Heavy metal contamination of mangrove sediments and the associated biota in Dar es Salaam, Tanzania

Mangrove wetlands are efficient in trapping pollutants that may have detrimental effects on mangrove dependent food chains. Mangrove ecosystems that are within urban areas are likely to suffer more from chemical pollution than those in rural areas. Heavy metals in mangrove plant parts, sediments and crabs from mbweni, msimbazi and mtoni mangrove ecosystems in Dar es salaam were analysed using an atomic absorption spectrophotometer, in order to assess the impact of heavy metal pollution on mangrove biota. Sediment samples from msimbazi and mtoni mangrove areas which are located within the city, had higher levels of pb, zn and cu than those from mbweni mangrove stand, which is far from the city centre. For instance, the concentration of pb was 31.6 ± 9.6 µgg-1 dw at msimbazi, 17.9 ± 10.7 µgg-1 dw at mtoni and 13.3 ± 3.5 µgg-1 dw at mbweni mangrove area. Crabs generally contained higher concentrations of heavy metals (pb, zn and cu) on dry weight basis compared with sediment and mangrove plant parts. Copper enrichment in crabs, for example, was more than six times compared with the concentration in sediment samples from msimbazi mangrove mangrove forest. Of the seven heavy metals (pb, zn, cu, co, ni, cr and v), only pb, zn and cu were found to be of anthropogenic origin. Overall, the mangroves and associated biota in mangrove forests, that are within the city, had higher levels of heavy metals compared with mangrove forests growing away from the city. Tanz. J. Sci. Vol.29(1) 2003: 61-7