Hard oligo-mesoptrophic lakes are characterised by water with a high base content, most often calcium, but very rarely magnesium, and are usually confined to areas of limestone and other base-rich substrates. These standing waters typically have very clear water and low nutrient status. They are therefore largely restricted to situations where the catchment or aquifer from which they are supplied with water remains relatively unaffected by intensive land-use or other sources of nutrients. Stoneworts (Charophytes) are a typical component of the flora and often form dense beds.

Two of the best examples in Wales are Kenfig Pool and Bosherston Lakes. Kenfig Pool is a shallow lake system within the extensive sand dune system of Kenfig, where high calcium values are probably derived from the remains of marine shells in the sandy substrate. The aquatic flora includes shining pondweed Potamogeton lucens, hairlike pondweed P. trichoides, rigid hornwort Ceratophyllum demersum, fan-leaved water-crowfoot Ranunculus circinatus and the charophytes Chara aspera var. aspera and Nitella flexilis var. flexilis. In contrast, Bosherston Lakes is a shallow marl lake system created by damming a limestone river valley in the late 18th to mid 19th centuries. The lakes are fed in part by calcium-rich springs and are isolated from the sea by a small sand dune ridge. Charophytes are represented by bristly stonewort Chara hispida which forms dense beds up to 1 m high, and by C. globularisC. virgata and C. vulgaris. Extensive white water lily Nymphaea alba beds also occur, mainly in the western and central arms.