Diel variations in nitrogen and carbon dioxide fixation by the blue-green alga Rivularia in an upland stream

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Abstract
Field and laboratory studies are described on nitrogen fixation (acetylene reduction) and 14CO2 fixation during summer by a population of Rivularia growing in Red Sike, a stream in Upper Teesdale, northern England. An in situ diel study of carbon dioxide fixation showed a significant correlation (r = 0.84, P < 0.01) between fixation rates and light flux, but a relatively low correlation between nitrogen fixation and light flux (r = 0.23, P > 0.05). Although the rates of nitrogen fixation were much higher by day than night, between 8 and 16% of total activity occurred at night (5–8 h dark); a small nocturnal peak was evident in one of the three diel surveys. When colonies were shaded to reduce incident light to 15% of full irradiation, total daytime activity was doubled. This effect was also shown clearly in a laboratory study. Carbon dioxide fixation decreased with decreasing light intensity, whereas nitrogen fixation showed a bimodal response to light, with a low intensity peak at a photon flux ...