NAMAGROEN EMPR Amendment.pdf - DLIST Benguela

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Addendum to EMPR for Concessions 8(a), 9(a) and 8(b) 23in contrast to nearshore areas, where recovery rates of three years are suggested followingmining using conventional mining tools (Lane & Carter 1999). At present, no data are availablefor recovery after mining with TSHD technology, although a specifically designed benthicmonitoring programme is in place off southern Namibia.Sediment discharges from dredger overspill, and sediment re-suspension by the draghead nearthe seabed will lead to suspended sediment plumes in the water column. Distribution and redepositionof sediments from plumes are the result of a complex interaction betweenoceanographic processes, sediment characteristics and engineering variables that ultimatelydictate the distribution of suspended sediment in the water column. Ocean currents, both aspart of the meso-scale circulation and due to local wind forcing, are important in distributionof suspended sediments. Turbulence generated by surface waves can also increase plumedispersion by maintaining the suspended sediments in the upper water column. The maineffect of plumes is an increase in water column turbidity, leading to a reduction in lightpenetration with potential adverse effects on the photosynthetic capability of phytoplankton.Poor visibility may also inhibit pelagic visual predators. For example, localised plumes ofelevated turbidity in the water column may affect the feeding efficiency of seabirds either byobscuring their vision or by potentially reducing prey availability through avoidance responsesof prey species to turbid water areas. If the plumes are highly localised and of short duration(i.e. the dredging vessel is highly mobile and plumes disperse quickly), then the consequencesare likely to be negligible. Egg and/or larval development may be impaired through highsediment loading. Benthic species that may be impacted by near-bottom plumes includebivalves and crustaceans.The highly dynamic nature of the Benguela coastal region suggests that most of the marinebiota is well adapted to cope with large fluctuations in natural turbidity and short-termincreases in suspended sediments, and many can tolerate long-term burial. The response ofbenthic invertebrates to increased sediment concentrations is highly variable, and tolerancelevels range from tens of mg/l to several thousand of mg/l. Suspended sediment effects as aresult of dredging will thus occur mainly at the sublethal level (at concentrations of about100 mg/l), as lethal effects are seen at much higher concentrations (>7,000 mg/l) and atexposures of several weeks. This suggests that in areas of moderate to high water movement,adult populations are likely to survive elevated suspended sediment concentrations, and onlywhere sediment deposition occurs will sediment intolerant species be excluded. However,whereas the adults of many species will tolerate extended periods of sediment burial orelevated suspended sediment concentrations, such conditions may detrimentally affect larvalsettlement and/or juvenile survival. Smothering of reef substrate reduces both settlementstimuli, as well as the food supply for juveniles, thereby affecting the recruitment success ofthe disturbed macrobenthos and ultimately impacting the community structure. In generalthough, the impacts from suspended sediment plumes are regarded as being of low significancedue to the typically short duration times, local scale and fast dispersion rates.The resuspension of sediments and their subsequent deposition can cause smothering ofbenthic communities adjacent to the dredging target areas. Smothering involves a reduction innutrients and oxygen, clogging of feeding apparatus, as well as affecting choice of settlementsite, and post-settlement survival. In general terms, the rapid deposition of material from theNamagroen Prospecting & Investments October 2008
Addendum to EMPR for Concessions 8(a), 9(a) and 8(b) 24water column (as would occur during overburden stripping and dumping in a sacrificial dumpsite) is likely to have more of an impact on the soft-bottom benthic community than gradualsedimentation to which benthic organisms are adapted and able to respond. However, thisresponse depends to a large extent on the nature of the receiving community. Studies haveshown that some mobile benthic animals are able to migrate vertically through more than30 cm of deposited sediment (Maurer et al. 1979; Newell et al. 1998; Ellis 2000). In contrast,sedentary communities may be adversely affected by both rapid and gradual deposition ofsediment. Filter-feeders are generally more sensitive to suspended solids than depositfeeders,since heavy sedimentation may clog the gills. Impacts on highly mobile invertebratesand fish are likely to be negligible since they can move away from areas subject to redeposition.Some studies have found a positive effect of dredging in that the disturbance of sedimentsreleases sufficient organic material to enhance the species diversity and population density oforganisms outside the immediate zone of deposition of suspended material (Jones & Candy1981; Poiner & Kennedy 1984). Crushed organisms returned to the sea with the overspill mayalso cause organic enrichment (Newell et al. 1998). The upwelling regions off the SouthernAfrican West Coast are frequently exposed to hypoxic conditions (
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