- Location: North West Scotland
- Type: Lake
- Catchment area: 897 ha
- Annual rainfall: c. 3300 mm
- Lake area: 11.6 ha
- Lake altitude: 125 m
Situated in north west Scotland Loch Coire nan Arr was chosen as the original control site for the Network. The loch lies on the Torridonian sandstones of the Applecross peninsula. The catchment is extensive (897 ha) and dominated on three sides by the steep corrie cliffs which rise to a maximum altitude at Beinn Bhan of 896 m. The floor of the corrie is overlain by peaty podsols whilst above the flatter ground subalpine soils and bare rock predominate.
The loch and its catchment receive an annual rainfall of c. 3300 mm.
The catchment is unafforested (a small block of conifers is planted on the eastern slope, but has failed to mature successfully) and is characterised by acid moorland species, notably Calluna, Erica, Vaccinium and Eriophorum. This vegetation is grazed at a low intensity in summer by sheep, and all year round by red deer, but there is little evidence of any other active land management. The loch lies within the Beinn Bhan Site of Special Scientific Interest and Special Area of Conservation.
Loch Coire nan Arr lies in a deep sandstone corrie at 125 m altitude. The loch covers an area of 11.6 ha and receives drainage at its northern end from the Allt Coire nan Arr and elsewhere along the loch from a series of small precipitous inflows. The loch drains 2 km to the sea via the Russell Burn. The loch has a very simple bathymetric profile with a single basin reaching 12 m at the deepest point.
In 1991 a temporary dam was placed on the outflow in order to preserve the water supply for a fish farm located downstream from the loch. This structure was replaced by a permanent sluiced dam that has raised water levels within the site by at least 0.5m. Since this dam was constructed water levels have fluctuated far more than in the past, resulting in significant changes in the loch's aquatic macrophyte communities. This disturbance is the reason that a second site in the same remote area of north west Scotland was chosen for monitoring, Loch Coire Fionnaraich, which has now fully replaced Loch Coire nan Arr.
Loch Coire nan Arr was one of the six UK sites represented in the UNECE International Co-operative Programme on Assessment and Monitoring of Acidification of Rivers and Lakes (UNECE - ICP Waters).