Encystment of Zygabikodinium lenticulatum (Dinophyceae) during a summer bloom of dinoflagellates in the southern Benguela upwelling system

A sediment trap was placed off Lambert's Bay in the southern Benguela upwelling system for 20 days in March 2001 to investigate the flux of dinoflagellate cysts from the upper mixed layer. A dinoflagellate bloom dominated by the small autotroph Gyrodinium zeta, developed in late March in association with intense stratification of the water column. The bloom included several heterotrophic species, in particular Zygabikodinium lenticulatum. The mass sedimentation of cysts of Z. lenticulatum, indicated by their dominance in the sediment trap, coincided with the maximum abundance of the vegetative stage. Observations of few cysts in the upper mixed layer indicated that cysts were formed over a short period and sank rapidly in the water column. Current patterns revealed predominantly northward flow in surface waters and southward flow in bottom waters, with current shear noticeable between 20 and 30 m depth. The formation of cysts by Z. lenticulatum under these patterns of flow serves to retain the population, preventing washout from the coastal environment. Analysis of sediment samples revealed that Z. lenticulatum also dominated the cyst assemblage of the sediments. Experimental results indicated a dormancy period of approximately 48 days, however, only a small fraction of cysts (20–28%) germinated under experimental conditions.