Toxic gas threat as rotting sea lettuce piles up
THOUSANDS of tonnes of a marine plant, which produces a dangerous gas as it rots, is accumulating on beaches because of poor water quality.
The Environmental Protection Agency (EPA) says as much as 10,000 tonnes of sea lettuce is being washed ashore every year.
It produces the toxic gas hydrogen sulphide when it decomposes.
New wastewater treatment plants, tackling pollution from septic tanks and curbing agricultural run-off, are required to deal with the problem, according to the environmental watchdog.
The issue was first highlighted five years ago in a report from the Sea Lettuce Task Force, but the situation remains as bad today.
Problems have been identified in Co Cork at Inchydoney and Youghal, at Dungarvan in Co Waterford and at the Tolka Estuary in Dublin.
“The situation is relatively the same,” EPA marine scientist Dr Robert Wilkes said. “There hasn’t been much in the way of change in terms of environmental impacts. They haven’t build the sewage treatment plant for Courtmacsherry (in Cork) yet, and the diffuse sources are still the same, with agricultural run-off and septic tanks.
“From surveys, we can see we’ve got similar volumes (of lettuce). The distribution is slightly different, but we’ve had a relatively dry, hot summer and overall the amount is the same,” he added.
Sea lettuce occurs in areas where there are excess nutrients, principally nitrogen, which come from domestic, agricultural and industrial sources. Large accumulations can decompose rapidly, and give rise to public health concerns due to release of hydrogen sulphide (H2S), which produces a gas which smells like rotten eggs. This been linked to fatalities, as it also occurs in slurry tanks in farmyards.
The lettuce can also hamper swimmers, foul fishing gear and become entangled in propellers of boats.
“It’s the same gas in slurry tanks on farms,” Dr Wilkes said. “You only get that in huge accumulations where it starts to rot. The seaweed forms a crust, and if you break it you can get a sudden rush of gas. If you can smell it, you’re probably ok. If it’s above a certain level, it knocks out your sense of smell.”
Local authorities have been removing the lettuce from beaches, before composting it or spreading it on land. However, Dr Wilkes said it will take five years to see a noticeable improvement. He called for the full implementation of EU rules aimed at improving water quality.
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Hovercraft helping to monitor water quality
MONITORING water quality and the growth of seaweeds is a time-consuming task but use of a hovercraft allows the Environmental Protection Agency (EPA) to gather data much more quickly than by foot.
It would take three people up to a week to examine Sligo Bay. With the small hovercraft in use since 2008, it takes one person just a day.
Marine scientist Dr Robert Wilkes monitors 320 water bodies across the country – an area of some 14,000 square kilometres. Water samples are taken and the growth of sea lettuce measured. The work is carried out between May and October.
He places a quadrat, or steel grid, on the sand to record the growth of the marine plant. Any increase indicates a problem with water quality, as the weed feeds on nutrients from industrial, commercial and domestic sources. If sea grass is dying back, it also suggests a problem. The work is vitally important, not only to comply with EU regulations but also to protect human health.
“We look at maps of sea grass and compare it year on year. If the seagrass is under stress, it will die back and shrink. That could be because of pollution or physical disturbance,” he said.
He warned that shellfish can accumulate the toxins inside their flesh making them unsafe to eat for several months.
“It won’t kill the mussel, but will kill the person eating it,” he said.
The agency tests for plankton and water quality four times a year.