State of the Bay Report 2010-Final - Anchor Environmental

Anchor Environmental
required in the whole of the Saldanha-Langebaan area. It is imperative that monitoring of bacterial
contaminants in the Bay and Lagoon continue into the future and that serious consideration be given
to further expanding and upgrade the sewage and storm water treatment facilities in these areas.
Concentrations of trace metals in marine organisms (mostly mussels) in Saldanha Bay are
routinely monitored by the Department of Agriculture, Forestry and Fisheries (DAFF) and by the
mariculture farm owners. The DAFF Mussel Watch Programme records concentrations of cadmium,
copper, lead, zinc, iron and manganese present in the flesh of mussels at several sites along the
shoreline of the Bay. Data supplied by the Mussel Watch Programme show that concentrations of
Lead in mussels at the monitored sites are consistently are above guideline limits for foodstuffs for
at least the last 10 years, while concentrations of Cadmium frequently exceed these limits, and those
for Zinc do so occasionally. Concentrations of Copper are, however, well below specified levels. No
clear trends over time are evident for any of the trace metals, although recent data (post 2007) are
lacking. High concentrations of trace metals along the shore is very clearly of concern and points to
the need for management intervention that can address this issue as it poses a very clear risk to the
health of people harvesting mussels from the shore. It is vitally important that this monitoring
continue in the future and that data are made available to the public for their own safety.
Data on trace metals concentrations in shellfish from the mariculture farms in the Bay were
also obtained from DAFF (courtesy of the farm operators). These results show that trace metal
concentrations away from the shore are much lower than those in nearshore water and mostly meet
guidelines for foodstuffs for human consumption. The reasons for the lower concentrations of trace
metals in farmed mussels compared with those on the shore may be linked with higher growth rates
for the farmed mussels, and the fact that the cultured mussels are feeding on phytoplankton blooms
in freshly upwelled water that has only recently been advected into the Bay from outside, compared
with those along the shore that filtering water that has been in the Bay for a longer period and may
contain a greater quality of suspended sediment and associated contaminants.
Sediments
The distribution of mud, sand or gravel within Saldanha Bay is influenced by wave action,
currents and mechanical disturbance (e.g. dredging). Under natural circumstances the prevailing
high wave energy and strong currents would tend to flush fine sediment and mud particles out the
Bay, leaving behind the heavier, coarser sand and gravel. Obstructions to current flow and wave
energy can result in greater deposition of finer sediment (mud). Large-scale disturbances (e.g.
dredging) of sediments, re-suspends fine particles that were buried beneath the sand and gravel.
Contaminants (trace metals and toxic pollutants) are largely associated with the mud component of
the sediment and can have a negative impact on the environment. Accumulation of organic matter
in benthic sediments can also give rise to problems as it depletes oxygen both in the sediments and
surrounding water column as it decomposes. Historically, it was reported that the proportion of
mud in the sediments of Saldanha Bay was very low, to the extent that it was considered negligible.
Reduced water circulation in the Bay and dredging activities has resulted in an overall increase in the
mud fraction in sediments in the Bay. The most significant increases in mud content in the surficial
(surface) sediments has been observed following dredging events, however, with time (several
years), significant proportions of the mud has either been flushed out or re-buried beneath sand and
gravel, and the sediment composition has returned to one mostly dominated by sand and gravel.
The most recent studies investigating the sediment particle size in Saldanha Bay (2004-2010)
indicate that the sediment in the Bay is currently predominantly made up of sand and is not
considered to contain high levels of contaminants, except in the most sheltered parts of the Bay (e.g.
Yacht Club Basin and multipurpose quay).
Particulate Organic Carbon (POC) and Nitrogen (PON) are present at elevated levels in the
sediments in certain areas of the Bay, notably near the Yacht Club basin and the Mussel Farm. It is
State of the Bay 2010: Saldanha Bay and Langebaan Lagoon 24