Key Ideas about Toxic Algae Blooms
- Shellfish, like oysters, mussels, and cockles, feed on very
small plants that float in the sea (the phytoplankton).
Dinoflagellates are one kind of microscopic algae and of
the thousands of species, only a few kinds release water-soluble
poisons that can kill other species of marine life. When shellfish
eat the toxic dinoflagellates, the poisons become concentrated
in them. If people eat the shellfish, they can become very sick
and even die from the concentrated poison.
- To see the various kinds of toxic dinoflagellates in New Zealand,
- Dinoflagellates are plants but they also swim using a long
whip-like extension called a flagellum. They are very small,
about 0.02 to 0.04 millimetres in diameter, and shaped like a
soccer ball with a groove around the equator.
- In 1982-83 thousands of fish and penguins died in the Hauraki
Gulf. The significance of the event went unremarked at the time
except for the Ornithological Society. Later reports of dead fish,
blamed on fishing trawlers, coincided with reports of numb lips
and swollen tongues from shell fish lovers in Waiheke Channel
and the Firth of Thames estuary. Then, in 1986 and 1987, across
the Tasman Sea in Tasmania, a massive outbreak of shellfish poisoning
was traced to toxic dinoflagellates formerly thought to be limited
to the east coast of the United States and Japan's Inland Sea.
- Some researchers believe the toxic species were accidentally
introduced. Many ships arrive in Australia and New Zealand from
Asian ports with very little cargo. To keep their propellers in
the water, large ships pump up to 50 million litres of sea water
into their ballast tanks. The water contains plankton from some
of the most polluted harbours in the world. When they arrive,
the ballast water is pumped out and cargo is loaded. Researchers
sampled the ballast water of one visiting ship and found more
than 30 million live toxic dinoflagellates.
- In 1989, a New Zealand salmon farm on Stewart Island, with
$12 million dollars worth of salmon, was wiped out in a few days
by a toxic algae called Heterosigma akashiwo. In 1992,
outbreaks of massive numbers of toxic dinoflagellates, broke out
in many areas of New Zealand. More than 200 cases of poisoning
were reported in the summer of 1993. Symptoms included diarrhoea,
vomiting, muscular aches, dizziness, loss of memory, tingling,
numbness of the lips and respiratory problems.
- Toxic dinoflagellate blooms continued during the summer months
of 1993, 1994 and 1995. More than six species of dinoflagellates
were identified in these episodes. The most common species was
formerly limited to the south-eastern United States.
- Dinoflagellates have an efficient reproductive strategy. When
conditions favour growth, they reproduce swiftly, by binary fission.
But when conditions are bad, they reproduce sexually and form
cysts which settle on the sea floor and await - often for years
- return of favourable conditions. When nutrients and temperatures
favour the organisms, the cysts burst open and the population
soars, filling the sea with thick, soupy masses of these swimming
plants. They excrete toxins to kill zooplankton and fish predators.
The toxins vary from species to species but are among the most
virulent poisons known. Thousands of fish and other organisms
may be killed when population densities become very high.
- Researchers suggest ocean outfalls of sewage from urban areas,
agricultural fertilisers, and cocktails of biotoxins from agricultural
areas all contribute to ideal conditions for the dinoflagellate
blooms. Although ships have been bringing toxic species into New
Zealand waters for many years, and toxic species probably occur
here naturally, the set of conditions for massive outbreaks seems
to be relatively new.
- When there is a bloom of algae, there can be more than 900,000
of them in one litre of sea water, and they colour the water brown
or red depending on what species are present.
- The Ministry of Fisheries and local health boards test shellfish
meat for marine biotoxins to alert people to dangerous toxic algae
blooms. Cawthron Institute in Nelson carries out plankton analyses
to alert the oyster industry of potentially hazardous conditions.
When poisons are found in the shellfish, or blooms of toxic dinoflagellates
reach dangerous levels, the areas are immediately closed to further
collecting and the public is warned not to eat shellfish until
the poisons have vanished again.
- Green Shell Mussels can rid themselves of toxins in about
three weeks, but Tuatua and Pipi may retain some kinds of toxins
for years after a serious toxic algae outbreak. Biologists believe
the Tuatua and Pipi might actually concentrate the poisons in
their siphons to discourage birds and fish from biting them.
Resources to use
Wild South Videos: Giants of Ningaloo (Whalesharks
off the reefs of Australia.)
School Journal: Mussel Farming-1984 Part 3 no.3
Oysters------------1975 Part 1 no.2 9-10
New Zealand Geographic Magazine #3 Oysters and #18
Microsoft Oceans CD. Section on Plankton and Shellfish
Newspaper stories about poisonous shellfish, especially in 1993.
Most references to toxic algae are of Tertiary level and difficult
to find. Schools interested in working with Lesley Rhodes on Toxic
Algae Blooms can write for copies popular publications on Toxic
Algae. Address requests to: Jenny Ragg at the Cawthron Institute
Private Bag 2, Nelson