State of the Bay Report 2010-Final - Anchor Environmental

5.6 Trace Metal Contaminants in the water column
State of the Bay 2010: Saldanha Bay and Langebaan Lagoon
Anchor Environmental
There is an increasing global trend emerging in countries like Canada, Australia, New Zealand
and South Africa to monitor the long‐term effects of water quality by assessing the impacts thereof
on specific marine species or species assemblages. Mussels and oysters, i.e. filter feeding organisms,
are considered to be good indicator species for the purpose of monitoring water quality as they tend
to accumulate trace metals, hydrocarbons and pesticides in their flesh. Mussels are sessile
organisms (anchored in one place for their entire life) and will be affected by both short‐term and
long‐term trends in water quality. Monitoring the contaminant levels in mussels can therefore
provide early warnings for poor water quality and dramatic changes in contaminant levels in the
water column.
Trace/heavy metals are often regarded as pollutants of aquatic ecosystems. However, they
are naturally occurring elements, some of which (e.g. copper & zinc) are actually required by
organisms in considerable quantities (Phillips 1995). Aquatic organisms accumulate essential trace
metals that occur naturally in water as a result of, for example, geological weathering. All of these
metals, however, have the potential to be toxic to living organisms at elevated concentrations
(Rainbow 1995). Human activities greatly increase the rates of mobilization of trace metals from the
earth’s crusts and this can lead to increases in their bioavailability in coastal waters via natural runoff
and pipeline discharges (Phillips 1995). Dissolved metal concentrations in water are typically low
(and therefore present analytical problems), have high temporal and spatial variability (e.g. with
tides, rainfall events etc.) and most importantly reflect the total metal concentration rather than the
portion that is available for uptake by aquatic organisms (Rainbow 1995). Measuring metal
concentrations in sediments resolves some of the analytical and temporal variability problems (as
metals accumulate in sediments over time & typically occur at higher concentrations than dissolved
levels), but still does not reflect their bioavailability. Measuring metal concentrations in the tissues
of aquatic organisms appears to be the most suitable method for assessing ecotoxicity as the metals
are frequently accumulated to high (easily measurable) concentrations and reflect a time–integrated
measure of bioavailable metal levels (Rainbow 1995).
Filter feeding organisms such as mussels of the genus Mytilus have been successfully used as
bio-indicator organisms in environmental monitoring programs throughout the world (Kljakovic-
Gaspic et al. 2010). These mussels are abundant, have a wide spatial distribution, are sessile, are
able to tolerate changes in salinity, are resistant to stress, and have the ability to accumulate a wide
range of contaminants (Phillips & Rainbow 1993, Desideri et al. 2009, Kljakovic-Gaspic et al. 2010).
Elevated levels of cadmium reduce the ability of bivalves to efficiently filter water and
extract nutrients, thereby impeding successful metabolism of food. Cadmium can also lead to injury
of the gills of bivalves further reducing the effectiveness of nutrient extraction. Similarly elevated
levels of lead result in damage to mussel gills and increased growth deficiencies and mortality.
Elevated levels of zinc are known to suppress growth of bivalves and at levels between 470 to 860
mg/l and can result in mortality of the mussels (South African Water Quality Guidelines for Coastal
Marine Waters, Mariculture).
In 1985 the Directorate: Marine and Coastal Management (MCM) of the Department of
Environmental Affairs and Tourism initiated a “Mussel Watch” Programme whereby mussels (either
brown mussels Perna perna or Mediterranean mussels Mytilus galloprovincialis) are collected every
six months (Apr/May and October) from 26 coastal sites. Mussels have been collected from five
stations in Saldanha Bay since 1997. Data from Saldanha Bay Mussel Watch programme are
currently, however, only available between 1997-2001 and 2005-2007 due to a backlog in processing
of samples. No new data were received for the 2010 period. The mussel samples are analysed for
the metals cadmium (Cd), copper (Cu), lead (Pb), zinc (Zn), iron (Fe) and manganese (Mn),
hydrocarbons and pesticides. A new automated method for sample preparation, including