Although the majority of microalgal species reproduce asexually for large parts of the growth season, most population genetic studies have rarely found clones in microalgal blooms. Instead, population genetic studies have identified large intraspecific diversity in most microalgal species. This paradox of frequent asexual reproduction but low number of clones creates challenges when interpreting the proportion of clones and distinct genotypes in natural microalgal populations. To estimate the proportion of clones and genotype richness, we created a computer model that simulates the composition of microalgal populations after a defined period of exponential growth. We simulated the probability of picking clones of the same genotype from this population as a function of initial genotype diversity, intraspecific differences in growth rates and sample size. This model was applied to five microalgal species for which high-resolution population genetic data and population growth rates based on monitoring data were available. The number of distinct genotypes in each population was extrapolated from the model outputs and the observed proportion of clones in the respective population genetic studies. The estimates from our simulation suggested that the genotype richness in most blooms exceeds several thousand distinct genotypes with very high variability among microalgal species. The highest numbers of distinct genotypes (500,000 and 2,000,000 genotypes) were estimated for species with very low numbers of observed clones in population genetic studies (< 5%), but genotype richness was also strongly impacted by intraspecific variability in growth rates. Furthermore, the probability of finding clones and presumably sampling a representative fraction of genotypes increased significantly with higher sample sizes, challenging the detection of differences in genotype diversity between sub-samples with few isolates.

The Pacific Arctic region is characterized by seasonal sea-ice, the spatial extent and duration of which varies considerably. In this region, diatoms are the dominant phytoplankton group during spring and summer. To facilitate survival during periods that are less favorable for growth, many diatom species produce resting stages that settle to the seafloor and can serve as a potential inoculum for subsequent blooms. Since diatom assemblage composition is closely related to sea-ice dynamics, detailed studies of biophysical interactions are fundamental to understanding the lower trophic levels of ecosystems in the Pacific Arctic. One way to explore this relationship is by comparing the distribution and abundance of diatom resting stages with patterns of sea-ice coverage. In this study, we quantified viable diatom resting stages in sediments collected during summer and autumn 2018 and explored their relationship to sea-ice extent during the previous winter and spring. Diatom assemblages were clearly dependent on the variable timing of the sea-ice retreat and accompanying light conditions. In areas where sea-ice retreated earlier, open-water species such as Chaetoceros spp. and Thalassiosira spp. were abundant. In contrast, proportional abundances of Attheya spp. and pennate diatom species that are commonly observed in sea-ice were higher in areas where diatoms experienced higher light levels and longer day length in/under the sea-ice. This study demonstrates that sea-ice dynamics are an important determinant of diatom species composition and distribution in the Pacific Arctic region.