Red Snow at Height
Red snow on the hill… At the sight, the chest might tighten for a second - blood on the snow? Thankfully, no. This isn't the scene of some past grisly accident. The colouring instead indicates the presence of one of nature's more unusual mountain phenomena, the spectacle known as 'watermelon snow'. And we need your help to find it.
The Blood of Glaciers
In 1787, the natural philosopher and explorer Horace Bénédict de Saussure made the third recorded ascent of Mt Blanc, following the first ascent the previous year. Having gained his professorship at the age of 22, de Saussure had dedicated his life to the pursuit of science and alpinism. Alongside his ascents, de Saussure made detailed observations of what the locals termed 'Sang des Glaciers' – the Blood of Glaciers. In his accounts, he expressed his amazement at the extent and vibrancy of red snows on Alpine snowfields. He noted, quite perceptively, that the pigment appears to come from the snow itself, rather than from avalanche or wind debris. Between 1760 and 1780, de Saussure carried out detailed investigations on materials collected from around the Alps, concluding that the red powder collected from the snow was, most likely, plant-based in nature.
Four decades later, in 1818, Captain John Ross found himself in Baffin Bay, off the western shores of Greenland, attempting the first navigation of the Northwest Passage. The expedition proved unsuccessful but brought back a wealth of oceanographic, geological, and botanical bounty. Amongst them were numerous samples of red snow, taken from the impressive crimson cliffs that lined Baffin Bay. The Times headlines on Captain Ross' return covered with amazement the "Red Snow from the Arctic Region" driving a frenzy of scientific interest and speculation. Were these strange fungi? Or meteoric iron deposits? It was the trip's botanist, Robert Brown - later credited with coining the term "nucleus", as well as the concept of "Brownian motion" - who guessed at the true culprits. Brown suggested in a footnote to the expedition that the coloured snows, rather than being atmospheric debris or a sign from the gods, may in fact have been caused by photosynthetic algae living and blooming in the icy fields. Robert Brown was correct.
Prismatic Snows
De Saussure's accounts of red snows closed with a question: "Might red snows be found elsewhere in the world?" His speculation was well-founded. Since his investigations, red snow blooms have been recorded throughout the world's mountain and polar regions: scarlet snowfields in the high Sierras, hot pink snow-banks on the sides of Mt Fuji, wine-red wells amongst Nieves Penitentes on Atacaman volcanoes. Darwin himself, during his explorations of the Patagonian Andes, noted the red footprints left in the snow by the expedition mules. And if snows can bloom red, pink, and purple, why not more? In 1838 the French botanist Charles Martins made the first report of green snows, from the snow fields of Spitzbergen. Since then, scientists and explorers have observed a rainbow spectrum of bloom colours in the snow. Fields of green on Antarctic shores, folds of yellow in old Svalbard snow cliffs (always a dangerous colour with snow...), and apricot oranges tingeing the seasonal snows of the South Shetlands. There is a saying in Irish – "o Níorbh iontaí liom an sneachta dearg ná é" – taken literally: "Red snow would not be more surprising to me than that." Green and orange snow, then, would be quite a shock, though perhaps not for Horace Bénédict de Saussure.
By the turn of the 20th century, the underlying cause of these coloured snows had become established. In each case, the pigmentation in the snow was produced by members of the loosely-termed group known as algae. Since then, our understanding of the breadth and diversity of organisms that make up these blooms has expanded rapidly. Today, over 50 species of photosynthetic algae have been described in relation to snow blooms, often uniquely from snow and ice habitats. The red, round cysts of the blood snow, Sanguina; green filamentous coils of Raphidonema; the ruby, swimming cells of Chlainomonas; and the deep-purple sausage-chains of the ice algae, Ancylonema - more closely related to land plants, than to the 'classical' green microalgae. Photosynthesis is the connecting thread. In each case, evolution has given the cells the tools to survive and thrive in the harshest of polar and mountain environments. To photosynthesise and multiply not just in the waters and soils of cold climates, but on the snow and ice itself.
Crystal forests: the view, under microscope, of the surface of an Antarctic Glacier. The red cells are likely Sanguina and Chlainomonas, and the purple-black sausages, the ice alga Ancylonema
Snow-pack swimmers, ice-cap painters
Amongst the characters in this cold circus, the species Sanguina nivaloides represents one of the most widespread in polar and mountain regions. Sanguina, previously reported as Chlamydomonas nivalis in many cases, is thought to be the main culprit behind the extensive red snow-fields which occur in the Alps and Rockies in late summer, as well as the red tinge seen on exposed glacier surfaces from Greenland to Antarctica. Although still debated, one theory explaining the lifecycle and formation of Sanguina blooms suggests that the cells start their seasonal cycle in the spring snow-pack as small, green, actively 'swimming' cells, with two swimming 'tails' or cilia. These green algal cells are exceptional water-ice climbers, with the cilia propelling the cells up through metres of snow in search of the best light and nutrient conditions. Once those conditions are found, the algae can multiply and divide in huge numbers and in rapid time.
As the season progresses, increased sun and the melting of the upper snow layers expose the algae to increased light stress. In response, Sanguina turns to the production of orange and red pigments which act to protect the cell's photosynthetic organs from the worst of the UV. In a final step to survive the eventual thaw and full exposure to the light, Sanguina begins a process of encysting, shoring up its defences for the coming winter with thicker cell walls and a cocktail of secondary carotenoids: pop-coloured pigments such as beta-carotene and astaxanthin. It is the onset of melting, and the subsequent accumulation of protective pigments in the algae, which gives watermelon snows their characteristic red hue in the summer snow-pack.
Not all snow algae move from green to red in their seasonal wardrobes. Nor do all species mature from vigorous swimmers to the dignified senatorial cysts of late season. Many snow algae exhibit very different lifecycles and adaptations to cold climes. The green filaments of some Raphidonema, for instance, show a preference for the slushy margins and melt zones of snow and ice-packs, forming intense emerald blooms, which boom and bust rapidly on the margins of the ice. The purple ice-algae, Ancylonema, on the other hand, prefers the cold stability of a glacier surface, growing and dividing in chains like strange glacial balloon creatures.
The Living Snow: A riot of colour and algal activity in a sample from an Antarctic ice-cap. The active green cells are typical of swimming Chlamydomonas-like species. The green spaghetti strands are various forms of the filamentous algae Raphidonema.
The production of UV-protective pigments is a common tactic across snow and ice algae. But it's not just their role as sunscreens which make these pop-coloured pigments so interesting from our point of view. The production of the coloured pigments also makes the algae engineers of their own homes, modifying the snow environment to their needs, and in the process, affecting the structure and melt-profiles of entire snow and ice fields. The colouring effects of algal blooms act to change the reflective index, or albedo, of the snow, effectively increasing absorbance of the sun's energy and driving increased melting around the algal cell. To the algae, this is a good thing, providing meltwater for the crucial functions of photosynthesis and nutrient supply. Recent research, however, has shown that the melting effects of darkening algal blooms on glacier and snow surfaces can significantly increase the melting and loss of ice in already retreating glacier systems. Ice algae on the Greenland Ice Sheet for instance, were estimated to be directly responsible for 9−13% of the surface melting in the ice-sheet dark zone in 2016. Understanding the melting influence snow and ice algae might have in an already warming world might then help in developing improved models of snow and ice retreat in polar and alpine environments.
Last of the Summer Snows
Though found worldwide, one of the early scientific records of snow algae was made much closer to home, in Garbh Choire Mhor of Braeriach. The scientists who made this observation, J J Light and J H Bulcher, made the journey in September of 1967, noting the extensive size of the main choire snow patch that year. They described "a general red colour, with some concentration into parallel vertical streaks covering several square metres", with the melted snow samples containing "many flocculent red masses". Closer inspection under microscope revealed the diversity of cells inhabiting the snow patch: ruby spheres of Chlamydomonas nivalis, dotted with the rarer ellipses of Scotiella cryophile and Scotiella nivalis, and the small green twiglets of Raphidomena nivale.
Today, many of those cells identified have different names. Light and Belcher's Chlamydomonas nivalis is likely Sanguina, and Chodatella most likely a Chloromonas species. Our understanding of the diversity of algal species globally has expanded greatly since 1967. And yet, the report from Light and Belcher still remains the most detailed observation of snow algae in the UK. In half a century since then, records have shown a dramatic decrease in the size and survival of Scotland's summer snow patches. The efforts of Dr Adam Watson, Iain Cameron and snow-hunting colleagues have provided a painstaking chronicle of Scotland's slow unwintering - the final days of those venerable snow patches of the Cairngorms and Ben Nevis that, until perhaps 1933, had survived untold centuries. To put it into perspective, since 2003, we have seen seven of only ten years of complete snow patch loss in Scotland on record, and 2023 has continued that trend. The question then is: how will the algal species that survive on the snow-pack persist as Scotland's summer snow cover diminishes?
Many of Scotland's snow-natives might already have been lost. Glacial specialists, such as Ancylonema, may well have been present on the UK's last ice sheet 11,000 years ago (or even in the - hotly debated - mini-glaciers of Coire an Lochain from the Little Ice Age 400 years ago). Examples such as these are of academic interest, perhaps. But the loss of today's snow ecosystems represents a real and continuing loss of biodiversity in Scotland's mountains. Species such as Sanguina nivaloides represent conspicuous and charismatic members of mountain ecosystems, with real influence on the physics of snow-packs, as well as the carbon and nutrient cycling of snow melt in their environment. In turn, the photosynthesis and primary productivity driven by snow algae has been shown to support an even wider network of snow-associated organisms, including insect-like springtails, crimson snow mites and a host of microbial 'grazers'; rotifers, ciliates and the ever-charismatic tardigrades. The knock-on effects of the loss of Scotland's snow algae on these communities, as well as wider mountain lichen, moss and grassland ecosystems, remains unknown and the window for describing them and capturing at least a glimpse of their world is shrinking.
Over 50 years have passed since Light and Belcher made their initial report on Braeriach's hidden world of algae. Yet little to no records or observations exist in between and mentions of snow algae sightings in UK mountain literature and journals remain obscure and uncompiled. It's an intriguing thought that red snows might once have been a much more frequent sight in Scotland's hills. Some of Scotland's prominent snow patches retain their names on the map - A' Chuithe Chrom, the Crooked Wreath on Lochnagar for instance - testament to their imprint on the cultural landscape of Highland communities. Might recurrent red snows have held similar significance in the past? A red tile on the cultural patchwork of the upland landscape? It is curious then that Gaelic, a language so infused with and perceptive of landscape, should have no mention (known to the authors) of the red snows of the hills.
Scotland's Red Snows
Worldwide, interest in snow algae is on the rise, and with it our understanding. Locals and visitors to the western Rockies and to the Alps are familiarising themselves with their algal snow neighbours through projects such as the Living Snow Project and ALPALGA. The work is driven not only by ecological curiosity and a duty towards recording biodiversity on the brink, but also the potential societal benefits that might come from these strange biological systems.
For Scotland's snow algae, though, the work has only just begun. In 2023, our team from the Scottish Association for Marine Science, in collaboration with the Highlands and Islands Environment Foundation (HIEF), started a project looking to map the occurrence of snow algal blooms in the Scottish mountains and raise awareness of the phenomena in walkers and climbers entering the mountains in the summer. The project hopes to better understand the distribution of Scottish snow algae through historical records and sightings and to uncover the diversity of species in Scottish snow algal communities by studying and characterising samples taken from the hills.
This article then was not just a lecture (wake up at the back!), but a request for your help. Have you seen occurrences of red snow in the past? Could you identify where? Even better, would you have pictures? If you haven't seen the phenomena before, or you didn't know it existed, perhaps this can be another reason to keep your eyes peeled when in the hills this summer. What better way to one-up the Bothy Bore and his tales of mountain-top auroras than with a score of watermelon snow sightings!
So if you're interested in helping to uncover one of Scotland's least understood ecosystems then consider one of the following, or share this article with others who might be keen to help out:
- Get in touch with any info on sightings or previous records and photos. Likewise for cultural records. If you can think of that mention of 'sneach rhu' in the poems of Duncan Ban MacIntyre, or remember an obscure passage on red snows from an old SMC journal, let us know!
- Keep an eye out too on the hill this summer and record any sightings by getting in touch at ecosnow@sams.ac.uk or tagging us on twitter at @ecosnow.
- Even better, sign up to the Living Snow project app, and join an amazing community of citizen scientists worldwide who are documenting these mountain marvels - iNaturalist is also a great app for these observations.
Photos of snow algae blooms are always magical, but whatever you choose to do, don't go putting yourself or others at risk for that close up shot… And, as ever in the mountains, be mindful of your impact. The habitats found in and around summer snow-pack hollows are unique and often fragile, so take photos from the path, or from a distance, where possible.
Whether you're a seasoned mountain microbiologist, an avid collector of natural phenomena (this gets similar points to sea sparkle), or simply like licking colourful snow (please don't, watermelon snow is rumoured to be a potent laxative and that's the least of your worries with yellow snow), take the knowledge of watermelon snow with you next time you're in the hills, and think of the small but unique contribution that these snow algae add to the biodiversity of the Scottish mountain landscape.
Acknowledgements:
Many thanks to Saz Reed and Kirsty Pallas for enthusiasm, comments, and early tip offs of red snows in Scotland. To Yvan Cautain and Alex Detain for joining initial reccy trips. To Robin Kodner for pioneering the Living Snow project. And to Naomi Thomas, Matt Davey, and the CCAP team for their algal expertise and teamwork.
Thanks to NatureScot for help and guidance on working in upland environments. And to MASTS SFC-Saltire programme and the SAGA - UKRI NERC grant NE/V000764/1 for additional financial support.
Finally, a huge thanks to HIEF for supporting the project financially. Payment from UKC/UKH for this article will be donated back to HIEF in support of their work across the Highlands and Island region. You can read more about their work, and donate to their efforts.
