State of the Bay Report 2010-Final - Anchor Environmental

6.2 Particulate Organic Carbon (POC) and Nitrogen (PON)
State of the Bay 2010: Saldanha Bay and Langebaan Lagoon
Anchor Environmental
Particulate organic carbon (POC) and particulate organic nitrogen (PON) accumulates in the
same areas as mud (cohesive sediment) as most organic particulate matter is of a similar particle size
range to that of mud particles (less than 60 µm) and settles out of the water column together with
the mud. The most likely sources of organic matter in Saldanha Bay are from phytoplankton
production at sea and the associated detritus that forms from the decay thereof, fish factory waste
discharged into the Bay, faecal waste concentrated beneath the mussel and oyster rafts in the Bay,
and treated sewage effluent discharged into the Bay from the waste water treatment works at
Saldanha and Langebaan, and leakage from sewage transfer stations, septic tanks and conservancy
tanks. POC and PON is most likely to accumulate in sheltered areas with low current strengths,
where there is limited wave action and hence limited dispersal of organic matter. Elevated
concentrations of PON in the sediments may indicate the presence of relatively fresh organic matter
originating from phytoplankton. High carbon to nitrogen ratios in the past suggests that the matter
was nitrogen depleted which may indicate that the matter was of fish waste origin (Monteiro et al.
1997), at least at this time anyway. Accumulation of organic matter in the sediments doesn’t
directly impact the environment, however, bacterial breakdown of the organic matter may lead to
anoxic conditions. Under such conditions anaerobic decomposition prevails, which results in the
formation of sulphides such as hydrogen sulphide (H2S). Sediments high in H2S concentrations are
characteristically black, foul smelling, and toxic for most living organisms.
The percentage of POC and PON recorded in the sediments in Saldanha Bay and Langebaan
Lagoon is presented in Figure 6.6 and Figure 6.7. The concentrations of both POC and PON are
highest in the deeper parts of Saldanha Bay, particularly around the ore jetty and in the north east
corner of the Bay near the yacht basin. POC levels in Saldanha Bay have not always been as high as
they are now, and were in fact very low (between 0.2 and 0.5%) throughout the Bay prior to any
major development (pre-1974). The next available POC data was collected in 1989 after the
construction of the iron ore jetty and the establishment of the mussel farms in Small Bay. At this
stage all sites monitored had considerably elevated levels of POC with the greatest increase
occurring in the vicinity of the Mussel Farm. POC levels peaked at a record high (16.9%) at this site
in 1990. The reason for this extremely high POC percentage is uncertain. Through all subsequent
years of POC monitoring (1990, 1999, 2000, 2001, 2004, 2008, 2009 and 2010), levels have remained
higher than those reported prior to development.
The sediments from the Yacht Club basin, Mussel Farm and Multi-purpose Quay have
consistently had the highest POC content of the six sites sampled over the last 20 years. High
organic carbon concentrations at the Mussel Farm site is attributed to the deposition of faecal
pellets and biogenic waste, while high organic carbon content of the sediments at the Yacht Club
basin and Multi-purpose Quay is due to long term deposition at these sheltered sites. Organic
carbon content of the sediments at the end of the ore jetty and within Big Bay show irregular
temporal trends, with concentrations spiking and dipping from one year to the next. These
fluctuations are most likely due the higher wave energy and tidal forcing at these sites, which would
prevent long term accumulation of organic matter by continually scouring and flushing sediments.
All POC levels recorded in 2004 were lower than in 2000 and 2001, except at the Mussel Farm site,
suggesting some degree of recovery in Saldanha Bay between 2001 and 2004. However, the POC
content increased at all sites in 2008, except the Mussel Farm, with the greatest increases observed
in Big Bay, at the Yacht Club basin and the Multi-purpose quay. The increase in POC at the two latter
sites corresponded to an increase in fine grained sediments in 2008.
POC at the Mussel Farm increased from 1.3% in 2008 to 3.4% in 2009 and decreased to 1.1%
in 2010. The increase between 2008 and 2009 may have been due to increased deposition of faecal
matter from the mussel rafts given that mussel production in the Bay was at its highest during 2008.
Similarly the reduced level of POC recorded at the Mussel Farm site in 2010 may be due to the