Temporal and vertical changes in biological communities within snowpacks during melting season in Northern Japan

Abstract. During the snowmelt season, diverse cold-tolerant microbes thrive within snowpacks. Snow conditions in forested areas change temporally with air temperature and budburst of trees. However, their effects on relevant biological communities are not well documented. Based on periodic sampling throughout the snowmelt season (March–May, 2021), this study describes the temporal and vertical changes in biological communities, including snow algae, microinvertebrates, and snow fungi, within snowpacks in Northern Japan. The melting season was divided into three periods: when the daily minimum air temperature was below the freezing point (Period A), when it was above the freezing point and before the budburst of beech trees (Period B), and after the budburst over the snow surface (Period C). During Period A, two types of algae and one of fungus were ubiquitously observed in the snowpack. During Period B, the abundance of microbes increased in the surface layer and green algal blooms visibly emerged. Later in this period, nutrients (NO₃^-, NH₄⁺, and PO₄^3-) depleted, likely inhibiting algal growth and consequently restricting the microinvertebrate population. Surface layer nutrient concentrations increased again during Period C, thereby increasing the abundance of algae and microinvertebrates. This increase in nutrients was likely due to the rainwater and tree-derived litter deposited on the snowpack. Analyses of snow pits and cores revealed that the active layers of microbes were distinct between snow algae/fungi (surface layer) and microinvertebrates (subsurface layers), probably because of their preferable conditions for the snowpack layers. This study highlights potentially important patterns in the dynamic interactions between microbial communities and environmental changes within snowpacks, revealing how tree phenology and snowmelt conditions jointly shape the vertical distribution and seasonal succession of snow-ice microbes.