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

Abstract

The distribution and seasonal dynamics of cyst populations of the spring bloom dinoflagellate Scrippsiella hangoei were studied in surface sediments on the southwest coast of Finland, Baltic Sea. In situ germination was assessed by monitoring the fraction of empty cysts and chlorophyll a fluorescence in cyst populations at different coastal sites throughout the annual cycle. 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 number in winter and the increases occurring in late spring reflected the dynamics of germination and encystment of the species. Chlorophyll fluorescence appeared in mid-winter in -40% of cysts from well-aerated basins and 6-15% of cysts from temporarily anoxic sediments. A generally low increase in the proportion of empty cysts indicated that only a part of the potentially germinable cysts actually germinates. Given the high cyst concentrations in the sediments, the potential for germination is considerable, despite the environmentally and physiologically determined losses. In contrast, the size of the vegetative inoculum is very low, indicating that the survival 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 under the unpredictable meteorological conditions in winter and early spring, which often lead to the sudden onset of favourable growth conditions.