Most of the upland lakes in Wales would be in the oligotrophic category, with a small number supporting native populations of Arctic charr (Salvelinus alpinus) and a distinctive flora typically comprising species including water lobelia (Lobelia dortmanna), quillwort (Isoetes lacustris) and shoreweed (Litorella uniflora) and occasionally the rarer awlwort (Subularia aquatica). The water in these lakes tends to be clear and soft but suffered increased acidification during the mid to late 20th Century. A more current issue is the increasing frequency of extensive algal blooms as a consequence of increased summer temperatures.