The local authority is considering whether it could be removed by hand dredging or blasted with selective weedkiller.
Dr Colin Reynolds, a senior research officer at the Institute of Freshwater Ecology at Far Sawrey, Windermere, Cumbria, said: 'The weed has been in the lake for 20 years or more but it has increased explosively in the last year and become a nuisance.'
The weed, Elodea nuttalli, is a flowering plant, although the flowers themselves are green and scarcely noticeable to the naked eye. A close relative, E. canadensis, invaded Britain in the 1840s and became such a pest that economic disaster was predicted when it blocked canals.
'The Canadian weed dwindled in abundance for no apparent reason and has ceased to be a problem. But now we have its relative which first became evident in the mid-Seventies,' Dr Reynolds said. 'It is virtually impossible to tell the two species apart except by a microscopic examination of the flowers.'
The weed has also been found in Grasmere, Rydal Water, Derwent Water and Bassenthwaite Lake. Dr Reynolds is afraid it could also explode in these lakes, which have similar characteristics to Windermere. They are relatively rich in nutrients coming in from farmland or sewage.
Pure crystal lake water can still be found in Wast Water, Ennerdale Water, Crummock Water and Buttermere. These lakes are fed by mountain water which contains very few nutrients, so few water plants can thrive there.
The introduction of mains water and sewerage to the Lakeland valleys in the Sixties, with increased discharge into the lakes, allowed algae to bloom in the polluted water.
Blooms began to be seen on some lakes, and blanket weed, a bright green stringy alga, began to grow abundantly on rocks and jetties near the water surface.
Last year North West Water began to introduce special processing to remove phosphates from the sewage discharge.
This has been monitored by the Institute for Freshwater Ecology, which has found that there is now a severe shortage of phosphate in the lake. However, there has not yet been any reduction in blanket weed or E. nuttalli growing in the water.
'The lake has been altered by the discharge and it may take 15 years or more for it to return to its original state,' Dr Reynolds said. 'As a result of treatment, about two-thirds of the phosphate is being removed from the processed sewage. There is now a severe shortage of phosphate in the lake.'
However, E. nuttalli is not directly affected by the shortage of phosphate because it has roots which allow it to pick up insoluble phosphate from the bed of the lake. Dr Reynolds believes that the weed has succeeded in growing in the lake because it has rooted in places where the blanket weed, Cladophora, has already attached itself.
'Cladophora is dependent on phosphate in the water. If it begins to be reduced in quantity then we may see the Elodea begin to go too,' Dr Reynolds said. 'But it could take a long time because a large amount of phosphate has been fixed by plants in the lake and it will take years for that to be washed out.'
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