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

State of Saldanha Bay & Langebaan Lagoon 2011
Benthic macrofauna
Depth and the percentage mud appear to be the principle abiotic factors that correlate with,
and most likely lead to distinctions in the benthic macrofauna community composition between
Langebaan Lagoon and Saldanha Bay. Saldanha Bay is deeper and the sediments contain a higher
percentage of mud compared to Langebaan Lagoon. This higher proportion of fine grained particles
also correlated with trace metal content. This can clearly be seen at site SB1 and SB14, which had
the highest mud content and the highest trace metal content. The benthic community compositions
at these sites were clear outliers. Site SB1 represents an impoverished community with a very low
abundance and diversity of benthic macrofauna (only 5 species and 6 individuals recorded in 2011).
This site was also identified as impoverished in previous surveys, most likely owing to the high
concentrations of organic matter and anoxic conditions within the sediments. Indeed, this site has
elevated cadmium levels relative to all the other sites sampled. In addition it has relatively high
concentrations of lead, copper, nickel, organic carbon (TOC) and mud. Site SB14 had a higher
number of species (14) but was completely dominated by one species, the deposit feeding bivalve
Tellina gilchristi.
The concentrations of trace metals did correlate with the distinction in community
composition seen between sites in Small Bay and adjacent to the Ore Terminal in Big Bay, and sites
in the central and southern reaches of Big Bay. Any further distinction between the other sites
sampled in Small Bay according to benthic macrofauna community composition does not clearly
correlate with any of the other environmental variables measured. It is likely that natural
community interactions and possibly other environmental variables not measured in this report are
having an influence on the community composition in Small Bay.
Figure 7.13, Figure 7.12 and Figure 7.14 indicate that Langebaan Lagoon is characterised by
shallow water depths, and sediments with low mud, particulate organic carbon and very low to
negligible concentrations of trace metals (with the exception of Ni). This suite of abiotic factors
clearly correlates with the cluster of Langebaan sites that have been grouped according to benthic
macrofauna community structure. This indicates that this particular suite of abiotic factors strongly
influences the benthic macrofauna communities. More fine scale differences between the benthic
communities within the lagoon are clearly not shaped by the abiotic variables considered here and it
is likely that other factors such as water circulation patterns and community interactions influence
the species composition within the lagoon.
Figure 7.12. MDS of Saldanha Bay and Langebaan Lagoon benthic macrofauna abundance (2011) with
superimposed circles representing depth (Increasing circle size = deeper)
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