Blankets of toxins carried in on the tide

IN spring, a great upwelling of water brings minerals from the ocean floor to nourish the growth of phytoplankton, the single celled, microscopic plants that, directly or indirectly, feed most of the animals living in the sea.

The plants need to buoy themselves up in the sunlit layer of water. Some of them - the diatoms - do it with tiny droplets of oil. Others, on the borderline between plants and animals, are swimmers, with long, whip like tails.

These are the dinoflagellates, and among them are the tiny number of phytoplankton species that produce potent poisons: nobody quite knows why. Perhaps they are waste products that just happen to be poisonous; perhaps they favour the plants' survival by killing the animals - the zoo plankton - that feed on them.

All over the world, and recorded since Biblical times, toxic dinoflagellates have been killing fish and, on occasion, warm blooded animals, including people. Developed countries, like Japan, New Zealand, the US and Ireland, protect their shellfish industries and public health by regular monitoring of the level of toxic algae. The recent warning not to eat shellfish from Cork Harbour was typical of temporary, precautionary "closures" of bays and estuaries.

Toxic algae can kill ocean animals in two different ways. The most dramatic are associated with a "red tide" (which can actually be orange. brown or even blue, depending on the species involved). What happens is that huge numbers of the dinoflagellate plants are concentrated in one area by coastal currents - sometimes up to several millions of cells per litre of water.

Then, if they use up all the nutrients in the water, or the temperature suddenly falls, the whole population can die simultaneously. Their decomposition uses up so much oxygen that masses of other animals die too.

A red tide can also poison other organisms directly, with toxins that cause the blood cells in fish gills to burst or, in warm blooded animals, affect the nervous system, bringing paralysis and death. The notorious red tides of Florida can leave its western shores piled with dead fish and irritate the lungs of people on the coast. This spring, toxic algae were also held responsible for the deaths of 158 of Florida's manatees, already an endangered sea mammal.

The red tide that crept around Florida in 1994 killed fish across 5,000 square kilometres of open sea - everything from groupers and snappers to tarpon and sharks were floating on these game fishing waters.

That year, as it happened, was also the worst "toxic season" on record in Ireland. Since monitoring (by the Fisheries Research Centre of the Marine Institute) began in 1984, there have been regular closures of shellfish growing areas, usually between June and September. But in 1994, toxins were detected in bays on the south west coast in mid May and weekly bioassays of shellfish samples were still finding poison until February 1995.

THE problem is that toxic algae seem not to do bivalve shellfish any harm. Mussels and oysters feed directly on the phytoplankton, sieving out the cells from many litres of water every day. The poison accumulates in the bivalves and can concentrate quite low levels of toxins into doses that can make human shellfish eaters seriously ill.

Thus, it doesn't need a red tide to set off closures and health warnings. Killary Harbour, for example, has had its mussels quarantined this summer after a finding of only 200 cells of Dinophysis species per litre of seawater; these are toxic algae associated with diarrhetic shellfish poisoning.

These dinoflagellates occur regularly in Irish waters during the summer and a "pulse" of Dinophysis acuta reached 20,000 cells per litre in inner Bantry Bay in August 1994. Diarrhetic poisoning, or DSP, is our main potential problem - but at least we don't have the neurotoxins of the southern US or the amnesic shellfish poisoning further north.

PSP, or paralytic shellfish poisoning, is the most severe of all, producing progressive muscular weakness and breathing difficulties; it can kill. The last recorded cases in Ireland are from the 19th century. Alexandrium, the dinoflagellate species detected in Cork Harbour, makes PSP problems all round the world.

There certainly seems to have been a global increase in harmful algal blooms (HABs, in the trade - there's even a Harmful Algae News, published by the Intergovernmental Oceanographic Commission of UNESCO). But is this a real phenomenon or just a reflection of greater scientific interest in plankton events and the increasing importance of the shellfish industry?

If toxic blooms really are increasing, many will look to the effects of global warming. By that hypothesis, last year's long, hot summer in Western Europe might have been expected to produce unusual algal blooms.

Along the coast of Brittany, indeed, blooms of the dinoflagellate Gvmnodinium produced spectacular kills of fish (including farmed trout) and massive mortalities of sea urchins and worms. In Ireland, on the other hand, it was not an exceptionally "toxic" year, judged by the monitoring of the Fisheries Research Centre.

Eutrophication is another suggested cause for the increase in blooms, especially in semi enclosed waters. Greenpeace, already agitated about the toxic blue green algae killing fish on Lough Derg, in the Shannon, may well take the PSP threat in Cork Harbour as a further sign of that estuary's overload of nutrients.