Accumulation and biological cycling of heavy metal elements in Rhizophora stylosa mangroves in Yingluo Bay, China

Absorption, accumulation and dynamics of 7 heavy metal elements were studied in mature Rhizophora stylosa mangroves in Yingluo Bay, China. The concentrations of Cu, Pb, Zn, Cd, Cr, Ni and Mn in the sediments of the mangrove forest were determined to be 18.9, 10.0, 46.6, 0,077, 9.27, 14.6 and 89.5 μg g^-1 dry wt, respectively. Evident differences of heavy metal concentrations have bean observed in different parts of the plant, and the weighted means were 0.98. 0.86, 4.91, 0.11, 0.50, 1.06 and 14.60 μg g^-1 dry wt for Cu, Pb, Zn, Cd, Cr, Ni and Mn respectively. In natural habitat conditions, the ability of R, stylosa to absorb each heavy metal except Cd was very low, and the weighted average accumulation indices of Cu, Pb, Zn, Cd, Cr, Ni and Mn in the plant to those in soil were only 0.05, 0.09, 0.11, 1.40, 0.05, 0.07 and 0.16, respectively. The heavy metal concentrations in residues on the floor were much higher than those in the plant and in litter falls. The total amounts of Cu, Pb, Zn, Cd, Cr, Ni and Mn in the standing crop of the forest were 28.73, 25.25, 143.68, 3.14, 14.61, 30.87 and 430.39 mg m^-2, among which about 90 to 96% of the total amounts was stored in the parts difficult to consume by the animals. The storages in residues on the floor were 271.7, 323.5, 1983.8, 8.2, 34.4, 63.0 and 1776.9 μg m^-2, respectively. With the development of leaves from young to old, Cu, Zn and Ni can be translocated and reused before defoliation, while amounts of other heavy metal elements increased in older leaves. In the biological cycle of 7 heavy metals in the forest the following was observed: (1) the annual uplakes of Cu, Pb, Zn, Cd, Cr, Ni and Mn by the forest were 1351.7, 1613.1, 8808.4, 240.7, 759.5, 1627.7 and 53692.0 μg m^-2; (2) annual retentions were 842.5, 806.9, 4694.2, 95.0, 464.9, 1054.0 and 14038.9 μg m^-2, respectively; (3) annual returns were 509.2, 806.2, 4114.3, 146.0, 294.6, 573.7 and 3965.3 μg m^-1, respectively. The estimated turnover periods were 56, 31, 35, 22, 50, 54 and 11 yr for CU, Pb, Zn, Cd, Cr, Ni and Mn, respectively.