NZSM Online

Get TurboNote+ desktop sticky notes

Interclue makes your browsing smarter, faster, more informative

SciTech Daily Review

Webcentre Ltd: Web solutions, Smart software, Quality graphics

Feature

Bloomin' Hell

Toxic algal blooms have seen New Zealand shellfish put
off-limits once again, and probably not for the last time.

Veronika Meduna

The summer and autumn of 1994 have seen a repeat of the events of last year's toxic algal bloom, where minute marine organisms polluted New Zealand's coastal waters and shellfish with a range of highly toxic substances.

Mussels, cockles, scallops, tuatua and pipi, and even paua and crustaceans such as crayfish and crabs which normally do not accumulate these toxins, were contaminated to health-threatening levels for humans. The harvest and consumption of all shellfish was prohibited along a significant part of New Zealand's coastline for many weeks.

Although the number of toxic blooms causing shellfish poisoning incidents has increased world-wide over the last two decades, the 1993 toxic algal bloom was the first major outbreak for New Zealand. Before that New Zealanders only had to deal with locally restricted minor blooms such as the 1989 incident which killed farmed salmon in Big Glory Bay.

This year the events of the 1993 bloom recurred and were just as severe. They lasted longer and covered different and broader geographic areas of New Zealand's coastline. Some areas even remained closed over winter.

Not a Surprise,
Not the Last

The repetition was no surprise for scientists and they suggest New Zealanders should expect to see this kind of contamination occurring regularly. Dr Lincoln Mackenzie, a scientist at the Cawthron Institute in Nelson, believes that "it's going to keep on happening...it might cause a whole run of problems and then tail off for another few years and you get another burst. I suppose it'll be something like all natural things...with large fluctuations."

This year's bloom and shellfish contamination was detected at a very early stage. The Marine Biotoxin Surveillance Unit, a body set up towards the end of last year's bloom and guided by the Marine Biotoxin Management Board, has a regular sampling programme which monitors 150 sites along the coast on a weekly basis.

One of these samples, collected from oyster beds in Foveaux Strait around mid-January, tested positive for a fat-soluble toxin. No cases of human poisoning had been reported at this stage, the sample was merely part of a routine analysis. But it indicated the risk of a toxic bloom and gave scientists the opportunity to follow the development from the very beginning.

Tests of the plankton composition followed and revealed higher than normal numbers of a dinoflagellate of the genus Gymnodinium, which resembled the description of Gymnodinium mikimotoi very closely.

Over the following weeks the level of toxicity in the oysters increased with the rising numbers of Gymnodinium cells in the water. At the peak of the bloom, the plankton was overwhelmingly dominated by at least four types of Gymnodinium, resembling species descriptions for Gymnodinium mikimotoi and Gymnodinium breve, the latter organisms well known from regular and major "red tide" blooms along the coast of Florida. High cell numbers (150,000 cells per litre) were distributed along the entire water column, straight down to the sea floor. The test results led to the closure of the region.

The bloom gradually worked its way along the East Coast to the Marlborough Sounds where it arrived in May 1994 -- and with it the closures of the shellfish harvest areas. The toxin levels in the shellfish tissue and the presence of the plankton organism were monitored regularly and showed good correlation. The toxicity levels were still increasing when the number of the plankton organism had started to decline, with peak toxicity levels occurring a few days later than the peak of the bloom of Gymnodinium.

Red Tide at Timaru

On the way up the coast Gymnodinium had caused a "red tide" offshore at Timaru. The dinoflagellate cells reached extremely high numbers and stained the water brown-red, an effect well known to citizens of Florida. Off Timaru, white-fronted terns died in the hundreds. Gymnodinium cells produce a fat-soluble neurotoxin which causes the symptoms of neurotoxic shellfish poisoning (NSP): irritations of the nervous system, coughs, sickness. Shellfish accumulate this toxin in the tissue and some species, particularly clams and oysters, are able to retain it at hazardous levels for up to several weeks.

The other major culprit of this year's bloom was Alexandrium, a group of dinoflagellate species which bloomed in smaller geographic areas but which produced a potentially lethal toxin. Alexandrium species found in New Zealand were Alexandrium minutum, Alexandrium tamarense and Alexandrium ostenfeldii. These cells produce a mixture of water-soluble toxic substances ranging under the description of paralytic shellfish poisoning (PSP) or saxitoxins.

Several derivatives of the saxitoxin molecule have been described and most species produce a certain number of these derivatives in different concentrations, thus producing different levels of toxicity. But, in some cases, the production of the toxic cocktail seems to be influenced by the region. Cells of Alexandrium ostenfeldii, taken from different locations along New Zealand's coast and cultured successfully at the Cawthron Institute, display a wide range of toxicity from highly toxic cells sampled at Timaru Harbour to non-toxic cells from Wellington Harbour.

No cases of human illness resulted from the Alexandrium bloom but several shellfish harvest areas had to be closed. The concentration of toxin which exceeds permissible marketing levels is set at 80 micrograms of saxitoxin per 100 grams of shellfish tissue, which is below the concentration causing illness. These levels have been adopted from the US Food and Drug Administration (FDA) regulatory guidelines.

Monitoring Toxin Levels

While the levels for closing a shellfish area are strictly defined it is not always simple to determine how long an area should remain closed. The biotoxin monitoring programme has revealed significant differences between the rates at which various species rid themselves of toxin.

Two sites in the Bay of Plenty have been monitored not only for toxicity levels in the shellfish tissue but also for the presence of the plankton organism in the water since the Alexandrium bloom in January 1993.

Although Alexandrium cells had disappeared by February that year, samples of tuatua were still showing significant quantities of the toxin more than a year later in May 1994. In contrast, green shell mussels, contaminated during a bloom of Alexandrium in Anakoha Bay in summer 1994, completely eliminated the toxin within three weeks.

Research carried out overseas confirms these findings, indicating that surf clams such as tuatua and pipi are likely to retain the toxins for long periods. Analysis of the tuatua body showed that a major proportion of the toxin is contained in its siphon. In analogy to strategies applied by some terrestrial insect larvae, the clams' long-term retention of the toxin could be useful to discourage predation by sea birds or siphon-biting fish.

In New Zealand, the shellfish contaminated by PSP-related toxins has not yet reached levels which would cause serious illness, but if the blooms recur regularly, surf clams like tuatua are likely to quickly accumulate toxin to dangerous levels. Alexandrium species are not only present as swimming, photosynthetically active cells in the plankton but also as resting cysts on the surface of the sediment.

Concealment in Cysts

Cysts of Alexandrium ostenfeldii, for example, are common in sea floor sediments around the entire coast, but are only rarely found in the plankton. These resting stages, however, are capable of surviving unfavourable conditions for years and, given the right environmental stimuli, they can seed new generations of blooms.

In New Zealand sediments no resting stages of Alexandrium minutum could be found, according to Mackenzie, although this species was present during the blooms of 1993 and 1994.

Cysts of Gymnodinium cells could not yet be identified at all and it is not clear whether Gymnodinium species have the ability to form cysts. Karen Steidinger, dinoflagellate expert at the Florida Marine Research Institute suggests, however, that those species which form regular blooms are very likely to have some kind of resting stage to enable them to endure unfavourable conditions.

Spring and summer, with more light for the plankton organism's photosynthesis and rising water temperatures, are the most favourable seasons for algal blooms to start. They can be dominated by either harmless or toxic species.

With the ongoing monitoring of New Zealand's coastal waters the risk of unexpected cases of poisoning is reduced and new blooms will probably be detected early. But where and when they begin is still impossible to predict.

Veronika Meduna is a freelance writer with an interest in science.

Veronika Meduna is a freelance journalist with an interest in science stories.