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Monday, October 26, 2009
LAST week the first storm of winter in this part of the world, with the huge gunnera leaves in the garden almost flying off their stalks – they need controlling anyway – and things going bang in the night.
The torrential rain brought mud and gravel pouring down the by-roads and next morning council workers were out sweeping up the mess.
At low tide on the morning after, the ebbed estuary shone in the sunlight, with specks of gleaming white far out on the mud – shelduck, egrets, gulls, oystercatchers and lapwing. In places, sea lettuce had been thrown over the sea wall and lay shining on the footpath. I found a Painted Lady butterfly in full colour plastered to the wet concrete, and occasional earthworms, alive but stranded, presumably washed out of their burrows by the rain. Out of respect for their industry I dropped them onto the sanctuary of the wet grass. The green weed persists on beaches and in bays around the Irish coast. It is alarming to learn that few invertebrates can survive beneath it, while none grow on the upper surfaces due to the toxins it produces. Fish, crabs, and wading birds – many of them long-distance winter migrants – are denied vast areas of feeding grounds. The birds feed on areas of bare mud or between patches of weed. The mats also kill molluscs and eelgrass meadows, important nurseries for young fish.
The other morning the vast lawns of sea lettuces looked beautiful in the sunlight, like dark green golf courses or the Serengeti after the first annual rain. However, when it dries over the cord grass it appears like stretched grey skin with ribs protruding from beneath. The Serengeti suddenly seems to be ringed with corpses of dead elephants. Happily, most are washed away by the high tides.
In Ireland, the weed’s proliferation, and the problems it causes for tourism, bathers and boaters, were unknown until 10 years ago. However, the extent of its growth in places like Inchydoney, Ring, Harbour View and Coolmaine has so overwhelmed Cork County Council that two weeks ago a task force of experts met in Clonakilty to discuss plans for identifying the cause and drawing up a strategy to combat it ahead of next year’s summer season. The team included officials from the council’s scientific, environmental and engineering departments, along with officials from the Marine Institute, the Department of Agriculture, Fisheries and Food, the Health Service, the Environmental Protection Agency and the Parks and Wildlife Service.
The problem is by no means confined to Ireland. According to Dr Clyde L. MacKenzie, Jr. of the Northeast Fisheries Science Center in New Jersey, mat-forming sea lettuce is now present in the estuaries of North America, South America, western and southern Europe, Japan, Korea, and China, Australia and New Zealand, India and Pakistan. The increases are due to large inputs of pollutants, mainly nitrates.
The algaes occur on wide, gently sloping sand and mud flats where tidal circulations and wind-driven waves are weak. During the warm months the mats lie loosely on shallow sand and mud flats. Currents and winds carry them to the shore, where they accumulate in piles on sheltered beaches. Sea lettuce overwinters as buds attached to shells and stones, and in the spring it grows as thalli (leaf fronds). Mats eventually form that are several thalli thick.
An experiment by Dr MacKenzie, published by the US National Marine Fisheries, showed that two removals of sea lettuce in a summer from a shallow estuary maintained the bottom almost free of it. The thalli began to grow in late April and by early June had covered the bottom with a single floating layer. It was removed with a seine net, which extended from the surface to the bottom. The scattered pieces that remained began to grow again and in early September were again cleared. The remaining scraps again began to grow but did not cover the area before they began to decompose in the autumn. The method worked in water up to 1m deep. Could it also function beyond estuaries if applied at low tide?
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