THE IMPACT N:P RATIO ON SNOW ALGAE GROWTH RATES AND CELL SIZE

During the summer melt, snow algae can form large-scale blooms in areas with seasonal or year-round snow coverage. Snow algae decrease snow albedo and increase local melt rates, potentially contributing to glacial retreat. The causes behind the occurrence of these blooms are still not well understood. Snow algal blooms might be controlled by the availability of nutrients including nitrogen (N) and phosphorus (P) in the ecosystem. Here, we analyzed the growth of three commercial strains of snow algae. We conducted a laboratory experiment that included 24 different nutrient treatments, varying in both absolute and relative concentrations of nitrogen and phosphorus. After 38 days of incubation, we measured their total biomass and cell size of selected samples and estimated their EARS (effective albedo reduction surface). Snow algal species tended to respond similarly and achieved bloom-like cell densities over a wide range of N:P ratios. The preferred N:P ratios were between 4 and 7, indicating that P availability is limiting for growth in this experiment. For the N:P ratios where strains reached lower cell densities, significantly bigger cell sizes were observed compared to conditions where highest cell densities were recorded. Our results suggest a preference for conditions with relatively higher P availability compared to N. However, N:P ratios associated to more typical alpine environment conditions could lead to blooms with a more significant impact on albedo reduction per unit cell.