Abstract
Snow algae are globally distributed photosynthetic microorganisms that contribute to snowmelt by lowering albedo. As photoautotrophs, snow algae require inorganic carbon. Sources of inorganic carbon for snow algae include atmospheric CO
2
, biomass degradation (heterotrophy by co-occurring microbes), and carbonate dissolution (when carbonate-bearing bedrock is present). To assess sources of inorganic carbon supporting snow algae blooms, we analyzed carbon isotopes of snow and snow algae sampled from summer snowpacks across varying bedrock compositions in the Cascade Range, Rocky Mountains, and Colorado Plateau (USA). Across the dataset, snow algae cell abundance increased with increasing inorganic carbon availability, consistent with CO
2
as a limiting nutrient. Sites with more snow algae had higher inorganic carbon concentrations from carbonate dissolution, heterotrophy, or both, whereas sites where atmospheric CO
2
was the main carbon source had low snow algae abundance. These observations indicate that heterotrophic degradation of autochthonous and allochthonous carbon and the dissolution of carbonate minerals (when present) result in more readily available CO
2,
which can lead to larger blooms than would occur based on atmospheric CO
2
alone. However, larger blooms were not linked to lower albedo. Instead, in low CO
2
settings where snow algae were less abundant, we observed higher per-cell carotenoid:Chl-
a
ratios and a larger decrease in snow reflectance. Thus, carbonate dissolution, heterotrophy, or both can alleviate snow-algae CO
2
limitation, but cell abundance is decoupled from albedo impact. Accurate models of snow algae impact on snowmelt and water resources through albedo reduction require consideration of bedrock setting and both pigment composition and cell abundance.