* Click on the images for more detail

Outbreaks of red tide along the South African coast are reported periodically in the media, with warnings to the public about the dangers of collecting and consuming shellfish in the affected areas. However, few people understand how a red tide develops, or how they might be affected by this phenomenon.


The term red tide is misleading, because discolourations of the water may be brown, orange, purple or yellow, as well as red. These discolourations are caused by dense concentrations of the microscopic plants of the sea, the so-called phytoplankton. The discolouration varies with the species of phytoplankton, its pigments, size and concentration, the time of day and the angle of the sun.
Most red tides along the South African coast are caused by a group of phytoplankton known as dinoflagellates. These single-celled organisms are able to swim short distances by means of two whip-like appendages called flagella. Their ability to swim contributes to their success as red tide organisms, for reasons that will be explained later.


Red tides usually occur along the Cape west or south coasts in late summer and autumn. The prevailing southerly winds at this time of year cause cold, nutrient-rich water to rise up from the deeper regions of the ocean to the surface, a process known as upwelling. Swept along with this upwelled water are dinoflagellate cysts, the resting stages of the organism, which lie dormant in the sediments on the sea floor. The high nutrient concentrations in the upwelled water, together with ideal conditions of temperature, salinity and light, trigger the germination of the cysts, so that the dinoflagellates begin to grow and divide. The rapid increase in dinoflagellate numbers, sometimes to millions of cells per litre of water, is what is known as a "bloom" of phytoplankton. Concentration of the bloom by wind and currents, as well as the dinoflagellates' ability to swim to the surface, together lead to the formation of a red tide.
Red tides are therefore not a result of sudden population explosions of phytoplankton attributable to increased division and growth rates, but result from confinement or concentration of phytoplankton following normal population increases to such an extent that they discolour the water.
If conditions in the surface waters become unfavourable for the dinoflagellates, for example if the nutrients are depleted or the bloom is dispersed by wind and currents, the dinoflagellates will again form dormant cysts and sink to the sea floor.


Most red tides represent useful contributions to plankton production but some periodically produce harmful results.


Dense concentrations of red tide organisms can suffocate fish by clogging or irritating their gills, so that they cannot extract sufficient oxygen from the water. During 1962 the mortality of more than 100 tons of fish in False Bay was attributed to gill clogging by the dinoflagellate Gonyaulax polygramma. Other forms of physical damage include the recently discovered feeding on fish tissue by certain dinoflagellate species resulting in the death of the fish within a matter of hours. Such events are now thought to have been responsible for many unexplained fish kills in the past.


Red tides may also kill indirectly by depleting the oxygen dissolved in the water. The mass mortality of the red tide organisms once the nutrients have been depleted results in an increase in the number of bacteria which are responsible for decomposition in the sea. The activities of this huge population of bacteria soon deplete the oxygen concentration in the water, leading to the death of other marine animals. Low oxygen levels following such blooms are believed on a number of occasions to have caused rock lobsters to crawl from the sea. Such an event was observed in St Helena Bay in 1978 following a bloom of the photosynthetic ciliate Mesodinium rubrum.

* In March 1994, South Africa experienced its worst recorded marine mortality in the west coast area of St. Helena Bay. The event was caused by the entrapment and subsequent decay of an extensive red tide dominated by the non-toxic Prorocentrum micans and Ceratium furca, with the toxic species Alexandrium catanella and Dinophysis acuminata present in lower concentrations. Marine life died because of suffocation or hydrogen sulphide poisoning. The low oxygen conditions allowed anaerobic, sulphate-reducing bacteria to convert sulphates in the water column to hydrogen sulphide gas, which corroded metal objects and caused respiratory problems amongst some residents of the area. These chemical reactions also caused the sea to turn black, and the event was soon dubbed a ‘black tide’ by the media. Approximately 60 tons of crayfish and 1500 tons of fish, comprising about 50 species, washed ashore. The mullet, Liza richardsnii, made up the bulk of the fish mortality (1250 tons) with the remainder being dominated by sharks, and bottom-dwelling fish. Along the bay’s rocky shores, most of the mussels, limpets, sea urchins and other intertidal life had died, with the exception of the false limpet, Siphonaria capensis, which is capable of switching to anaerobic metabolism in the absence of oxygen.


Toxins produced by certain dinoflagellates are some of the most potent poisons known to man. The most notorious of the dinoflagellate toxins are the neurotoxins which disrupt normal nerve functions. Toxins of this nature have caused numerous marine mortalities on the South African coast. Virtually the entire adult mussel population in the Elands Bay area was destroyed by the dinoflagellate Gonyaulax catenella in 1980, while 30 tons of abalone were washed up in the HF Verwoerd Marine Reserve in 1989, following blooms of the dinoflagellate Gymnodinium nagasakiense, a recognized fish killer in the coastal waters of Japan.



Animals such as mussels, clams and oysters are particularly vulnerable to red tides, because they feed by filtering particles, including phytoplankton, from the water. Toxic phytoplankton accumulate in the digestive system of these filter-feeders and subsequently cause illness or death to consumers such as birds, marine mammals and man.
This then, is the category of red tide about which the public is periodically warned. It should be remembered that cooking only slightly lessens the toxicity of affected shellfish because the toxins are generally heat stable - in other words, they are not destroyed by heat. However, most cases of poisoning are restricted to filter-feeding shellfish, and other seafood may be consumed safely.

Four different types of indirect poisoning have been identified as harmful to man.

Paralytic shellfish poisoning (PSP)

PSP was discovered in the 1700s and is in many respects the most serious of the shellfish poisonings: several hundred human deaths have been recorded worldwide during the past 300 years. Along the South African coast the dinoflagellate Gonyaulax catenella is regularly responsible for PSP on the West Coast and has caused several human deaths.

A number of toxins are responsible for PSP. The most common is saxitoxin, which disrupts normal nerve functions. It is extremely potent and may become so concentrated that consumption of a single mussel can be fatal. The first symptoms are a tingling, prickling, stinging or burning sensation of the lips, tongue and fingertips within 30 minutes of eating poisonous mussels. Numbness of the arms, legs and neck follows. Other symptoms develop later and include dizziness, general muscle incoordination, headaches, vomiting and impaired respiration. Death is by respiratory failure and may occur within 2 - 24 hours.
Mussels may remain toxic for some time after the occurrence of this type of red tide. If the red tide disappears completely the mussels may take only a few weeks to flush the toxins from their systems. However, if the red tide organisms remain in the water at low concentrations the mussels may remain toxic for several months.

Diarrhetic Shellfish Poisoning (DSP)

DSP has only recently been recorded for the first time on the South African coast. The causative organism has been identified as the dinoflagellate Dinophysis acuminata, which produces the toxin okadaic acid. The symptoms, which usually occur within four hours but may persist for three days, include diarrhoea, nausea, vomiting, stomach ache and shivering. It is likely that DSP has gone unreported on many occasions because of the relatively mild nature of the symptoms. In addition, the symptoms may be confused with those of gastro-enteritis associated with the consumption of shellfish from polluted waters.

Neurotoxic Shellfish Poisoning (NSP)

Along the South African coast the dinoflagellate Gymnodinium nagasakiense is usually implicated in NSP. Most outbreaks have been reported from False Bay, where they are responsible for the olive-green discolouration of the seawater during autumn.
The symptoms of NSP are sensory abnormalities and include dizziness, tingling sensations, dilated pupils and hot-cold reversals. These symptoms usually disappear in three days and no human deaths have yet been documented. Red tides of this type may also cause irritations of the human eye, nose and throat through contact with sea spray.
As a result of the mass mortalities of marine animals associated with these red tides, the public should exercise caution in the collection of seafood.

Amnesic Shellfish Poisoning (ASP)

ASP was recorded for the first time off the coast of Canada in 1987 when three deaths and over 100 confirmed cases of acute intoxications followed the consumption of cultured mussels. Nitzschia pungens, a species of phytoplankton belonging to a group known as the diatoms, was identified as the causative organism, producing the neurotoxin domoic acid. Symptoms of ASP include abdominal cramps, vomiting and neurological responses involving disorientation and memory loss. Although ASP has not been recorded off the South African coast, the responsible organism is thought to occur in our waters.

* Aerosol (air-born) Toxins

In the summer of 1995-96, South Africa experienced a severe aerosol toxin problem in False Bay which later spread to the coastal resort of Hermanus in Walker Bay. Beachgoers and seaside residents were overcome by the discomfort of coughing, burning of the nasal passages, difficulty in breathing, stinging eyes and irritation to the skin. Although the discomforts experienced were considerable, symptoms were usually relieved upon leaving the area, and no long-term effects were noted. The aerosol toxin was linked to the presence of the bloom of a toxic dinoflagellate species Gymnodinium, first recorded in False Bay in 1988. This species appears to represent a new species of southern hemisphere Gymnodinium. Despite the species having bloomed on several occasions since then, never before have the noxious effects to humans been so evident as during the 1995-96 event. Faunal mortalities were however small, with the exception of the larval mortalities experienced by several land-based abalone farmers in the Walker Bay area. The presence of aerosol toxins which result in respiratory distress in human has thus far only been recorded from the west coast of Florida (for many years now) USA, and in 1993 from New Zealand.


In recent years there has been a growing awareness worldwide of the problems associated with red tides, largely due to the expansion of the shellfish industry and the increased risk to human health. In addition, scientists have concluded that red tides are occurring with increasing intensity and frequency over a wider global distribution, and that this may be a result of human activities. Nutrient enrichment through various forms of pollution, and subtle changes ascribed to the greenhouse effect, are thought to have influenced the intensity and frequency of red tides. Also, the transport of dormant cycts in the ballast tanks of ships is thought to have contributed to the spreading distribution of red tide outbreaks. It has therefore been recommended that an international research effort be undertaken to evaluate the global expansion of algal blooms and man's involvement in it.
The aim of this research includes finding a way of accurately predicting red tide outbreaks, so ensuring that steps can be taken to warn the public in advance. Simple predictive models have been developed for certain dinoflagellate species, but these can only be applied to specific areas. Good monitoring programmes are therefore still the most satisfactory means of providing an efficient warning system.
In South Africa the responsibility for monitoring red tide rests with the Chief Directorate Sea Fisheries of the Department of Environment Affairs. Researchers from the Sea Fisheries Research Institute (SFRI) regularly monitor the waters around our coast for red tide outbreaks in order to warn the public of potentially harmful blooms. Members of the public should notify the SFRI at 021-4396160 of unusual discolourations of the sea or illness following the consumption of shellfish. The Red Tide Hotline (021-4394380) is a 24-hour answering service that provides information concerning outbreaks of red tide detected by the SFRI.


The original text was written by Irma van der Vyver and Dr Grant Pitcher. Additional information courtesy of Sue Matthews* and edited by Gavin W. Maneveldt. Photographs by Deon Horstman and Dr Grant Pitcher. We kindly thank Dr. Pitcher for permission to reproduce his booklet for the Web.


The IOC Science and Communication Centre on Harmful Algae

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