Snow algal communities, albedo, and supraglacial biogeography in northwestern North America

Snow algae grow on the surface of alpine and polar snowfields in summer, forming blooms known as "red snow" or "watermelon snow". Snow algal blooms reduce albedo, thereby increasing the rate of snowmelt, potentially resulting in earlier loss of snow cover. Snow cover duration impacts summer water supplies for humans and ecosystems, and on glaciers earlier loss of snow results in increased bare ice ablation and loss of glacier mass. A key first step in understanding snow algae albedo, ecology, and biogeography is identifying the species that form snow algal blooms, and mapping the bloom distribution is prerequisite to quantifying their snowmelt impact. The aims of this thesis were twofold: (1) to identify which snow algal species form blooms in southern British Columbia, and (2) to map the spatiotemporal extent of snow algal blooms on glaciers in North America. To address these aims, I analyzed snow algal communities and single cells via amplicon sequencing, and applied machine learning to classify supraglacial blooms in satellite images across 2019–2022. The results of high-throughput sequencing (metabarcoding) showed blooms contained a diverse mixture of Chloromonadinia and Sanguina. Orange and green Chloromonas blooms were most predominant at lower elevations, whereas red Sanguina was most abundant at high alpine sites. Rosette-like cells, widespread in field samples, were long thought to be a life stage of the cosmopolitan and abundant Sanguina nivaloides. By isolating and sequencing individual cells from field samples, we discovered that "rosettes" in fact constitute a novel genus of Chloromonadinia comprising five new species. The results of the remote sensing analysis revealed blooms were perennial and extensive at mid-elevation, low-angle sites on glaciers on the interior side of the Pacific Coast Ranges and throughout the interior mountain ranges. Bloom extent, duration, and intensity was highly variable between years, with the highest algal biomass produced in years and regions with persistent summer snowpack. Across all four summers, blooms affected nearly 5% of the total glaciated area in northwestern North America, but in some regions and years percent cover was as high as 65% per glacier. Altogether, this thesis characterises the algal taxa forming snow blooms in southern British Columbia, and reveals extensive coverage on North American glaciers, suggesting that blooms may play a previously underestimated role in snow and ice loss in this region.