Many marine protists form resting stages that can remain viable in coastal sediments for several decades. Their long-term survival offers the possibility to explore the impact of changes in environmental conditions on population dynamics over multidecadal time scales. Resting stages of the phototrophic dinoflagellate Pentapharsodinium dalei were isolated and germinated from five layers in dated sediment cores from Koljö fjord, Sweden, spanning ca. 1910–2006. This fjord has, during the last century, experienced environmental fluctuations linked to hydrographic variability mainly driven by the North Atlantic Oscillation. Population genetic analyses based on six microsatellite markers revealed high genetic diversity and suggested that samples belonged to two clusters of subpopulations that have persisted for nearly a century. We observed subpopulation shifts coinciding with changes in hydrographic conditions. The large degree of genetic diversity and the potential for both fluctuation and recovery over longer time scales documented here, may help to explain the long-term success of aquatic protists that form resting stages.

Effective population size (Ne) determines the rate of genetic drift and the relative influence of selection over random genetic changes. While free-living protist populations characteristically consist of huge numbers of cells (N), the absence of any estimates of contemporary Ne raises the question whether protist effective population sizes are comparably large. Using microsatellite genotype data of strains derived from revived cysts of the marine dinoflagellate Pentapharsodinum dalei from sections of a sediment record that spanned some 100 years, we present the first estimates of contemporary Ne for a local population in a free-living protist. The estimates of Ne are relatively small, on the order of a few 100 individuals, and thus are similar in magnitude to values of Ne reported for multicellular animals: the implications are that Ne of P. dalei is many orders of magnitude lower than the number of cells present (Ne/N~10-12) and that stochastic genetic processes may be more prevalent in protist populations than previously anticipated.