The effect of nutrient addition on metal tolerance in Salvinia herzogii

Abstract

Disappearance of floating macrophytes was observed in two successively constructed wetlands for the treatment of wastewater that contained Zn, Ni and Cr at a tool factory in Argentina. Experimental work was developed to test the hypothesis that nutrient enrichment enhances the metal tolerance of floating macrophytes. Relative growth rates, root biomass and chlorophyll concentration in Salvinia herzogii (water fern) were measured in greenhouse incubations exposed to different Zn, Ni and Cr concentrations, and compared with simultaneous treatments enriched with nitrogen and phosphorus. Relative growth rates were negatively correlated with metal exposures. Nutrient addition suppressed such effect. Root biomass was also negatively correlated with metal concentrations. Nutrient enrichment either attenuated (Cr and Zn) or suppressed (Ni) root biomass decrease in response to metal exposure. Chlorophyll concentration was negatively correlated with Ni exposures. Nutrient addition attenuated chlorophyll decrease in response to Ni exposure. Growth rate represented an early indicator of Cr and Zn toxicity, while growth rate and chlorophyll concentration represented suitable indicators of Ni toxicity. Metal concentration in leaves was correlated with metal exposure concentration. The effect of Zn exposure on Zn leaf concentration was attenuated by nutrient addition, apparently through P and Zn immobilization in the roots. Metal and nutrient concentration in water decreased throughout the experiments. Chromium removal from water was faster than that of Zn and Ni. Zinc and Ni were mainly sorbed by S. herzogii biomass, while Cr was also retained in the detrital fraction