Algal cyst dormancy: a temporal escape from herbivory

Many phytoplankton species form resting cysts and remain dormant for part of the year. The subsequent excystment is regulated by the external 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 morphology, forms summer blooms, whereas P. aciculiferum, which is vulnerable to grazers, grows underneath the ice during winter. In our study, herbivore abundance, and thereby grazing pressure, was low during periods when water temperatures were low, and the abundance of P. aciculiferum was high. In the laboratory experiment, excystment of C. hirundinella occurred at high temperatures irrespective of whether zooplankton exudate was added or not, whereas at intermediate temperatures, excystment was lower if zooplankton exudate was added. Germination of P. aciculiferum cysts was lower in the presence of exudate from a zooplankton culture than in controls at all temperatures. Our studies suggest that dinoflagellates use the presence of zooplankton in addition to temperature as a cue to determine when to excyst. Consequently, not only abiotic factors, but also the composition of the food web, may determine succession and composition of phytoplankton communities.