Encystment in a Dynamic Environment: Deposition of Dinoflagellate Cysts by a Frontal Convergence

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Abstract
The dinoflagellate Gyrodinium uncatenum forms massive summer red tides in Chesapeake Bay (USA) and tributary estuaries. These blooms are delimited in the downstream direction by estuarine fronts may serve to concentrate and recirculate the population. Toward the end of the bloom cycle, G. uncatenum sexual stages accumulate in the frontal convergence and are transported downward along the frontal interface. These stages are retained below the pycnocline in net upstream flowing bottom waters and settle out into the sediments along the subsurface transport pathway. Examination of sediments indicates that major deposits of cysts of G. uncatenum are bounded in an upstream direction by a benthic front (where the pycnocline intersects the bottom). Above this area, motile cells and sexual stages are absent from the water column and cysts are absent from the sediment. Streamflow-induced variations in the location of the estuarine front in 1979 and in 1980 result in deposition of cysts in different regions, predictable from examination of the location of the convergence. It is proposed that the convergence zone of the estuarine front and the associated pycnocline serve to transfer encysting dinoflagellate forms from surface waters to their ultimate seedbed locations.