An Investigation of Two Benthic Algal Communities in Malham Tarn, Yorkshire

Abstract

The algal flora growing on two sediments in Malham Tarn, Yorkshire, has been investigated over a period of nearly two years. One sediment is of a peaty nature and the other highly calcareous. Sampling was carried out every two weeks as far as conditions permitted. As many algae as possible were identified and an estimation of numbers made over this period. An attempt has been made to correlate the distribution of algae on the sediments and their seasonal periodicity with the chemical analysis of the lake water and a consideration of the physical and chemical nature of the sediments. The major groups represented in the flora are the Bacillariophyceae (forty-six living species), Cyanophyceae (sixteen species) and the Chlorophyceae (nineteen species). These are not equally distributed on the two sediments, the Diatomaceae being better developed on the calcareous sediment; only one member of the Cyanophyceae develops to any extent (Pseudanabaena catenata) on either sediment. Both the Cyanophyceae and the Chlorophyceae are present in greater variety on the peat than the calcareous sediment. The floras are shown to be greatly influenced by the calcareous and peaty natures of the sediments. There is a seasonal variation in numbers of all three groups: the Bacillariophyceae having peaks of growth in spring and autumn, the Cyanophyceae attaining a maximum growth in spring and the Chlorophyceae a peak of growth in May and a second in August. The seasonal periodicity of growth is related to the nutrient concentration of the lake water and seems to be unaffected by the sediments. In the case of the Bacillariophyceae a relation between silica concentration in the water and growth on the sediments is recorded.