding the mechanism of trophic transfer of heavy metal through the aquatic food web is critical to ecological exposure risk assessments in mangrove ecosystems. Zhanjiang Mangrove National Nature Reserve (ZMNNR) is the largest and biologically richest mangrove reserve in China, but has been exposed to heavy metal pollutants caused by the progressive industrialization and urbanization. We collected a variety of aquatic consumers, and primary producers, as well as sediments from the ZMNNR and analyzed them for heavy metal (Cd, Cr, Cu, Ni, Pb and Zn) concentrations, and for both δC and δN values to establish the trophic levels. The trophic magnification factors (TMF) of Cd, Cu and Zn are 0.19 (p < 0.01), 0.07 (p < 0.01) and 0.33 (p < 0.05), respectively, indicating significant biodilution in a simplified food web composed of bivalves, crustaceans and fish. There are also potential tendencies of biodilution for Cr, Ni and Pb. Comparison of heavy metals in representative fish and shrimp in the ZMNNR with those in worldwide mangroves indicate a low risk level for aquatic consumers in our ecosystem. Quantitative source tracking is conducted based on principal component analysis and cluster analysis, which indicate that Cr, Ni and Pb are mainly originated from natural geological processes, Cu and Zn from shrimp farming and agriculture activities, and Cd from the deposition of aerosol released by regional metal smelting industry.