Elevation shapes alpine snow algal blooms and their influence on albedo reduction.

Snow algae darken snowpacks and accelerate melt world-wide. Although elevation strongly structures mountain snowfields, its influence on snow algal traits and their effects on snowpack reflectance remains unclear. Here, we investigated snow algal blooms across an elevational range of 1059-3423 m above sea level (asl) in the Cascade Range (California, Oregon, Washington) and the Rocky Mountains (Utah, Wyoming, Montana). We analyzed 294 snow samples and quantified algal community composition, algal cell density, cell size, pigment concentrations, and snow albedo. We further examined elevation-dependent patterns within Sanguina nivaloides-dominated blooms (117 of 206 bloom samples). Across samples spanning clean snow and algal blooms, algal cell density emerged as the strongest biological predictor of albedo, whereas pigment-related variables showed no consistent effects. Within Sanguina nivaloides-dominated blooms, neither relative abundance nor algal cell density varied systematically with elevation. Instead, mean cell size increased with elevation, while per-cell pigment concentrations declined, leading to higher astaxanthin ratios driven primarily by reductions in Chla per cell. These elevation-dependent shifts in cell size and pigment balance were consistent across both mountain ranges, indicating phenotypic acclimation to increasing environmental stress with elevation. Together, these findings link cellular-scale acclimation of a widespread snow algae to radiative processes shaping mountain snowpacks.