About Loch Leven — a case study of global importance

Loch Leven, a large (13.7 km2), shallow (mean depth 3.9 m), eutrophic loch in the lowlands of Scotland, UK. It provides a wide range of ecosystem services across local, national and international scales. Over the past 150 years, this system has been managed in many different ways with the aim improving some of the key goods and services that the loch provides. These activities, which have been carefully documented in terms of their original aims and subsequent outcomes, have generated a wealth of historical information. This enables us to explore the effects of these management activities with the benefit of hindsight.

Loch Leven

Monitoring the environment

UKCEH, and its predecessors, have been monitoring Loch Leven at fortnightly intervals since 1968. The data collected form part of a wider monitoring programme undertaken by NatureScot, SEPA, Kinross Estate Company, University of Stirling and the River Leven Trustees. The dataset has been supplemented by information from shorter term collaborative projects such as palaeolimnological assessments of sediment records and data stewardship projects. The combined Loch Leven long-term dataset spans more than 150 years and includes records of about 150 variables including the weather; water temperature; conductivity; pH and dissolved oxygen concentrations; water level; inflowing water and discharge; phosphorus and nitrogen concentrations; phytoplankton, zooplankton and macroinvertebrate species composition and abundance; macrophyte species and distribution; fish species and abundance; and waterfowl numbers.

  Flow regulation and land-use Fishery performance Eutrophication
Pressures Downstream industries have relied on water from Loch Leven since the early 1800s; by the mid-1800s there was also a need for more agricultural land to feed a growing population. Loch Leven is a world famous trout fishery. In the early 1980s, measures were required to improve the performance of the fishery. Phosphorus (P) inputs to Loch Leven increased from around 5 tonnes of P per year in the early 1900s to about 20 tonnes of P per year by 1985. This caused a deterioration in water quality, including the proliferation of blue green algae (cyanobacteria) and a consequent decrease in the growing depth and diversity of underwater plants.
Management response Outflow regulation began in 1850 when sluice gates were installed; the loch level was lowered by about 1.5 m to create 265 ha of extra land for farming. The outflow is still managed to meet the needs of downstream industries. Brown trout were stocked between the 1880s and 1930s, and stocking restarted in 1983 (max. ~166,000 fish per year) after a dramatic decline in fishery performance. Brown trout and rainbow trout were stocked (max. ~30,000 fish per year) between 1993 and 2004. Phosphorus inputs to the loch were reduced between 1985 (ca. 20 tonnes per year) and 1995 (ca. 8 tonnes per year), by reducing outputs from a woollen mill and waste water treatment works, and by reducing runoff from farmland.
Environmental recovery Performance of the fishery decreased by 33% as a result of decreased littoral zone habitat. Downstream flooding increased in the short term. Ecological impacts included the loss of some aquatic plants (Isoetes and Chara spp.), the local extinction of Arctic charr, and fewer wading waterbirds. The performance of the fishery was not enhanced in the long-term. Rainbow trout stocking reduced the number of zooplankton grazing on the phytoplankton (algae), leading to greener water. Once the external input of phosphorus had been reduced, open water phosphorus concentrations fell, water clarity increased (especially in spring), and submerged aquatic plants recolonised deeper areas of the loch. However, full recovery took more than 15 years due to the internal release of phosphorus from the loch sediments.