A 2-year study on the spatio-temporal changes in trace metal concentrations in sediment, water and plants within the Walvis Bay Lagoon, Namibia

Abstract

Most pristine coastal ecosystems are at threat from pollution due to the increasing anthropogenic activities such as mining, recreational activities, and human settlements that discharge waste as effluents. In Namibia, the Walvis Bay Lagoon is regarded as one of the most important wetlands for birds along the Southern African coast and has been designated as wetland of international importance, which makes its protection from pollution very critical. To date, no studies have focused specifically on the potential accumulation of heavy metals in the Walvis Bay Lagoon environment under the current increase in industrial activities around the lagoon. It is therefore imperative to monitor the changes in industrial pollutants such as toxic heavy metals within the Walvis Bay Lagoon, which is critical in evaluating the overall lagoon ecosystem health. Therefore, this study monitored changes in heavy metals (As, Cr, Cd, Ni, Pb and Zn) in sediment, water and Salicornia spp. plants collected from the Walvis Bay Lagoon. The highest concentrations of the heavy metals were found in the sediment in transects located furthest away from the lagoon mouth, though with values below minimum threshold values. For sediment samples, the maximum average concentration recorded was Cr (31.3 mg/kg); Ni (8.03 mg/kg); Pb (27.9 mg/kg); Zn (59.4 mg/kg); As (6.14 mg/kg); whilst for water samples it was As (0.06 gm/L); Cd (0.11 mg/L); Ni (0.68 mg/L) and Pb (1.49 mg/L). It was noted that the stations further away from the lagoon mouth had higher concentrations of most metals, indicating the importance of lagoon recharge and discharge. Sedimentation with clay in areas furthest away from the lagoon mouth could be resulting in higher heavy metal retention within the lagoon and thus periodic monitoring of both heavy metals and clay sedimentation is required. Based on this baseline data, the heavy metal concentrations within the lagoon indicate a healthy lagoon, with no immediate risk posed by the heavy metals. However, this study was only undertaken over two years and more time will be required to monitor the long-term changes in these trace metals to have more policy influencing data, hence continued monitoring is recommended. Monitoring of sediment, water and plants should also be coupled with sampling of organisms like fish, crustaceans and birds, that are found within the lagoon to evaluate any correlations between the source concentrations and that in the organisms.