FACTORS INITIATING ALGAL LIFE-FORM SHIFT FROM SEDIMENT TO WATER

In enclosure and whole-lake experiments, I tested whether life-form sh ift (recruitment) is a passive process induced by turbulence. a season al phenomenon. or a behavior that can be induced by alterations in env ironmental variables. The number of algal cells recruited from the sed iment varied considerably during the experimental period. The most imp ortant migrating genera in this study were: Cryptomonas, Dinobryon, Go nyostomum, Gymnodinium, Peridinium, and Synura. An obvious conclusion is that it is not the same factor in each case that causes life-form s hift, but that different triggering factors operate in different algal species. Turbulence and temperature were similar in all treatments an d therefore did not cause the considerable fluctuations and trends in algal recruitment in the enclosures. This suggests that life-form shif t is not a passive process driven by wind and temperature-induced curr ents. In the enclosure experiment, alterations in the light regime exp lained a major part (up to 53%) of the variation in recruitment for mo st genera. For Gymnodinium this was corroborated in the whole-lake exp eriment, where the depth of the euphotic zone explained 41% of the var iation in recruitment. For Gymnodinium, however. 64% of the variation in recruitment was explained by the depth of the ''oxycline'', whereas 52% of the variation in recruitment of Synura were explained by the d epths of the euphotic zone and the oxyline. Peridinium pusillum and P. wisconsinense showed low recruitment at high zoo-plankton abundance a nd high recruitment at low zoo-plankton abundance in the lake experime nt, as well as in the enclosure experiment. Thus, the hypothesis that the presence of grazers can induce shifts in behaviour of some algal g roups cannot be rejected.