Effects of Zinc on the Growth of Bulrush ( Schoenoplectus californicus ) and Its Distribution Between Different Sediment Fractions

Abstract

During the last decades the discharge of heavy metals into water bodies has increased dramatically. There, they interact with organic and inorganic ligands thus precipitating and accumulating in the sediments; mainly adsorbed on the fine grain components. The sediments of the littoral zone represent a sink of heavy metals that in certain conditions, can be released or recycled by the aquatic macrophytes. The water bodies of the industrial belt arround Buenos Aires receive high inputs of contaminants, resulting in high concentrations of metals in their waters. Schoenoplectus californicus is a native emergent macrophyte and its presence in the margins of rivers with high metal concentrations in Buenos Aires suggests a great tolerance to the main local xenobiotic agents. The present contribution aims at assessing zinc toxicity to S. californicus under laboratory conditions, by evaluating its absorption, tolerance, effects on growth, and metal distribution in shoot and rhizomes and in different sediment fractions. Three treatments (control, TL (low Zn level) and TH (high Zn level)) were carried out. The growth rate showed significant differences between treatments and diminished as Zn dose increased. The final biomass showed significant differences between control and treatment TH, and between treatments TL and TH. Shoot contained 5 and 23 times more Zn in treatment TL and TH, respectively, than the control, attaining 38, 210 and 881 µg Zn ug/g, respectively. Zn rhizomes concentration increased with the zinc dose, but this increase was not proportional. Zinc concentrations in rhizome were always higher than those of shoot, and were 4 times higher than shoots in control and 2 times higher than shoots in treatment TL and treatment TH. The added zinc was mainly retained in the fractions associated to carbonates and to iron and manganese oxides which represent 37 and 27% of the total in treatment TL, and 32 and 49% of the total in treatment TH, as compared to 17 and 38% of the total in the control. Because of their wide distribution and its tolerance to high zinc levels, S. californicus can be considered as a promising organism to be used in constructed wetlands for effluent treatment. Although zinc was incorporated in the form of a soluble salt, only 0.05% of the added metal finally remained in water. Therefore, wetlands of S. californicus would very efficiently improve the water quality of effluents and/or contaminated water courses. Since high zinc concentrations may be attained in shoots and rhizomes in treatment TL without any discernible effect in the final biomass, constructed wetlands would also retain high amounts of zinc in plant tissue.