ENVIRONMENTAL AND ENDOGENOUS REGULATION OF CYST GERMINATION IN 2 FRESH-WATER DINOFLAGELLATES

The role of excystment in relation to seasonal succession was investig ated in two freshwater dinoflagellates, Ceratium hirundinella (O.F. Mu ller) Dujardin and Peridinium aciculiferum (Lemmermann). Field studies and laboratory experiments were performed to determine which factors regulate the timing of cyst germination. Environmental factors (temper ature, light, nutrients, and anoxia) and endogenous factors (maturatio n period and biological clock) were investigated. Our main results ind icate that temperature and internal maturation period determine when g ermination can, occur. C. hirundinella had a maturation period of 4.5 months and germinated in the laboratory and in the field at temperatur es above 6 degrees C. P. aciculiferum had a maturation period of 2.5 m onths and germinated in the laboratory and in the field at temperature s below 7 degrees C. In addition, our results indicated that both spec ies were regulated by a biological clock. Furthermore, anoxia prevente d the germination of C. hirundinella, contrary to results in earlier s tudies. To conclude, we could explain the appearance in plankton of th e two dinoflagellate species through two main factors regulating excys tment, that is, temperature and maturation period.