<p>Our aim was to study variation in diatom communities in urban boreal streams during winter and spring, and what are the key environmental variables connected to such variation as diatoms in cold seasons are still notably understudied. Samples were collected during winter and spring in 2023 in Helsinki region, Finland. Analyses included Spearman’s correlation coefficient, generalized linear mixed model (GLMM), Principal component analysis, dissimilarity analysis and Redundancy Analysis (RDA). During winter, water properties varied in time more than during the spring and had high conductivity and concentration of dissolved nitrate-nitrogen. Surprisingly, diatom species richness was higher in winter than in spring. GLMM showed that species richness was positively related with suspended solids and negatively with nitrate-nitrogen and water temperature. Diatom community composition varied temporally and spatially. RDA suggested that both climatic and water chemical variables explain the variation significantly, but communities did not show very clear seasonal turnover. This study shows that research on boreal stream diatoms could be extended to cold periods as well and typical indicator species are present both in cold and warmer seasons. Year-around studies are needed to get a better understanding of diatom seasonality in the northern regions.</p>