State of the Bay Report 2011-Final.pdf - Anchor Environmental

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State of Saldanha Bay & Langebaan Lagoon 2011 Benthic macrofauna Figure 7.13. MDS plot of Saldanha Bay and Langebaan Lagoon benthic macrofauna abundance (2011) with superimposed circles representing abiotic factors: Total Organic Carbon (TOC), and % Mud (Increasing circle size = larger measurement). Figure 7.14. MDS of Saldanha Bay benthic macrofauna abundance (2011) with superimposed circles representing concentrations of select metals: Cu, Cd, Pb and Ni. Circle size is proportional to magnitude of concentration (increasing circle size = larger concentration) 152 ANCHOR e n v i r o n m en t a l
State of Saldanha Bay & Langebaan Lagoon 2011 Benthic macrofauna 7.5 Discussion Macrobenthic community structure within Saldanha Bay has been the subject of several studies, most of which focus on anthropogenic impacts to benthic health. Kruger et al. (2005) studied the changes in epibenthos within Saldanha Bay between the 1960s and 2001, and found that there was a substantial change in benthic communities before and after harbour development. Severe declines in a number of species were reported, along with a change in the relative dominance of different trophic (feeding) groups, with a reduction in the number of suspension feeders and an increase in the numbers of opportunistic scavengers and predators (Kruger et al. 2005). Organisms that preferred sheltered habitats also became more common. These changes were attributed to the restricted flow, altered wave energy, deposition of fine sediments and increased organic matter, which resulted from harbour construction and fish factory and mussel farm effluents (Kruger et al. 2005). Previous studies also indicate that the most significant changes in benthic faunal structure occurred directly after dredging and deepening of the harbour from 1974 to 1976. Up to 25 million cubic meters of sediment were dredged from the Bay, and the dredge spill was used to construct the new harbour wall (Moldan 1978). Dredging directly impacts benthic community structure for a variety of reasons: many organisms are either directly removed or buried, there is an increase in turbidity and suspended solids, organic matter and toxic pollutants are released and anoxia results from the decomposition of organic matter (Moldan 1978). Within Saldanha Bay, many species disappeared completely after dredging (most notably the sea-pen, Virgularia schultzei), only to be replaced by opportunistic species such as crabs and polychaetes (Moldan 1978). Harbours are known to be some of the most highly altered coastal areas that characteristically suffer poor water circulation, low oxygen concentrations and high concentrations of pollutants in the sediment (Guerra-Garcia and Garcia-Gomez 2004). Beckley (1981) found that the marine benthos near the iron-ore loading terminal in Saldanha Bay was dominated by pollution-tolerant, hardy polychaetes. This is not surprising since sediments below the ore terminal were found to be anoxic and high in hydrogen sulphide (characteristically foul smelling black sludge). 7.6 Small Bay An assessment of the temporal variation in the composition, abundance, biomass and diversity of benthic macrofauna communities in Small Bay indicates two principle drivers influencing the benthic ecology in the area. The first and most obvious being the construction of the Ore Terminal and the Marcus Island causeway. These developments altered and reduced currents and wave energy in Small Bay. This permanent and ongoing impact has modified and reduced the capacity of the benthic environment to recover from large scale perturbations such as dredging. In addition, this has changed the nature of the environment and by doing so has changed the community structure and composition of species which can be supported by that environment. It is thus unlikely that benthic communities will ever recover to the pre-development state in Small Bay. However, this does not suggest the ecosystems in Small Bay cannot reach a “healthy” state. Indeed, increases in the abundance, biomass and diversity of benthic macrofaunal communities since 2008 do indicate the ongoing recovery of the area. Assessments of the community composition within Small Bay in 2011 reveal spatial variations with sites in the northern reaches (further from dredged areas and Ore Terminal) having larger species and slightly higher diversity values. This suggests that impact intensity and recovery rates vary over spatial scales within Small Bay. The second principle driver of change with fairly wide-scale impacts in Small Bay is dredging. Dredge events have generally lead to an increase in mud content and trace mental contamination levels coupled with a reduction in macrofauna biomass, abundance and diversity. In the years immediately after dredge events the benthic communities have been dominated by fast growing 153 ANCHOR e n v i r o n m en t a l
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