ALGAL CYST DORMANCY - A TEMPORAL ESCAPE FROM HERBIVORY

Many phytoplankton species form resting cysts and remain dormant for p art of the year. The subsequent excystment is regulated by the externa l environment and internal maturation processes. Here we assessed the excystment of the dinoflagellates Ceratium hirundinella and Peridinium aciculiferum in relation to herbivores and temperature in laboratory and field studies. C. hirundinella, which has a grazer-resistant morph ology, forms summer blooms, whereas P. aciculiferum, which is vulnerab le to grazers, grows underneath the ice during winter. In our study, h erbivore abundance, and thereby grazing pressure, was low during perio ds when water temperatures were low, and the abundance of P. aciculife rum was high. In the laboratory experiment, excystment of C. hirundine lla occurred at high temperatures irrespective of whether zooplankton exudate was added or not, whereas at intermediate temperatures, excyst ment was lower if zooplankton exudate was added. Germination of P. aci culiferum cysts was lower in the presence of exudate from a zooplankto n culture than in controls at all temperatures. Our studies suggest th at dinoflagellates use the presence of zooplankton in addition to temp erature as a cue to determine when to excyst. Consequently, not only a biotic factors, but also the composition of the food web, may determin e succession and composition of phytoplankton communities.