Macophyte cycling of zinc, copper, lead and cadmium in the littoral zone of a polluted and a non-polluted lake. II. Seasonal changes in heavy metal content of above-ground biomass and decomposing leaves of Phragmites australis (Cav.) Trin.

Abstract

The seasonal changes in heavy metal content of the above-ground biomass and decomposing leaves of Phragmites australis (Cav.) Trin. from the littoral zone of the eutrophic, sewage-polluted Lake Sortesø and the oligotrophic, non-polluted Lake Hampen, were investigated. It was found that the concentration of Zn in the leaves and stems and of Cu in stems is at a maximum during the growth season and decreases thereafter. The concentration of Cu in the leaves was more or less constant during the study period. The concentration of Pb in the leaves increased during and after the growth season, possibly as a result of atmospheric deposition. In decomposition experiments it was found that leaves decompose some three to seven times faster than the stems. Stems decayed significantly faster in Lake Sortesø than in Lake Hampen, whereas decomposition of leaves was only significantly faster in Sortesø during the first six months of the study period. Concentrations of Zn, Pb, and Cd increased significantly in Phragmites leaves during decomposition. The actual contents of Zn and Cu were either relatively constant or decreased, whereas those of Pb and Cd increased. A simplified model showing the cycling of heavy metals in the littoral zone of the two lakes indicates that Lake Hampen is more sensitive to pollution with heavy metals than Lake Sortesø.