An investigation of the distribution in time and space of the algae of a British water reservoir

Abstract

i. The results of a continuous investigation of the algal flora of a water reservoir over a period of two years are given. The flora includes a littoral community (chiefly represented by Cladophora glomerata), the phytoplankton, and a number of bottom-living species. ii. The plankton includes a considerable number of species which are typical of the pelagic plankton of lakes, but there are in addition a number of species which are essentially littoral in their usual occurrence (e.g. Pediastrum Boryanum, Scenedesmus spp., Nitzschia acicularis). Many phytoplankton species are derived from the supply water of the River Thames (e.g. Staurastrum paradoxum, Stephanodiscus astraea, Fragilaria spp., Microcystis aeruginosa), but other important forms are lacking in the river and must have been introduced from other sources (e.g. Quadrigula closterioides, Coelastrum microporum, Anabaena spp., Aphanizomenon flos aquae). iii. The only perennial forms in the plankton were Cryptomonas ovata, Pediastrum Boryanum, Fragilaria crotonensis and Asterionella formosa. Carteria globosa, Staurastrum paradoxum, Anabaena spp. and Aphanizomenon flos aquae produced spores, but the majority formed no obvious resting stages and no doubt persisted as occasional individuals. iv. Evidence is given for the view that Scenedesmus spp., Coelastrum microporum, Fragilaria capucina, Nitzschia acicularis and Anabaena spp. persisted from one period of occurrence to the other in the silt. The same may be true of Carteria globosa, the species of Pediastrum, Oocystis and Closterium, Planktosphaeria gelatinosa, Staurastrum paradoxum, Melosira granulata var. angustissima, Fragilaria crotonensis, Synedra acus, Microcystis aeruginosa and Aphanizomenon flos aquae, but the evidence is not so satisfactory. v. The plankton differed very markedly in the two years, in 1937 including numerous Chlorophyceae, in 1938 consisting largely of diatoms and Myxophyceae. These differences can be correlated with the closure of the inlet to the reservoir in March 1937 which resulted in marked changes in the chemical components of the water. vi. The periodicticy shown by the algal flora can, in part, be correlated with changes in the amount of light, and the concentrations of nitrogen, phosphate, and silica in the water. In winter diatoms were conspicuous, although the dominant species were different in the three winters. The diatoms attained their maximum growth in spring, the important species in 1937 being Synedra acus and Nitzschia acicularis, in 1938 Stephanodiscus astraea and Asterionella formosa. In the summer of 1937 Chlorophyceae were conspicuous and were followed by Myxophyceae, but in 1938 the former were rare, and the Myxophyceaen phase succeeded immediately upon that of the pelagic diatoms. During the summer the nitrate content was negligible, while phosphates and silica increased. The autumn phase, characterised by the presence of a few diatoms and desmids, accompanied an increase first of ammoniacal nitrogen and later of nitrate nitrogen. vii. The Chlorophyceae (apart from desmids) of the plankton appear to require higher nitrogen and lower prosphate contents than do the diatoms, while the blue-green algae attain their chief development when the concentration of phosphate is high and the amount of inorganic nitrogen negligible.