Spatial variation of copper and lead concentrations of water hyacinth plants in a wetland receiving urban run-off

Abstract

Copper and lead concentrations of roots of water hyacinth, Eichhornia crassipes (Mart.) Solms, in an urban wetland in Sydney, Australia ranged between 145 ± 15 and 1100 ± 145 μg g −1 dry mass for lead; and between 14.7 ± 7.0 and 303 ± 108 μg g −1 dry mass for copper, (mean ± sd, n = 6). Considerable variation at the scale of individual plants was found, about 20% relative standard deviation within a site of about 3 m radius. Samples of single or few plants are therefore unlikely to allow assessment of the pollution status of a wetland. Copper and lead had similar spatial distribution. Metal concentrations in the sediments, roots and leaves declined exponentially with increased distance from the inflow to the wetland. Distance from an inflow explained much of the variation in metal concentrations, up to: 87% for Pb and 85% for Cu in roots; 54% for Cu in leaves; 88% for Pb; and 45% for Cu in sediments. The decline in metal concentration over distance was steeper for roots than sediments. Concentrations of metals were greater for fine secondary roots than for coarse primary roots. The median and range of metal concentrations in root portions and sediments were as follows (μg g −1 dry mass): Cu fine: 21.0, 12.3–259; Cu mixed: 53.3, 18.8–296; Cu coarse: 60.8, 29.7–327; Cu sediment: 263, 190–377; Pb fine: 121, 21.8–843; Pb mixed: 257, 41.4–1270; Pb coarse: 265, 77.5–1160; Pb sediment: 1170, 590–1870. Regressions of log e-transformed metal concentrations of roots and leaves on log e-transformed metal concentrations in adjacent sediment were significant for copper in leaves and roots, and highly significant for lead in roots. This study highlights the need for estimation of within-site variation for comparisons among sites, and provides a predictive model of exponential decline of metal concentration over distance within a small urban wetland.