Phosphate remedy for acid lake

Environmental scientists have found a paradoxical way of solving the problem of acidic lakes - by adding to them a substance that causes the other major environmental lake problem, eutrophication.

Lakes which are naturally mildly acidic can become much more acidic as a result of acid rain. Such lakes are a problem because they cannot be used as water supply and there is little biological growth, so there are no fish.

Meanwhile, water at lower levels in agricultural areas faces the problem of eutrophication from alkaline substances.

Phosphate fertilisers and waste water containing phosphates from detergents enter the water system. They are taken up by algae which multiply. The lake produces green scum and occasionally toxic algae. There is loss of oxygen and light is barred from reaching life under the surface.

Bill Davison, professor of environmental chemistry at Lancaster University, has been experimenting with Seathwaite Tarn, a reservoir in the Lake District which suffers from acidity.

The conventional way of combating acidity is to neutralise it with lime. But to get the lime to work, it must reach saturation level. This leads to a highly alkaline lake.

So Professor Davison, with scientists from the Institute of Freshwater Ecology in Cumbria, has tried pouring into the lake the culprit of eutrophication: phosphate. The experiment seems to have worked. The phosphate stimulated growth in the lake; the resulting algae absorbed increased amounts of the nitrate; and the nitrate, once in the algae, changed composition.

Thus, the algae had effectively absorbed the crucial component of the acid rain. When the algae die they release the phosphate and it becomes available for other algae.

Professor Davison admitted that the system seemed paradoxical given the problems with excess phosphate elsewhere. He said that one advantage was that phosphate does not get out of control. It is gradually lost as water leaves the lake and must be topped up - which leaves him in control of the remedy.