Effects of Wastewater-borne Heavy Metals on Mangrove Plants and Soil Microbial Activities

Effects of wastewater-borne heavy metals on growth of young plants (9-month-old Bruguiera gymnorrhiza) and soil microbial activities in mangrove microcosms were evaluated. During the 26-week loading period, each mangrove microcosm received 31.2 litres synthetic wastewater of three strengths: normal, medium (5 times of normal strength) and strong (10 times of normal strength). Normal strength wastewater had Cu, Zn, Cd, Cr and Ni concentrations of 3, 5, 0.2, 2 and 3 mg l⁻¹, respectively. Plant growth and total plant biomass in wastewater-treated microcosms were lower than that in the control, and the maximum reduction was found in microcosms receiving strong wastewater. Alkaline phosphatase activity and ATP contents of the mangrove soils receiving wastewater were also reduced. More than 95% reduction in these two parameters was found in soils loaded with strong wastewater. Microtox test demonstrated that soil elutriates obtained from microcosms receiving strong wastewater were of the greatest toxicity (EC₅₀ was 23%). These results show that high concentrations of heavy metals present in strong wastewater were toxic and posed negative effects to both mangrove plants and soil microbial activities. Microbial activities were generally more sensitive to the toxicity of heavy metals than plants.