Some aspects of heavy metal contamination of mangrove ecosystems in Dar es Salaam.

Heavy metal concentrations were determined in sediment, mangrove roots and leaves as well as in crabs from Mbweni, Msimbazi and Mtoni mangrove ecosystems in Dar es Salaam. The heavy metals were analysed using an Atomic Absorption Spectrophotometer. The results show that the mean concentration of the metals in the sediment, mangrove plant parts and crabs decrease in the order: Cr>> V > Ni > Pb > Co > Zn > Cu. Sediment samples from Msimbazi and Mtoni mangrove areas which are located within the city, had higher levels of Pb, Zn, Cu, Cr and V than those from Mbweni mangrove stand, which is at the outskirt of the city. For instance, the concentration of Pb was 31.6 ± 9.6 mg/g dry wt at Msimbazi, 17.9 ± 10.7 mg/g dw at Mtoni and 13.3 ± 3.5 mg/g dry wt at Mbweni mangrove area. Friedman's test showed that the concentrations of Pb, Zn and Cu in sediments were the only ones that varied significantly between the study sites (Fr = 12.600, P = 0.0080, Fr =10.400, P = 0.0034 and Fr = 9.800, P = 0.0063 respectively). This suggested anthropogenic input of these heavy metals. Untreated domestic and industrial wastes which are normally discharged into the rivers that drain the city are the possible sources of heavy metal contamination of mangrove stands within the city. The concentrations of Ni, Cr, Co and V in the mangrove sediment did not vary significantly among the three mangrove forests, indicating their background values in the local soils. However, the concentrations of all analysed heavy metals in root and leaf samples varied significantly between the three sites. In crab tissue samples Pb, Ni and Co were the only metals whose concentrations varied significantly between the three sites. This study recommends developing a mechanism for controlling waste discharge from residential and industrial areas so as to ensure that living organisms in mangrove ecosystems are not adversely affected by contaminants.