Abstract
Abstract The diurnal migratory rhythm of the epipelic diatom association of freshwater streams has been investigated in laboratory studies. Movement of the diatom flora up to the surface of the sediment and down beneath the surface occurs once every 24 hours reaching a peak of cell numbers at the surface at approximately the same time each day. This migratory movement persists in the laboratory for at least eleven days under alternating light/dark conditions. It is also expressed in continuous darkness but the number of cells migrating is less than under conditions of alternate illumination and darkening. In continuous illumination the rhythm is disturbed. All the common diatom species behave in approximately the same way as the population. From the data, the existence of three separate rhythms of motility, phototaxis and geotaxis have been deduced. During the first half of the light cycle the cells increase their motility and become positively phototactic and during the latter half of this cycle they los...
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