Heavy metals in Metapenaeus ensis, Eriocheir sinensis and sediment from the Mai Po marshes, Hong Kong

The Mai Po Marshes Nature Reserve in north-west Hong Kong is a Ramsar site visited by some 60 000 migratory birds each year. Due to increasing industrialization around the Deep Bay area, the environment at Mai Po may be threatened by heavy metal contamination. In this study, concentrations of four heavy metals (chromium, lead, iron and zinc) were determined in sediment samples and in tissues of the commercial shrimp Metapenaeus ensis and the Chinese mitten crab Eriocheir sinensis. Samples were obtained from four gei wais (8, 11, 16/17 and 19) running along a north-south transect within the nature reserve. Measured concentrations (μg metal g^-1 dry wt.) in sediment, M. ensis and E. sinensis tissues were: for Cr, 49.6-559.1, 1.5-3.5 and 6.0-6.2; for Fe, 3168.1-15478.7, 91.8-180.3, 845.2-2304.0, for Pb, 100.5- 154.8, 8.4-11.7, 15.9-19.2 and for Zn, 226.4-421.1, 75.5-96.8 and 163.7- 187.1, respectively. Concentrations of the heavy metals in M. ensis were all below the legislation limit but those in E. sinensis were slightly in excess. Analysis of variance showed no significant differences between sites in the trace metal concentrations in M. ensis tissues. Significant differences between sites were, however, obtained for Fe concentration in E. sinensis and for concentrations of Fe, Cr and Zn in sediments. Discriminant function analysis on the metal concentrations in M. ensis and E. sinensis tissues separated gei wai 16/17 from the other gei wais, mostly on account of the lead concentration in M. ensis and the Zn concentration in E. sinensis. For the sediment samples, gei wais 8 and 11 were well separated from gei wais 16/17 and 19 with both Cr and Fe concentrations contributing significantly to the discrimination of gei wais.