Assessing the rate of recovery of benthic macrofauna after marine mining off the Namibian coast

Master Thesis


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University of Cape Town

The primary aim of this study was to assess the rate of recovery of macrobenthic communities after offshore marine mining. Three techniques, namely univariate, distributional and multivariate, were used to make this assessment. Two distinct areas, the northern and southern research areas, were investigated, and statistical and numerical analyses were conducted for each area independently. Data were aggregated to, and analysed at, the genus level. Replicates were arranged in temporal categories according to recent mining history. The northern research area appears to be affected by mining activity in terms of species composition, but not species diversity. Statistical testing detected significant differences between unmined replicates and all other temporal categories for this area, and this was also discernible in the cluster analysis and ordination plots. The overall picture generated suggests that the northern research area is affected immediately and severely by mining activity, resulting in rapid changes in species composition. However, the period of 15-19 months subsequent to mining is insufficient to allow the community to recover to a stable state. The southern research area, on the other hand, shows a slightly different scenario, with mining activity having a severe and immediate impact on both species composition as well as species diversity. Recently mined sites were found to be significantly different from both unmined sites and sites mined 43-51 months ago. The latter two categories were not found to be significantly different from each other. The results suggest that the road to recovery in the southern research area is a slow, but steady one in terms of species composition. This was particularly apparent in the results of the "SIMPER" analysis where the level of similarity between temporal categories increased steadily with time after mining. The overall picture suggests that the area has recovered substantially after 43-51 months, and that the community approximates that of the unmined area with regard to species composition. Geological analyses were also conducted, with results indicating a prevalence of fine surficial sediment particles in unmined sites, and coarse surficial sediment particles in recently mined sites. Percentage gravel, in particular, was found to be a reliable indicator of the condition of a site with regard to the level of disturbance. Altered stratigraphy and changes in particle size distribution as a result of mining activity are considered to have a noticeable effect on the structure of benthic communities. A number of taxa were found to be particularly reliable as indicator species. In both the northern and southern research areas, polychaetes (specifically Prionospio pinnata and the Lumbrineris genus) were abundant in unmined sites as well as in sites mined 43-51 months ago in the southern research area. Individuals of the genus Nassarius, on the other hand, were scarce in unmined sites, but abundant in recently mined sites. These taxa appear to be reliable indicators of the level of recovery attained in previously mined areas.

Bibliography: pages 94-99.