Mussels growing near some of SA’s iconic Blue Flag beaches in False Bay show elevated levels of heavy metal contaminants, a new scientific study shows.
However, the result is potentially good news because it proves mussels can be useful in cleaning up human mess. That is, so long as nobody eats them.
The study found metal concentration in mussel tissue were highest at Bikini Beach, a Blue Flag beach in Gordon’s Bay. Levels of arsenic, cadmium, lead and zinc exceeded the legal limit in shellfish.
There was further evidence of nutrient pollution in Strand, one of several densely populated areas along the False Bay Coast.
On the plus side, the study indicated impressive biodiversity, suggesting the Bay’s marine ecosystem remains relatively healthy, possibly thanks to water flow that disperses pollutants.
The criteria do not necessarily account for the quality of the water or the ‘health’ of the marine life.
False Bay is SA’s biggest natural bay and a top tourist destination. Its marine biodiversity received international recognition after the release of Oscar-winning documentary film My Octopus Teacher.
The mussel study is due to be published in the June edition of prominent scientific journal Ecological Indicators. Eleonora Puccinelli, lead author and researcher at the University of Cape Town oceanography department, said the study results were potentially significant for environmental management and aquaculture: “Mussels are used in many places to remove pollutants/organic matter from the water, mostly in northern Europe where there are lots of fish farms and having filter feeders below them would help in removing some of those pollutants.
“‘Bioaccumulation’ of metals in wild mussel populations was more problematic because the mussels formed part of the natural ecosystem and could not simply be removed.
“The best way to permanently remove (the accumulated metals) is to remove the mussels, which of course it would be ecologically irresponsible in the case of False Bay,” she said.
Mussels may also release some of this metal load when spawning.
The study results were consistent with the bay’s circulation, which has seasonal variation but generally involves a west-east flow. Metals and nutrients enter the bay via multiple sources such as runoff from rivers. The study identified antifouling paint used on boats at the Gordon’s Bay marina as one possible source of metal.
Upwelling in the bay, particularly during the summer months with the strong prevailing southeast wind, promotes water circulation and thereby helps mitigate the impact of pollution, the study suggested. Puccinelli said the study also had implications for the global Blue Flag Beach programme which grades beaches according to limited biological information — namely E coli and Enterococcus — and not metals.
“The criteria do not necessarily account for the quality of the water or the ‘health’ of the marine life,” Puccinelli said in response to Sunday Times Daily questions. “Heavy metals, nitrogen, or other organic compounds are not accounted for the (Blue Flag) classification.
“It is important to say this is not only a problem of SA, it’s a problem for the entire globe, as these are international guidelines,” she said.
Puccinelli said the mussel contamination results should not overshadow the generally positive health of the False Bay environment.
“The general picture we found, by having other measurements such as system productivity and biodiversity associated with the mussel beds — a good indicator of the ‘health’ of an ecosystem — suggest that the Bay is doing reasonably,” she said. “The natural environment has some coping mechanisms for anthropogenic (human) changes.”
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