Options for liming rivers to ameliorate acidity — A UK perspective

The options for large scale liming of river systems are reviewed with particular reference to the River Tywi in Wales, a major salmonid river with a reservoir in the upper catchment. For hydrological source area liming the cost of lime transport is high due to remoteness and inaccessibility. The concern of potential damage to wetland mires of conservation value is considered. Re-treatment intervals are expected to be 5–10 years, but results from sub-catchment experiments indicate that treatment longevity and the pattern of ecological recovery are uncertain. Flow-related dosing systems for rivers avoid impacts on wetlands but would treat only the main river. Reliability may be problematical though the reservoir provides a margin of safety against system failure. Reservoirs can be limed to treat the main river outflow. This avoids the problems of power supply. For the River Tywi, the financial costs of both direct treatment methods are lower than for catchment liming and comparable with estimates of the economic benefit to the fishery. The system currently operated on the Tywi is reservoir treatment. Results from the first 3 years demonstrate colonization of the main river by acid intolerant invertebrate taxa and clear increases in populations of juvenile Atlantic salmon, Salmo salar L. and sea/brown trout. Salmo trutta L.