The ecology of coastal wetland ponds created by diamond mining in southern Namibia. 2. Saltmarsh vegetation

Abstract

Diamond mining on the southern Namibian coastline has created multiple large coastal ponds of up to 380 000 m2 adjacent to the coastline, as the sea overtops erected seawalls or seeps into excavated areas. These ponds span ages of 1–38 years. We investigated whether the ponds offer an environment for the establishment, growth and dispersal of saltmarsh vegetation along the coast, which is otherwise devoid of natural wetlands apart from at the Orange River estuary and Lüderitz Bay. Most ponds supported saltmarshes, but they comprised only a single species, Salicornia natalensis. The abundance of this succulent, mat-forming, salt-tolerant plant was greatest around old ponds, but its health decreased with increasing age and hence salinity of the ponds. The orientation of saltmarshes around the ponds was correlated with prevailing wind direction, suggesting that wind determines dispersal of this plant along the coast. However, any saltmarsh communities that have developed will be disturbed by possible future mining activities. In addition, once mining ends, the saltmarshes will become stressed owing to rising salinities as ponds age. Nevertheless, the ponds are capable of supporting saltmarshes for up to 15 years, and new ponds will be created as mining progresses; this offers an ongoing opportunity for the ponds to serve as ‘stepping stones’ in the dispersal and establishment of S. natalensis along the coast.