Article: Pulfrich, A., Branch, G.M., Effects of sediment discharge from Namibian diamond mines on intertidal and subtidal rocky-reef communities and the rock lobster Jasus lalandii, Estuarine, Coastal and Shelf Science (2013)
For over a century, diamonds have been mined across western Africa, contributing significantly to the economy of those nations. These diamonds, however, have been making headlines with their links to civil wars and insurgency. As a result, there has been a global push to buy and support conflict-free diamonds. But we shouldn’t feel too good about ourselves yet, as diamond mining practices are impacting coastal ecosystems and communities.
Much like any industry, diamond mining produces a lot of waste that needs to go somewhere. In mining, the waste is referred to as tailings or overburden, and is created when the valuable ore is separated out. In diamond mines the tailings are typically a slurry composed of silts and sands that are carried away from the facility via pipelines.
The Elizabeth Bay Mine began production on the southwest coast of Namibia in 1991 (Fig 1). The mine has a discharge pipeline that deposits tailings onto a nearby beach on Elizabeth Bay. With decent foresight, the Namibian government launched a coastal biological monitoring program that looked into the long-term effects of tailings deposition on coastal communities. The program was initiated due to concerns that deposition may impact the economically and socially important rock lobster, Jasus lalandii (Fig 2). The first round of monitoring took place in the mid-1990s, and researchers found that tailings deposition was affecting communities in sheltered waters (areas safe from direct wave exposure), but less so in exposed (unprotected rocky shores) and semi-exposed waters(open coast, but expansive populations of wave-buffering species). Researchers concluded that this was likely due to greater physical stress from wave action and more mixing and dilution in the exposed areas.
In the early 2000s monitoring resumed due to the impending increase in production from the mine. This would allow researchers the opportunity to see the effects of tailings deposition before and after increased production. During this time, researchers found that discharge of tailings from the mine increased from 1.6 million tons per year to almost 2.9 million tons per year. This period of increased tailings lasted until about 2009, when the global economic crisis halted mining production. Now researchers had 3 distinct periods in which to study coastal community structure: typical mining production, increased production, and post-mining production.
This study looks at the impacts of different levels of mining production on marine community assemblages as well as the impacts on rock lobster. Researchers chose 10 sites around Elizabeth Bay looking at sub-tidal communities, intertidal communities, or both. 3 of the 10 sites are located north of Elizabeth Bay and acted as reference sites (Fig 1). Sites were described as sheltered, exposed, or semi-exposed. Over the course of 9 years, researchers surveyed sites using transects and quadrats to assess community structure. Quadrats are placed along a transect every 5-10 meters and researchers would take note of all present species within each quadrat. This method was used for both intertidal and sub-tidal zones. At intertidal sites species were categorized into one of ten functional groups, including grazers, trappers, seaweeds, mobile predators, etc. Percent cover of each functional group within the quadrat was determined. These functional groups were also used as designations in sub-tidal sites, but surveyors also took note of proportions of rock and sand and caught and measured any rock lobsters found.
In years of typical mining production, tailings only impacted sheltered intertidal sites with wave exposure alleviating deposition in exposed and semi-exposed sites. However, when mining production was ramped up the impacts of tailings were seen in all types of sites, including all semi-exposed sites and even some of the exposed sites. Overall, communities lost balance (Fig 3, 4, 5). Impacted sites lost predators, grazers, and crustose algae (algae that forms crusts on hard surfaces). Filter feeders (organisms that eat by filtering particles out of the water column, like mussels and tube worms) and foliose seaweeds (larger, attached algae that stand upright when submerged) became more dominant (Figs 3, 4, 5).
In intertidal communities researchers saw significantly greater species richness, a metric for diversity, at reference sites than at impacted sites (Fig 4). With increased disturbance came more opportunistic species, species capable of inhabiting a variety of environments and doing so rapidly, and a population decline of higher trophic level species, or species higher in the food web like predators and grazers. The connectivity in an ecosystem like this means that impacts to one level will ripple through the rest.
With a decline of grazers, foliose seaweed (Fig 6) took over and began to shade out encrusting algae, and increases in sediments benefitted filter feeders (Fig 7). The story from sub-tidal surveys at these sites mirrors that from the intertidal (Fig 5). Increased sedimentation played a large role in sub-tidal communities as organisms in sheltered and semi-exposed sites became smothered. Continued surveying during post-mining revealed that communities had not yet rebounded, in terms of health and diversity, from the years of increased production.
The main goal of this monitoring was to track impacts on rock lobsters, but researchers found that there was no significant effect on rock lobster populations, sizes, or even sex ratios. The impacts of mining then seem relegated to community diversity.
An unexpected finding from this study was that some of the sites were physically altered due to deposition. Some of the sites that were initially classified as exposed needed to be reclassified once rocky shores became sandier. Deposition from the millions of tons of sediments caused accretion, averaging 500 meters, beyond the original shoreline. Researchers found, in exposed sites that experienced heavy deposition, an increase in species diversity (Fig 3). This was chalked up to the Intermediate Disturbance Hypothesis, a classic theory in ecology stating that intermediate levels of disturbance caused diversity to be at its highest with both opportunistic and competitive species thriving.
Coastal ecosystems are fragile and the relationships between species in this ecosystem are valuable. Around the world, these ecosystems are threatened by point source pollution like that from the Elizabeth Bay Mine. This study provides valuable insight as to how marine communities are impacted across multiple habitat types and across years of varying production. By monitoring communities during years of typical, increased, and decreased production we can gain a better understanding of how communities fluctuate and their overall resiliency and ability to rebound from anthropogenic stress. Surveys allow us to infer direct and indirect relationships between organisms depending on the changing abundances and interactions. Long-term monitoring is crucial if we aim to truly grasp our impact on ecosystems. With years of increased mining effort species diversity is going to decline, and lack of diversity can have widespread effects. Maybe it’s time we expand our definition of conflict diamond.