Toxic algae season begins in Scotland

The toxic algal blooms which affected many waters in southern and central Britain so dramatically in 1989 (ENDS Report 176, pp 9-11) have now put in their first major appearance in Scotland. Following a spell of calm, dry weather, many Scottish lochs have produced an early crop of toxic algae, drawing attention to the expense and difficulty of restoring lakes which receive phosphate and nitrate inputs, and highlighting possible eutrophication problems in lochs which had been thought to be pristine.

The least surprising bloom event in June occurred on Loch Leven, a large loch near Kinross which is Scotland's most famous eutrophic water. Summer blooms of the cyanobacterium Anabaena have been common since the turn of the century.

The bloom peaked with impeccable timing on the evening before a waterside gala to launch the south-east region of Scottish Natural Heritage. A thick, paint-like scum coated the loch's western shore where the gala was being held. Water sports were cancelled and the consequences of the loch's enrichment with nutrients were on full display.

Unlike most Scottish lochs, Leven is relatively shallow, and drains intensively cultivated farmland. It is also a major trout fishery and a National Nature Reserve. It has received nutrients from industrial, sewage and agricultural sources for many years.

The loch has been studied intensively by the Institute of Freshwater Ecology, and nutrient loads have been well documented. Inputs of phosphate peaked in the early 1970s at 40 kilos per day. The sharpest recent reduction came in 1989 following a process change at a local woollen mill which removed phosphate from its effluent.

Streams now carry 20 kilos of phosphate per day into the loch, mostly from diffuse agricultural sources and minor sewage discharges. Three sewage works serving a population of 10,000 people account for the remaining 10 kilos per day.

It is now clear that the loch has shown no improvement following the elimination of the woollen mill's inputs. The Forth River Purification Board (RPB) has brought forward its plans to open discussions on further reductions.

A Forth RPB spokesman commented that the changes will have to be far-reaching and probably expensive. Phosphate stripping at sewage works alone will be insufficient. Buffer strips along stream margins and settlement lagoons to reduce nutrient levels in water entering the loch are being mooted as possible solutions.

Phosphate-rich sediments in the loch will pose a long-term problem, but dredging or chemical treatment will need to be considered carefully because of the effects on wildlife.

Forth RPB is also concerned about eutrophication of other lochs in its area which are naturally low in nutrients. As Loch Leven has shown, the consequences of delaying nutrient controls may be long-term damage which is expensive to remedy, but Scottish regional councils work on fixed capital allocations, making the introduction of phosphate stripping slow.

Many cyanobacterial blooms have also been reported in the Strathclyde region this year, often in waters low in nutrients which have been assumed to be little affected by eutrophication. At Loch Awe, four dogs were poisoned after eating algal scum, and two subsequently died. Water sports have been curtailed at Kilbirnie Loch in Ayrshire following a bloom.

Although these lochs produce small blooms which either form on the shoreline or are concentrated by the wind, their nutrient inputs and assumed pristine status will now have to be re-examined.

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