Vertical-migration patterns of flagellates in a community of freshwater benthic algae

Abstract

The population densities of sediment-inhabiting flagellates sampled from a shallow eutrophic lake in April 1986 were investigated at intervals of 1 or 1.5 h over a twenty hour period in the laboratory under natural irradiance and in controlled conditions. In natural irradiance the flagellates exhibited a vertical migration rhythm up onto the sediment surface after dawn and down into the sediment during the afternoon. Details of the timing of the migration movements and period of time flagellates were present on the sediment surface differed between species and five flagellate groups: trachelomonads, green euglenoids, chlamydomonads, chrysophytes and cryptomonads. During daylight, twenty-seven species maintained population maxima at the surface of the sediment on eight occasions. The species composition of these maxima differed and eight species were found to have two maxima on the surface of the sediment at different times of day. Numbers of three species of chrysophycean flagellates and Rhodomonas minuta increased again on the sediment surface after dark. Under continuous irradiance at 10°C, the migration cycle of all five groups of flagellates was affected. Some evidence for an endogenous nature of this rhythm was found for green euglenoids and chlamydomonads. Numbers of chrysophytes, particularly Synura spp. increased in constant light in the surface layers of sediment.

Thus a mosaic of vertical migration patterns was described in an epipelic community of algae dominated by five groups of flagellates. The importance of this in the survival strategy and ecology of these sediment-inhabiting algae is discussed.