LLYN MAWR is one of three lakes within a Site of Special Scientific Interest, high up on the moorland plateau above the Carno Valley. The reserve comprises the 7.7ha (19 acre) lake itself and a narrow margin of surrounding land adding a further 4.4ha (11 acres).
It is an important site for wildlife conservation of aquatic plants and of bird life in both summer and winter. Visits by the public are welcome but please keep to the southern shore and do not attempt to walk right around the lake.
The lake is described as mesotrophic - having a moderate amount of nutrient in the water, derived from the surrounding moorland. It therefore has a relatively rich growth of plants and a correspondingly large number of animals.
At the western end particularly it illustrates a 'textbook example' of succession of vegetation from that of the open water, through a swampy zone of marsh plants to willow carr (willow trees growing in a waterlogged soil) then up onto the dry heathy bank.
The open water plants include shoreweed growing on the lake floor, along with the superficially similar quillwort. But the former is a flowering plant, whilst quillwort is related to the club mosses and ferns. More conspicuous are the marginal plants growing up above the water surface which include yellow water lily and broad-leaved pondweed, both with floating leaves.
The rather stiff but distinctive bog bean sends up aerial shoots off underwater runners. The attractive maroon flowered marsh cinquefoil spreads through the marshy edge in a similar way, helping to create a mat of vegetation which includes the insectivorous sundew and butterwort. Bog asphodel is noticeable all the year, as yellow flowering spikes in summer or dead brown flower stalks which last through the winter. Although Llyn Mawr is noted for its wetland birds, breeding success in recent years has been poor. The traditional black-headed gull population appears to be deserting Llyn Mawr for nearby Llyn y Tarw.
However the marshy land and tussocky grass around the lake are ideal for waders. Both snipe and curlew breed here. The haunting call of the curlew is one of the special features of this moorland landscape, but undisturbed nesting sites are getting fewer. Reed buntings nest amongst the rushes along the lake edge.
In the winter the lake is a popular roost with a number of waterfowl. Ducks such as pochard, wigeon, goldeneye, and the fish-eating goosander come here along with whooper swans - migrants coming south to escape the Arctic winter.
A number of management tasks occupy the Trust here, but potentially the most significant - keeping the swamp and willow carr from encroaching too far on the open water - appears to be under natural control. Possibly a relatively stable situation has been reached, with deeper water and wave action preventing outward growth of the shoreline.
There is an extensive network of bridleways that give access over the moorland and rough grazing of Mynydd Cerrigllwydian, one of which passes Llyntarw, another of the trio of Caersws lakes. This is good curlew country as well as offering hunting grounds for birds of prey including buzzard, kestrel and occasionally merlin and hen-harrier.
GRID REF : SO 013970
ACCESS : Open at all times. Access restricted to narrow marginal zone which does not allow easy access right round the lake.
PARKING : On the roadside verge but beware of ditch.
TIME : A brief visit will tell you whether there is much to see on the lake itself. Allow 1.5 hours if you want to explore the lake side.