Manganese and land-use in upland catchments in Scotland.

Manganese (Mn) in surface waters is a micronutrient, but elevated concentrations are toxic to fish and impair drinking water quality. In Scotland, undesirable Mn concentrations (> 0.05 mg l(-1)) occur predominantly in upland freshwaters because the acidic pH and organic nature of catchment soils favour Mn mobilisation. The relationship between upland land-use in Scotland and Mn concentrations in surface waters is reviewed. Conifer afforestation is associated with enhanced Mn in runoff. Mn is leached from conifer foliage and litter, and mature conifers enhance acid deposition and loss of Mn from acidified catchment soils. After harvesting, increased soil pools of water-soluble Mn and elevated Mn concentrations in runoff have been observed. Liming, fertiliser addition, drainage ditch construction and ploughing to improve upland pastures, and muirburn on grouse moors may also increase Mn concentrations in runoff, but the evidence is less clear-cut. The extent to which land-use influences Mn concentrations in upland catchments in Scotland is modified by catchment hydrology and soil type. Catchment geology, instream processes and standing water stratification are probably lesser influences on Mn concentrations in surface waters of upland catchments in Scotland. The location of land-use in upland catchments, especially in the riparian zone, is critical in determining its effect on Mn in runoff. Climate change is expected to increase Mn concentrations in runoff from upland catchments in Scotland because of predicted changes in soil hydrology.