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

State of Saldanha Bay & Langebaan Lagoon 2011
Sediments
at different rates. The elevated POC at Donkergat and Salamander Bay are likely to be a result of the
dredging activities that took place in this area between 2009 and 2010. The mud content in this area
also indicates that it is not an area subject to a high deposition or retention rate. It therefore follows
that the POC in the area is of local origin from a fairly recent event.
The ratios of POC: PON are high for all sites, with the exception of site BB20, a deep site at
the opening to the Bay, where the ratio is 2:1. The POC was very low at this site in relation to other
sites in the Bay and Lagoon suggesting that this site may be less influenced by organic matter from
anthropogenic sources. Indeed this site is positioned at the greatest distance from anthropogenic
activities likely to contribute to the POC (waste water treatment works, fish factories and septic
tanks).
5.3.2 Temporal trends
5.3.2.1 Particulate organic carbon
A total of six sites have been sampled and POC compared at various stages between 1974 and 2010.
The sediments from the Yacht Club Basin (SB1), Mussel Farm (SB9) and Multi-purpose Quay (SB14)
consistently had the highest POC content of the six sites sampled since 1989. The much elevated
organic carbon content of the sediments at the Yacht Club Basin has most likely been due to a
combination of input of organic matter from dredge events and the fish factories and a high
retention rate due to the sheltered nature of the area. The elevated organic carbon concentrations
at the mussel farm site were attributed to the deposition of faecal pellets and biogenic waste. The
elevated organic carbon concentrations at the Multi-purpose Quay is also most likely attributable to
the historical dredging that took place at the site and a relatively higher retention rate of organic
matter and fine sediments, given the depth and the sheltered nature of the site. The historical data
revealed that organic matter concentrations increased following dredging events and decreased in
years following the dredging. This suggests the re-suspension of organic matter from deeper
sediments and the subsequent settling of this matter is a primary contributor to organic matter in
surface sediments in the Bay. The only exception to this trend was that of the mussel farm site. This
suggests that the mussel farm activities had a stronger local influence at that particular site than that
of the dredging activities.
As mentioned previously no direct comparison of the percentage particulate organic carbon
could be conducted for 2011, however a comparison of spatial trends between 2010 and 2011 was
conducted. The concentrations of organic carbon were, as in 2010 and previous years, greatest at
the Yacht Club Basin and at the Multi-purpose Quay. Interestingly the POC at the mussel farm sites
was moderate to low in relation to other Small Bay and Big Bay sites, suggesting that the mussel
farming activities have had a lower influence on the POC than in previous years. Relatively
moderate to high concentrations of POC were detected at the deeper sites within Big Bay, while the
lowest concentrations were detected at the opening to Big Bay. These spatial variations are most
likely a reflection of the flushing capacity at these sites. Flushing of sediments is influenced by the
current strength and the depth at the sites. Interestingly these spatial variations in Big Bay were not
detected in 2010. This change in the spatial pattern suggests that factors influencing retention of
organic particles are having a greater influence over the concentration of POC than in previous
years. On the basis of trends revealed in previous years as well as the consideration of the reduced
mud content in the Bay it is understood that the system is in a state of recovery and that this
recovery varies based on the extent of exposure and depth at different sites. This variation is
revealed in the spatial analysis of the 2011 POC results.
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