Effects of sediment discharge from Namibian diamond mines on intertidal and subtidal rocky-reef communities and the rock lobster Jasus lalandii

Extensive terrestrial diamond mining occurs on the southern coast of Namibia, and at Elizabeth Bay near Lüderitz sediment tailings totalling about 2 million tons.yr−1, have been discharged onto the beach. We report here on monitoring spanning 2004–2012 to assess (1) the impacts of increased tailings discharges following an expansion of the mine in 2005, and (2) recovery after discharges halted in 2009. Sampling covered three levels of wave exposure, and compared impacted sites with comparable unmined reference sites. Benthic communities were quantified on both intertidal and subtidal reefs, and kelp densities and rock-lobster abundances, lengths and sex ratios on subtidal reefs.Prior to intensification of mining, deposition of tailings significantly influenced intertidal communities only at sheltered localities where wave action was insufficient to disperse them. Following the mine expansion, effects spread to both semi-exposed and exposed sites. After mining was suspended, recovery of the biota was limited, even three years later.Reductions of intertidal diversity and grazers, proliferation of macroalgae, and increased dominance by filter feeders were recorded at the impacted sites and were persistent, but the affects of wave exposure on community composition generally exceeded those of mining discharges.On subtidal reefs, tailings deposition reduced predators and grazers, increased filter feeders and ephemeral green algae, and decreased all other algae, possibly driven by light reduction due to plumes of suspended fine sediments. Increased discharges post-2005 also substantially influenced bathymetry, wave and current regimes, transforming 2 km of previously wave-exposed rocky coastline into a semi-exposed sandy beach. Tailings discharge appeared to influence community composition in four ways: (1) inundation and blanketing; (2) increased suspended particulate materials; (3) indirect top-down ripple effects, and (4) light reduction.Throughout the period 2004–2007, tailings-deposition had no detectable effects on the sex ratio, sizes or density of rock lobsters, but following suspension of mining activities, densities in 2010–2012 at impact sites exceeded those at reference sites. High natural variability in the abundance of rock lobsters may mask mining impacts, but the data strongly indicate an absence of any negative effects on rock lobsters.