Relationship between vegetative cells and cyst production during Alexandrium minutum bloom in Arenys de Mar harbour (NW Mediterranean)

English. A recurrent Alexandrium minutum bloom in the Arenys de Mar harbour (Catalan coast, North WesternMediterranean) was monitored in order to establish the relationship between vegetative cells and cystproduction. The bloom lasted from January 21 to February 24, 2002 and reached cell concentrationsof up to 47 x106 cell L-1. Two aspects related to the resting cysts deposition were studied:(i) production of resting cysts during the bloom period (by means of sediment traps) and (ii) distributionof resting cysts in the sediment after the bloom (May 2002). Cyst formation in Arenys clearly started ina period with high vegetative cell densities in the water column. Once production was initiatedencystment fluxes remained constant for two weeks, and covering the periods of maintenance and decline of the bloom. High cyst fluxes (up to 6000 cysts cm-2 day-1) were quantified as a result of the high vegetative cell concentration. Moreover, encystment occurring in less than 1% of the total population indicates that most of the cells are not involved in resting cysts formation. A comparison of the resting cyst flux values obtained from the sediment traps and the resting cyst concentrations in surface sediment (628–3270 cysts cm-3) three months later, revealed that the number of cysts in the sediment decreased during that time. The studies of excystment showed a high germination percentage (91%) and germling viability (100%). These data, together with the resting cyst distribution in the sediment, are important in assessing the role of resting cysts in the bloom dynamics of A. minutum in confined waters.

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