Effects of cyst resuspension on germination and seeding of two bloom-forming dinoflagellates in the Baltic Sea

The implications of cyst resuspension on germination and subsequent seeding of the 2 spring-bloom dinoflagellates Scrippsiella hangoei (Schiller) Larsen and Peridiniella catenata (Levender) Balech from the Baltic Sea were investigated in a field study and laboratory experiments. Sedimentation of resuspended cysts was monitored by an automated sediment trap in 2 consecutive winters prior to and throughout the germination period off the SW coast of Finland. The effects of increased irradiances and water motion on germination and germling survival were tested by incubating cysts at different light levels and in turbulent water, Cyst fluxes of both species were low during the calm and cold winter of 1998/1999. In 1999/2000, heavy storms caused strong resuspension of S. hangoei cysts, Light significantly increased the germination frequency of S. hangoei cysts and supported germling survival and cell division. In P. catenata, the percentage of excystment was not significantly influenced by light and germination was successfully completed in both darkness and light. Subsequent growth of the species, however, required light, although maximum cell numbers were encountered at an irradiance as low as 10 mu mol m(-2) s(-1). Small-scale turbulence reduced the germination frequency of S. hangoei but did not affect excystment in P. catenata. No negative effects on subsequent growth were detected. The favourable effects of light on germination and germling survival of S. hangoei emphasize that resuspension would be advantageous for the bloom initiation of this species. Cyst resuspension seems to be less important in P. catenata population dynamics, since germination can be successfully completed in darkness and the amount of cysts transported to the water surface is insignificant even with strong turbulent mixing. It is concluded that cyst resuspension may be advantageous for dinoflagellate bloom initiation, depending on its extent and timing and the specific germination requirements of the respective organism.