The Ecology of Epipelic Algae of Five Welsh Lakes, With Special Reference to Volvocalean Green Flagellates (Chlorophyceae)1

The ecology of epipelic algae on the marginal sediments of five Welsh lakes was studied over an annual cycle. The lakes, Llydaw, Cwellyn, Padarn, Maelog and Coron ranged from very oligotrophic to nutrient‐rich. Attention was focussed on chlamydomonad flagellates, diatoms, blue‐green algae and euglenoids and the different proportions of these in algae in the epipelon of lakes of contrasting water quality. A total of 75 algal taxa was found in the five lakes, 25 were species of volvocalean flagellates. Mean annual population density of these flagellates differed by an order of magnitude between the lakes. The greatest population density was recorded forChlamydomonas anticontataSchiller in nutrient‐rich Llyn Maelog. Twenty species of pennate diatoms were recorded frequently in the epipelon. In the nutrient‐rich lakes, Maelog and Coron, pennate diatoms were dominant on the sediments, where they exhibited population maxima in spring and autumn. Increase in numbers of epipelic diatoms was recorded when silica concentrations were minimum in the overlying lake waters.Navicula hungaricaGrun. achieved the maximum population density, 260 000 cells · cm−2. Euglenoids formed large epipelic populations during late‐summer and autumn in these nutrient‐rich lakes. Blue‐green algae were more important, proportionally, in the nutrient‐poor mountain lakes, which had sediments of higher organic content. Chlamydomonads were the major algal component of the epipelon in the mountain lakes, Llydaw and Cwellyn, where the sediments were characterized by larger particle size, and higher organic content. In the nutrient‐rich lakes, where the sediments had higher calcium content, chlamydomonads formed significant populations only during spring and summer, when nutrient levels were minimal in the overlying lakewaters.