| Summary: | Growth and physiological experiments were carried out using Scenedesmus subspicatus and Chlorella vulgaris as representative species typically found in waste stabilisation ponds. These experiments were designed to test the ability of the organisms to survive and grow under a range of different temperatures and light intensities that might occur in mid to high latitude regions. Growth was assessed using optical density and photosynthetic rate for a combination of temperatures of 5, 10, 15 and 20 °C at light intensities of 7.8, 15.7, 31.3, 47, 62.7 and 78.3 mmol m-2 sec-1. C. vulgaris had a higher rate of growth and photosynthetic activity than S. subspicatus at low temperatures but had reached its maximum growth rate at 15 °C. S. subspicatus showed a higher growth rate than C. vulgaris at higher temperatures, and did not achieve its maximum growth rate over the range of temperatures studied. For both species light was not limiting to growth above 47 mmol m-2 sec-1. Survival of the two species under dark conditions was tested at 4 °C and -20 °C using direct plating and growth tests. C. vulgaris was able to survive at 4 °C for a much longer period than S. subspicatus and a portion of the population was able survive -20 °C. The different responses of the two species to dark and cold conditions are indicative of the range that may occur across a wider population, and show why in practice some species may appear earlier and compete more effectively in early spring but then lose advantage as the temperature and light intensity increases into the summer.
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