Long-term water-quality monitoring at Loch Leven

Loch Leven is one of the CEH’s most important long-term monitoring sites and now forms part of the Environmental Change Network (ECN). Chemical and biological water quality data have been collected here since 1968. These data have provided the scientific basis for lake and catchment management strategies aimed at solving Loch Leven’s eutrophication problems here. These problems are most visible as cyanobacterial blooms, which affect amenity value and have a marked impact on the local economy. The most famous example occurred on "Scum Saturday" in June 1992, an event that attracted considerable media attention (see algal bloom photo on right).

The CEH dataset suggested a direct link between the occurrence and severity of algal blooms and the availability of phosphorus (P). So, in 1985, the CEH undertook a detailed study to identify and quantify the main sources of P within the catchment. The study estimated the P load from the catchment to about 20 tonnes per year, with 58% of that load emanating from point sources. As a result of this work, discharges from point sources (e.g. sewage works, industry) were targeted for control, incurring costs of more than £4 million. In 1995, a further study showed that, as a result, the contribution of P from point sources had fallen from 11.6 tonnes per year to 3.3 tonnes per year. The amount of P retained within the loch had also fallen from 11 tonnes per year in 1985 to 1 tonne per year in 1995. Over time, this should result in a reduction in the amount of phosphorus re-cycling from the sediments and lead to an improvement in water quality.

Recent data, such as measurements of phosphorus concentrations, indicate that Loch Leven is beginning to respond to the reductions in P load from its catchment. Although algal blooms are still common, summer peaks in soluble reactive P (SRP) concentrations have progressively fallen since 1995. This suggests that the amount of P being re-cycled from the sediments has started to decrease. In addition, there is some evidence that rooted macrophytes are beginning to colonise deeper waters. This also indicates an improvement in water quality. CEH staff are continuing to monitor recovery processes at Loch Leven.

Background information on Loch Leven

1. Loch Leven is owned by the Montgomery family. Management and conservation of the Loch and surrounding area is the responsibility of a wide range of people and organisations including Scottish Natural Heritage, Royal Society for the Protection of Birds, Scottish Environment Protection Agency, Kinross Estates, the Loch Leven Trustees, and Perth and Kinross Council.

2. Loch Leven is the largest shallow loch in lowland Scotland. It is an internationally important nature reserve, not only because it attracts the largest concentration of breeding ducks found anywhere in the UK, but also because it provides an autumn and winter refuge for thousands of migratory ducks, geese and swans.

3. By the mid 1980s, phosphorus pollution from industry, sewage and agriculture was entering the loch. This caused a serious decline in water quality. Nutrient levels increased, toxic algal blooms became more common and water clarity became reduced. This resulted in both ecological and socio-economic deterioration of Loch Leven which led to questions being raised in parliament. As a result, there were widespread demands for the situation to be improved. A variety of conservation measures were put in place based on scientific evidence collected during the long term monitoring programme carried out by the Centre for Ecology & Hydrology. These reduced the phosphorus input to the loch by about 60 per cent. Recovery was not immediate because phosphorus that had built up in the lake sediments over the years was released slowly into the water over the next 10-15 years. However, eventually, the combination of good management practices and patience paid off. Algal blooms reduced in frequency, the water became clearer and underwater plants began to thrive again in deeper water.

4. Loch Leven has a surface area of 13.3 km2, a mean depth of 3.9 m, a maximum depth of 25.5 m, and a surrounding catchment area of about 145 km2. Catchment landuse is mainly rural or agricultural.

5. Mary Queen of Scots was imprisoned in Loch Leven Castle in 1567-68, but she was probably more worried about recovering her crown than the state of the surrounding loch’s ecology.

See also: Loch Leven on the road to recovery after 40 years of science.