The ecology of coastal wetland ponds created by diamond mining in southern Namibia. 1. Physical conditions

Abstract

Coastal diamond mining in southern Namibia involves constructing seawalls to hold the sea at bay, and seaward accretion of the shoreline by up to 800 m opens what was previously the surf zone for excavation and extraction of bedrock alluvial diamonds. This has created large coastal wetland ponds of up to 380 000 m2 as the sea overtops the seawalls or seeps into the excavated areas. The ages of these ponds span 1–38 years. We investigated physical conditions in the ponds to determine whether they can function as saline wetlands equivalent to blind estuaries. Water temperatures were 6–10 °C higher than in the sea, as expected of shallow enclosed waterbodies. Dissolved oxygen was 82–137%, peaking at midday owing to photosynthesis, and the ponds were never hypoxic. Correlated with oxygen levels, pH values spanned 7.7–8.3, and always exceeded the pH of seawater. Chlorophyll a concentrations matched or exceeded the levels in seawater, reaching 76 μg l⁻¹. The southern and central ponds had salinities close to those of seawater, but the salinity of northern ponds exceeded 80 after ~15 years, thus limiting their capacity to support wetland communities. Apart from this, these ponds are viable habitat that can support flora and fauna typical of saline wetlands, a habitat that is scarce along this arid coastline.