Harmful Algal Blooms on the South African Coast

Transcription

1 Harmful Algal Blooms on the South African Coast Harmful Algal Blooms, often referred to as red tide, are periodically reported on the South African Coast, with warnings to the public about the dangers of collecting and consuming shellfish in affected areas. Many misconceptions relating to red tide and how people may be affected by this phenomenon persist. This document serves to provide an explanation of red tide and its consequences. WHAT IS RED TIDE? Red tide refers to the discolouration of seawater caused by dense concentrations of the marine microorganisms known as the phytoplankton (those microscopic plants that float and drift under the action of ocean currents). Red tides are most common during late summer and autumn and are usually attributed to a group of phytoplankton known as the dinoflagellates. These single-cell organisms are able to swim by means of two whip-like appendages called flagella, and this ability contributes to their success as red tide organisms in that it allows them to accumulate in particularly high concentrations at the sea surface. Red tides are most common to the west of the country as they are often associated with upwelling, a process driven by the prevailing winds on the West Coast, which causes nutrient-rich waters to rise to the surface. These nutrients fuel the growth of phytoplankton, which also form the basis of the production supporting the large Bright green discolouration of the waters of Saldanha Bay in January 2003 was of concern to port authorities. Investigations determined that the bloom was dominated by an unknown species of Tetraselmis. Although these blooms show no signs of toxicity and pose no serious threat, they are often considered aesthetically unpleasant and are in many cases perceived to be associated with eutrophied or polluted waters. For this reason, blooms such as these have significant negative effects on the recreational uses of coastal areas. [Photographs L. Metcalf] 1

2 fisheries on South Africa s west coast. The term red tide may be misleading in that the discolouration of the seawater associated with these blooms of phytoplankton varies and may include shades of red, orange, yellow, brown and green. For this reason the term Harmful Algal Bloom is now preferred. CONSEQUENCES OF A RED TIDE The harmful impacts of red tides are associated with either their dense concentrations (high biomass) or the toxigenicity of some species. As a result, they are able to impact both commercial and recreational interests in the coastal region, causing fish kills and contaminating seafood with toxins, which result in serious public health problems, or altering ecosystems in ways that are perceived by humans as harmful. OXYGEN DEPLETION Low oxygen events, and in some cases the production of hydrogen sulphide, have for many years lead to spectacular mortalities of marine life on our West Coast. The inshore decay of organic-rich material derived from red tides after the exhaustion of nutrients is considered to be the cause of oxygen depletion. The massive strandings of tons of rock lobster on the West Coast have been attributed to these low oxygen events. In extreme cases of oxygen depletion, the bacteria causing decay turn to sulphur as a substitute for oxygen and begin to produce poisonous hydrogen sulphide. In other cases, dense concentrations of phytoplankton merely clog or damage the gills of fish, resulting in mortalities. Discolouration of an inner basin of Cape Town Harbour in November 2003 was caused by a bloom of the toxic dinoflagellate Alexandrium minutum. This species is known to produce paralytic shellfish poisoning toxins and the bloom persisted in the harbour for several weeks. This was the first record of this species on the South African coast. [Photographs D. Calder] 2

3 DIRECT POISONING Toxins produced by marine phytoplankton include some of the most potent poisons known, some of which may impact directly on marine life. The fish-killing dinoflagellate Karenia cristata was, for example, responsible for large abalone mortalities on the South Coast during the 1980s. This species also produces an aerosol toxin responsible for respiratory and skin disorders in humans. Not all toxin-producing phytoplankton are dinoflagellates. The raphidophyte Heterosigma akashiwo is renowned for its toxic properties and has been responsible for fish kills in various parts of the world. Although it has been known to occur on the South African coast for many years, only recently have fish mortalities in Saldanha Bay been associated with this species. INDIRECT POISIONING One major category of public health impact from Harmful Algal Blooms occurs when toxic phytoplankton are filtered from the water by shellfish such as clams, In March 1994, large marine mortalities were experienced in St Helena Bay on the South African west coast as a consequence of hydrogen sulphide poisoning. This followed the decay of a red tide dominated by the dinoflagellates Ceratium furca and Prorocentrum micans. Oxygen concentrations were maintained at less than 0.5 ml l-1 in the bottom waters of the Bay and hydrogen sulphide, generated by anaerobic bacteria in the absence of dissolved oxygen, was recorded in excess of 50 µmol l-1. Although common in the northern Benguela, this was the first recorded case of hydrogen sulphide poisoning in the southern Benguela. [Photographs G. Pitcher and D. Horstman] 3

4 mussels, oysters and scallops, which then accumulate the algal toxins to levels that are potentially lethal to humans and other consumers. Of particular concern on the South African coast are the shellfish poisoning syndromes known as Paralytic (PSP) and Diarrhetic (DSP) Shellfish Poisoning. Paralytic Shellfish Poisoning (PSP) On the West Coast, blooms of the dinoflagellate Alexandrium catenella, responsible for PSP, are common and have historically been the primary concern of public health officials. Although some of the earlier possible accounts of PSP on the South African coast may date back to the 1880s, PSP was only confirmed in 1948 when a man died after eating mussels collected at Blaauwbergstrand. Alexandrium catenella is thought to be restricted to the West Coast, and this distribution is confirmed by the incidence of PSP-contaminated shellfish, with no records of PSP east of Cape Point, whereas the highest incidence of contaminated shellfish is found on the southern Namaqua coast. Here monitoring has indicated that Alexandrium catenella appears almost every year, typically during the latter part of summer. Alexandrium catenella can be found at extremely high cell concentrations on the South African coast (millions of cells per litre) and these blooms not only render shellfish toxic to consumers, but are also seemingly responsible for fish and shellfish mortalities. The PSP toxins, comprising saxitoxin and its derivatives, disrupt Approximately 30 tons of abalone were washed ashore in the Betty s Bay region in 1989 following a bloom of the toxic dinoflagellate Karenia cristata. In addition to marine mortalities, this species also produces an aerosol toxin that causes skin and respiratory irritations in humans. [Photographs L. Botes and D. Horstman] 4

5 normal nerve functions. These toxins are extremely potent and may become so concentrated that the 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 shellfish. 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. Diarrhetic Shellfish Poisoning (DSP) The other very common form of shellfish poisoning on the South African coast is DSP, usually attributable to the dinoflagellates Dinophysis acuminata or Dinophysis fortii. Dinophysis species often form relatively minor components of blooms dominated by other dinoflagellates. Despite considerable interannual variation, cell densities tend to peak each year in the late summer and autumn, when their distri- The first fish mortality in the Benguela attributed to Heterosigma akashiwo was observed in Saldanha Bay in March High concentrations of this raphidophyte caused a yellow brown discolouration of the water, and although the mortalities were not particularly extensive, large shoals of disorientated fish were observed in shallow water. [Photographs J. de Goede] 5

6 Vegetative and cyst stages of Alexandrium catenella responsible for PSP on the west coast of South Africa. Dormant cysts may form under unfavourable conditions and are able to settle to the seafloor, where they can survive for several years. When favourable conditions return, cysts are able to germinate and reinoculate the water with vegetative cells that can again bloom. Recognising the role of cysts in bloom initiation, growth and termination is critical to our understanding of Harmful Algal Bloom phenomena. [Photographs G. Pitcher and L. Joyce] bution is often widespread. For both Dinophysis acuminata and Dinophysis fortii, okadaic acid has been identified as the primary toxin. Time-series data from the Namaqua Coast of okadaic acid concentrations in mussels reveals that toxin concentrations during summer and autumn frequently exceed the harvestable limit, indicating the severity of the problem posed by DSP, particularly on the South and West coasts. The symptoms, which usually occur within four hours, but may persist for three or four 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. Dinophysis acuminata (left) and Dinophysis fortii (right) are the dinoflagellate species most often responsible for DSP in the southern Benguela. DSP on the South African coast was identified for the first time in 1991 and attributed to D. acuminata. Monitoring has revealed that DSP is commonplace on both the West and South coasts and several other Dinophysis species known to cause DSP have been recognised as a component of the phytoplankton on the South African coast. [Photographs G. Pitcher] 6

7 Other algal species reported to form red tides or to be toxic on the South African coast, are listed by Pitcher and Calder (2000). The Yessotoxins are a lesser known group of toxins produced by the dinoflagellate Protoceratium reticulatum, a species that is known to form blooms on the West Coast. Although there are no reports of human intoxications caused by the Yessotoxins, they are acutely toxic to mice. As a result, regulatory limits to these toxins in shellfish are imposed and have been the cause of extended closures to the harvesting of shellfish on the South African coast. Amnesic Shellfish Poisoning (ASP) Also, common on our coast, but only of potential harm, are various species of Pseudo-nitzschia, known to cause Amnesic Shellfish Poisoning (ASP) in other parts of the world. Pseudo-nitzschia australis, for example, has been identified as a very common bloom-forming species on the West Coast, but to date no trace of the toxin domoic acid has been found in cultures of this species and no form of shellfish poisoning has been associated with these blooms. MONITORING OF RED TIDES In South Africa, the Department of Environmental Affairs and Tourism s Marine & Coastal Management branch, is the lead agency responsible for the detection of Harmful Algal Bloom events and the management of harvesting closures. The toxicity of phytoplankton species may vary with their global distribution. Species of Pseudo-nitzschia belong to a group of phytoplankton known as diatoms and are the cause of Amnesic Shellfish Poisoning (ASP). Although these species are common on our coast, they do not appear to produce the toxin domoic acid and have yet to be associated with any form of shellfish poisoning. [Photograph G. Pitcher] 7

8 Monitoring for this purpose is divided into: Monitoring of Wild Shellfish Utilised in Recreational and Commercial Fisheries The primary objective of this programme is to provide a warning and information system to the public and the aquaculture and fishing industry. As part of this programme a 24-hour answering service provides information relating to Harmful Algal Blooms on our coast (Red Tide Hotline ). Owing to the extensive nature of our coastline, this programme is unable to serve the purpose of ensuring seafood safety. Monitoring of Shellfish from Aquaculture Establishments As part of the South African Molluscan Shellfish Monitoring and Control Programme, toxins and toxin-producing phytoplankton are monitored in each of the aquaculture growing areas in order to prevent the placement of contaminated shellfish on the market. 8

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