Laboratory experiments suggests limited impact of increased nitrogen deposition on snow algae blooms

During the summer melt, snow algae commonly form large-scale blooms in areas with seasonal or year-round snow coverage. Snow algal blooms, in which the dominating species belong to Chlorophyta (green algae), decrease snow albedo and increase local melt rates. However, the causes behind the size and frequency of these blooms are still not well understood. One factor that is likely contributing to snow algal blooms is nutrient availability, specifically nitrogen (N) and phosphorus (P). However, the nutrient requirements of the taxa responsible for these blooms is not known. Here, we assessed the growth of three commercial strains of snow algae under 24 different nutrient treatments that varied in both absolute and relative concentrations of N and P. After 38 days of incubation, we measured total biomass and cell size and estimated their effective albedo reduction surface (EARS). Snow algal strains tended to respond similarly and achieved bloom-like cell densities over a wide range of NP conditions. However, the molar ratio of N:P at which maximum biomass was achieved was between 4 and 7. Our data indicate a high requirement for P for snow algae and suggest that additional N inputs into the ecosystem may not significantly impact the productivity and abundance of snow algae blooms. This highlights P availability as a critical factor influencing the frequency and extent of snow algae blooms and their potential contribution to snow melt through altered albedo.