| Abstract: | Photoresponsive behavior in phytoplanktonic flagellates potentially controls depth regulation, vertical migration, and the accumulation of cells in preferred conditions and hence has major implications for photosynthesis, growth, and species competition. This study used microscale laboratory chambers with cell track analysis and mesoscale lake studies to investigate the responses of five phylogenetically contrasting freshwater flagellates to gradients of light. Laboratory results demonstrated that these species differed in their light preferences despite being grown at the same photon irradiance. Preferred photon irradiances were 20–120, 20, 65–120, 4–20, and>200 μmol photons·m?2·s?1, respectively, for Ceratium furcoides (Levander) Langhans, Chlamydomonas moewusii Gerloff, Dinobryon sertularia Ehrenberg, Euglena gracilis Klebs, and Plagioselmis nannoplanctica (Skuja) Novarino. Analysis of the response of individual cells showed that in all species, photoresponsive preference was the result of positive and negative phototaxis combined with step‐up and step‐down photophobic reactions. There was no evidence for photokinesis or cell memory. Only in C. furcoides did the preferred photon irradiance alter with growth conditions. In C. furcoides, D. sertularia, and P. nannoplanctica, irradiance preference matched the optimal irradiance for growth, whereas in the two remaining species a lower than optimal irradiance was preferred, suggesting that light may be used as an indicator of other ecological conditions. Mesoscale experiments in a lake demonstrated that the laboratory microscale measurements provide information relevant to understanding ecological distributions. Behavioral responses to light contribute to the delineation of vertical niche separation and provide a method for predicting the spatial and temporal distribution of flagellates. |
