Algal blooms are a natural phenomenon in the Benguela region off the coasts of Namibia and South Africa, where wind-induced upwelling results in nutrient enrichment of coastal waters. However, some algal blooms have harmful effects such as shellfish poisoning and marine mortalities, which can adversely affect coastal tourism, mariculture operations and fisheries. There has also been speculation that recent fish kills off the Kenyan coast were caused by a harmful algal bloom. It is quite possible that some of the species responsible for harmful algal blooms were introduced to African waters in the ballast of visiting ships, and there are fears that new and more problematic species might arrive in the future.
Phytoplankton such as diatoms, dinoflagellates and ciliates contain various photosynthetic pigments, and when they ‘bloom’ they reach such high concentrations that they discolour the water. These blooms are commonly known as red tides, although they may be various shades of red, purple, green, yellow or brown, depending on the pigments involved.
Some phytoplankton are able to produce powerful toxins, and blooms of these species are highly dangerous. Filter-feeding shellfish such as mussels, clams and oysters accumulate the toxins, and while they may be only slightly affected themselves, humans and animals that feed on them may become seriously ill, or even die. There are four different shellfish poisoning syndromes:
Paralytic shellfish poisoning in the Benguela region is associated with the chain-forming dinoflagellate Alexandrium catenella, which is widely distributed in cold temperate waters of the world. The toxin, known as saxitoxin, disrupts nerve functions and causes neuromuscular paralysis. In severe cases it results in death by respiratory failure.
Diarrhetic shellfish poisoning is usually caused by dinoflagellates of the genus Dinophysis, which produce the toxin okadaic acid. As the name suggests, symptoms include diarrhoea as well as nausea, vomiting and abdominal pain.
Neurotoxic shellfish poisoning has been linked to the dinoflagellate Gymnodinium cf. mikimotoi, which was recorded for the first time in South African waters in 1988. Members of the genus produce polyether toxins that interfere with the transmission of nerve impulses, causing sensory abnormalities such as dizziness, numbness, tingling sensations and hot-cold reversals. Noxious gases associated with such blooms also cause skin and respiratory irritations.
Amnesic shellfish poisoning was recorded for the first time in Canada in 1987, and is attributed to a toxin called domoic acid that is produced by a number of pennate
diatoms, including species of Pseudonitzschia. Symptoms include vomiting, disorientation and memory loss. No cases have ever been recorded in the Benguela region, but
the responsible species are thought to occur in these waters.
Toxic red tides occasionally cause mass mortalities of marine life. For example, in 1980 an estimated 5 million white mussels were washed onto the beach at Elands Bay on the west coast of South Africa following a bloom of Alexandrium catenella. Gymnodinium cf. ikimotoi poses a particular threat to South Africa’s abalone industry, since it caused the death of approximately 30 tons of abalone in 1989 and larval mortalities at several abalone culture facilities in 1996. Gymnodinium galatheanum releases powerful neurotoxins into the water, and has been implicated in fish mortalities off the Namibian coast.
Marine mass mortalities can also result from non-toxic harmful algal blooms. Fish sometimes suffocate to death after their gills become clogged or damaged by the phyto-plankton, rendering them unable to extract sufficient oxygen from the water. More commonly, bacterial decom-position during the bloom’s decay depletes oxygen in the
water. Such low-oxygen events frequently cause mass strandings and subsequent mortalities of rock lobster in the Benguela region.
Toxic red tides periodically halt harvesting at shellfish mariculture operations, but a re-occurring non-toxic algal bloom has caused a dramatic decline in production at mussel and oyster farms in South Africa’s Saldanha Bay since January 1997. These blooms are known as ‘brown tides’ because they colour the sea golden-brown. The tiny algae reach concentrations of over 3 million cells per millilitre, and inhibit bivalve filter-feeding. The responsible species, Aureococcus anophagefferens, was first described in 1988, after similar blooms impacted bays along the mid-Atlantic coast of the United States, crippling the local scallop fisheries and mussel farms. There is therefore speculation that the species was introduced to South African waters from the United States via ballast water.
The worst marine mass mortality ever recorded in South Africa occurred in March 1994, in St Helena Bay on the west coast. The respiration and subsequent decomposition of a mixed bloom of dinoflagellates – trapped in the bay by gentle onshore winds – exhausted the oxygen in the water. This created an ideal environment for anaerobic, sulphate-reducing bacteria to convert sulphates in the water to hydrogen sulphide, which filled the air with the stench of rotting eggs. It also turned the sea black, with the result that the event was dubbed a ‘black tide’ by the media.
Marine organisms in the bay died because of suffocation or hydrogen sulphide poisoning, or from being stranded on the shore after moving into shallow water in search of oxygen. A 30 kilometre stretch of shoreline was littered with the carcasses of over 1500 tons of fish and rock lobster.
Reference: Matthews S. & Brandt K. Africa Invaded: The growing danger of invasive alien species Global Invasive Species Programme 2004