False Bay’s seabed is becoming a reservoir for pharmaceuticals, pesticides and metals but scientists say pollution levels are not yet causing obvious ecological damage.
The findings suggest the bay remains resilient despite widespread contamination, although pollutants are accumulating in marine sediments and the long-term consequences remain uncertain, according to the new study, which was led by researchers from the University of Cape Town (UCT).
The study, published in Marine Pollution Bulletin, investigated the presence and ecological effects of selected pharmaceutical compounds, herbicides and metals across 19 sampling stations in False Bay between April and June 2021.
Researchers detected pharmaceuticals such as diclofenac (sold under the brand name Voltaren), acetaminophen and carbamazepine, along with the herbicides atrazine and metolachlor, in both seawater and marine sediments throughout the bay.
“The present study confirms the widespread occurrence of commonly used pharmaceuticals … alongside agricultural herbicides, personal care products, antimicrobials, hormones and metals,” the researchers wrote.
The contaminants are linked to wastewater discharges, stormwater runoff, harbour activities and urban development around Cape Town.
One of the key findings was that pharmaceutical concentrations were far higher in sediments than in seawater, suggesting the seabed acts as a long-term storage reservoir for pollution.
“We found that pharmaceuticals and herbicides are widespread throughout False Bay and that sediments appear to retain these contaminants over extended periods,” said Dr Buyani Mazeka, the lead author and a postdoctoral fellow in UCT’s Department of Biological Sciences.
The highest seawater contaminant loads were recorded in the north-eastern sector of the bay near Gordon's Bay, while the greatest sediment accumulations were found near Muizenberg and parts of the north-eastern coastline. In some areas, sediment concentrations exceeded 230 nanograms per gram.
Among the pharmaceuticals detected, diclofenac, a widely used anti-inflammatory medicine, stood out because of its persistence and repeated occurrence across sampling sites.
“Diclofenac emerged as a compound of concern due to its relatively higher concentrations, reflecting continuous input of poorly treated sewage effluents in False Bay,” the researchers wrote.
Mazeka said the drug’s persistence and continual introduction through wastewater systems made it a useful indicator of sewage-derived contamination.
The study also measured concentrations of copper, iron, manganese, lead and zinc in marine sediments. Elevated levels were recorded in areas influenced by urbanisation, wastewater discharges and harbour activities, particularly near Gordon's Bay.
The researchers linked some of the metal concentrations to vessel maintenance operations, maritime activities and land-based pollution sources. Copper was the only metal that approached internationally recognised guideline thresholds, although it remained below levels expected to cause acute ecological harm.
Despite the widespread presence of contaminants, researchers found little evidence that pollution levels are causing immediate ecological damage.
The team examined benthic communities — organisms living on and in the seafloor — including larger bottom-dwelling animals and microscopic nematodes, which are widely used as indicators of ecosystem health.
Their analysis found no significant relationship between contaminant concentrations and measures of abundance, diversity or community structure. Instead, natural factors such as sediment grain size appeared to play a much larger role in determining where different species occurred.
Environmental risk assessments also showed that contaminant levels remained well below internationally recognised thresholds of concern. “Nematode indices indicated good to high ecological quality,” the study found.
“The concentrations measured during this study were generally insufficient to produce consistent changes in benthic community structure,” Mazeka said.
However, the researchers caution that the absence of immediate impacts should not be interpreted as evidence that the contaminants are harmless.
“Chronic effects and the presence of unmonitored contaminants cannot be excluded,” they wrote.
As the study was based on a single sampling period and assessed only a limited range of chemicals, subtle effects on reproduction, growth or behaviour might have gone undetected.
The scientists note that pharmaceuticals accumulating in sediments might be transferred through marine food webs. Previous studies in False Bay have documented the uptake of some pharmaceutical compounds by mussels, limpets, sea snails, sea urchins and starfish.
“We therefore describe pharmaceutical residues as contaminants of emerging concern that warrant continued attention,” Mazeka said.
The study is the first field-based assessment of its kind in Africa and among the first in the Southern Hemisphere to simultaneously examine pharmaceuticals, herbicides and metals alongside biological indicators of ecosystem health.
False Bay is one of South Africa’s most important coastal ecosystems, supporting rich biodiversity, fisheries, tourism and recreation, while also receiving inputs from wastewater treatment plants, rivers, stormwater systems and surrounding urban development.
The researchers recommend long-term monitoring, broader contaminant screening and more detailed biological assessments to better understand the cumulative ecological risks facing the bay.