The increasing nutrient loading of freshwaters from urban and agricultural sources results in the occurrence of algal blooms (Devai MoldovAn 1983, Grigorszky et al. 2000, 2003a). Among organisms that cause nuisance blooms, dinoflagellates are important. They often have a dormant (resting) stage in their life cycle. Recently, many aspects of dinoflagel-late blooms, such as timing, location and duration, have been linked to the encystment-excystment cycle of species that form resting cysts (Flaim et al. 2004). The cyst stage is considered important in dinofla-gellate biology making the analysis of environmental triggers of encystment meaningful. The important functions of such cysts are: short- and long-term survival under unfavourable conditions, blooming initiation, species dispersal, and preservation of genetic variability (Grigorszky et al. 2003b). Many studies involving the cyst formation both in culture and in situ have been primarily concerned with elucidating the life histories of dinoflagellates (Doucette et al. 1989). Results of these studies suggest that nutrient deficiencies were found to trigger encystment in some species (Chapman Pfiester 1995). Quantitative studies of encystment in cultured dinoflagel-lates have been performed for only a small number of species (Park Hayashi 1993). None of these experiments found evidence for encystment induced by a decline in either cell numbers or growth rates. The purpose of this study was to examine the encystment of Peridinum cinctum Stein and to determine whether the formation of cysts of this species follows the environmentally induced patterns observed in other species.
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