Distribution, dynamics and in situ seeding potential of Scrippsiella hangoei (Dinophyceae) cyst populations from the Baltic Sea

The distribution and seasonal dynamics of cyst populations of the spring bl oom dinoflagellate Scrippsiella hangoei were studied in surface sediments o n the southwest coast of Finland, Baltic Sea. In situ germination was asses sed by monitoring the fraction of empty cysts and chlorophyll a fluorescenc e in cyst populations at different coastal sites throughout the annual cycl e. Scrippsiella hangoei resting cysts were widely distributed in the study area and occurred in exceptionally large numbers (magnitudes of 10(4)-10(6) cysts cm(-3)) at all sampling locations between the innermost parts of the coastal archipelago and the open Gulf of Finland. The decreases in cyst nu mber in winter and the increases occurring in late spring reflected the dyn amics of germination and encystment of the species. Chlorophyll fluorescenc e appeared in mid-winter in similar to 40% of cysts from well-aerated basin s and 6-15% of cysts from temporarily anoxic sediments. A generally low inc rease in the proportion of empty cysts indicated that only a part of the po tentially germinable cysts actually germinates. Given the high cyst concent rations in the sediments, the potential for germination is considerable, de spite the environmentally and physiologically determined losses. In contras t, the size of the vegetative inoculum is very low, indicating that the sur vival of germlings is problematic under harsh winter conditions. This is an unusual life cycle strategy; however, the early release of cells into the water column provides a high probability for successful bloom initiation un der the unpredictable meteorological conditions in winter and early spring, which often lead to the sudden onset of favourable growth conditions.