Spatial and temporal variations of heavy metal contamination in sediments of a mangrove swamp in Hong Kong

Spatial and temporal variations of heavy metal contamination in sediments of a small mangrove stand in Hong Kong were examined by laying two transects perpendicular across the shore, Surface sediment samples were taken along the two transects running landward to seaward at intervals of 5 or 10 m during December 1989, and March, July and September 1990, Total concentrations of Cu, Zn, Mn and fb did not show any specific trend along each transect, although the maximum concentration of heavy metals tended to occur at the landward edge, There was a high level of variability among locations within each transect; for instance, the Cu concentrations fluctuated from 1 to 42 mu g g(-1), Certain sites contained exceptionally high levels of total metals, Total concentrations of Cu, Zn, Mn and Pf,as high as 42, 150, 640 and 650 mu g g(-1), respectively, were recorded, implying contaminated sediment. A comparison of the two transects indicated that the sediments of Transect B seemed to contain higher total Zn but lower Cu and Mn concentrations than those of Transect A, Most of the heavy metals accumulated in the sediments were not extractable with ammonium acetate and no Cu or Pb was detected in these extracts, The concentrations of extractable Zn and Mn mere low, less than 10% of the total metal concentration in the sediment, and appeared to decrease from the landward to seaward samples, For both total and extractable metals, there were significant seasonal fluctuations for both transects, but no specific trends could be identified, These spatial and temporal variations suggest that the scale and representativeness of sampling require careful planning, and a single sample might not give a satisfactory evaluation of the levels of heavy metal contamination in mangrove ecosystems.