Over-use of nitrogen-rich fertilisers combined with the effects of global warming are behind a host of recent seaweed blooms around the south-west coast, including a colony so large it is visible from space.
The most recent outbreak has seen the return of the red-tide bloom, Alexandrium tamarense, at Fal Estuary. It causes paralytic shellfish poisoning, which in humans leads to symptoms such as tingling, numbness, shaking, slurring of speech, burning of the stomach and fever. There is no antidote.
Scientists are still investigating the extent of the outbreak, but the Government is expected to impose a blanket ban on dredging shellfish in the next few days.
Another algae, Emiliania huxleyi, which is harmless, stretches more than 100 miles along Devon and Cornwall and 20 miles out into the Channel.
Their arrival follows a ban on scallop fishing off the west coast of Scotland when government officials ordered what is thought to be the world's biggest closure of a fishing area after toxins causing amnesiac shellfish poisoning were discovered. ASP causes irreversible brain damage.
Dr Martin Angel, a marine scientist who advises Ospar, which monitors algal blooms in EU waters, said last night that they are becoming more frequent because of fertiliser run-off into the water courses.
Nitrogen- and phosphate-rich fertilisers are ideal food for some of the more toxic blooms such as Red Tide.
"More and more fertiliser is being used and is finding its way into the system making the coastline a perfect breeding ground for these toxic blooms," he said. "They occur naturally, but in the past 20 years the amount of available nitrogen has more than doubled ."
His concerns are mirrored both by Ospar and Britain's Environment Agency. Ospar is conducting research into the eutriphaction, or nutrient enriching, of the north-east Atlantic and North Sea and has set levels for reduction in nitrogen pollution by 2010.
Since 1991 the Environment Agency has recorded a steady rise in the number of algal blooms. It now tests 470 bathing sites in England and Wales 20 times a year for toxic algae.
An agency spokesman said: "It's still very much a jigsaw puzzle for the scientific community. We don't know really what combination of sunlight levels, water temperature or pollution is responsible for the steady emergence of these blooms. "It is something that does concern us. Not only do you get massive fish kills when these blooms die off and use the oxygen but they threaten humans through poisoning of shellfish."
So far, Britain has been spared the worst of the algal blooms that plague tropical and sub-tropical waters. But if conditions continue to get warmer and richer, the south coast could be host to the worst bloom of all, Dr Angel warned.
Pfiesteria piscicida, only discovered this decade and resident in the waters of the US eastern seaboard, is so toxic it poisoned at least 13 scientists sent to research it, leaving them with brain damage.
Unlike other algal colonies it does not just float on the surface of the water; it actively hunts prey and paddles towards fish and other marine life.
"It's unlikely but we just don't know how all the climatic factors will interact in the future and whether pfiesteria could make it here and survive," Dr Angel said. "We think our waters are still too cold."Reuse content