The role of the cyst stage in the seasonal growth of the dinoflagellate Ceratium hirundinella within a small productive lake

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Abstract
Large populations of the dinoflagellate Ceratium hirundinella occur regularly in a small productive lake and their decline during September and October is normally associated with mass encystment. Evidence for the major role of benthic cysts in providing the inoculum for subsequent growth is given from morphological and quantitative studies over two contrasting years of cyst production, 1980 and 1981. For both years there were two distinct phases of population increase in the plankton. Low winter densities were followed by rapid rates of increase during February and March at the same time as empty cysts appeared in the sediment. These changes occurred when water temperatures rose from approx. 3 to 5°C. After near-stationary periods in April and May 1980 and April to June 1981 there followed phases of exponential growth but at appreciably slower rates of population increase than during recruitment from excystment. That excystment was the major contributor to the rapid vernal increase of planktonic cells is supported by the observation in the plankton of high percentages of preceratium (recently excysted) cells and young vegetative cells with carbohydrate in their posterior halves. Moreover, a calculation of growth rate in situ, from the phasing of cell division when water temperatures were approx. 8°C, indicated that vegetative growth was very slow relative to the earlier observed rate of increase. Growth rates in culture at temperatures below 10°C were also very slow. Instead of the usual encystment in 1980 there was an early mass mortality of cells. This failure to form cysts did not prevent recruitment from benthic cysts in 1981 due to the large reservoir of viable cysts in the sediment from earlier years.

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