Temporary cyst formation of<Emphasis Type="Italic"> Heterocapsa triquetra</Emphasis> (Dinophyceae) in natural populations

Heterocapsa triquetra (Ehrenberg) Stein is a phototrophic marine dinoflagellate with wide coastal distribution. It is known to be capable of mixotrophy and diel vertical migration. The species was particularly abundant in the Gulf of Finland (the Baltic Sea) during the summers of 1996 and 1998, leading to discolouration of water on the south-west coast of Finland. Large-scale (50 m³) coastal mesocosm experiments in the north-west Gulf of Finland (the Baltic Sea) in the summers of 1996 and 1998 with daily mineral nutrient additions provoked a biomass increase of phytoplankton dominated by H. triquetra. From the first days of the experiment temporary cysts of H. triquetra were found in the bottom sediment water of the mesocosms. Maximum temporary cyst production rates reached values up to 20×10⁶ cysts m^–2 day^–1, accounting for <1% of the depth-integrated motile population size. The environmental features favouring temporary cyst production remain uncertain; zooplankton grazing and nutrient stress are potential factors. Temporary cysts of H. triquetra were observed in a unialgal culture (f/2 medium) isolated in summer 1999 from Eel Pond (Woods Hole, Mass., USA).Communicated by M. Kühl, Helsingør