Don't Eat the Red Snow: Monitoring Melt Due to Red Algae

The Kenai Peninsula in Southcentral Alaska is home to the largest continuous icefield in the United States. The Harding Icefield covers over 700 square miles, it stretches into mountain valleys, glacial lakes, and reaches into the Gulf of Alaska with its over 30 glacial arms. A jewel of the peninsula, the Harding breathes life into rivers, whips katabatic winds, and with its recession reveals pristine valleys and fjords. The prominence of the Harding icefield makes it seem an unchanging natural monolith, but with each passing year snowpack melts, glaciers recede, and the icefield shrinks. Climate change and rising global temperatures play a role in the increasing melt rate of the Harding Icefield and other icefields. However, as with any complex ecosystem, the range of affective players doesn't end with anthropogenic changes. To fully understand the future of the Harding Icefield we must get out our microscopes and look at an organism that is responsible for up to 17% of observed melt.

Chlamydomonas nivalis, commonly referred to as ‘watermelon snow,’ is a single-celled photosynthetic red algae that makes its home on the snowpack of mountains and glaciers. Not many organisms are found in these harsh environments and those that are must have extreme adaptations to match their extreme homes. This algae’s red pigment is one such adaptation.

Ironically, the biggest limiting factor for red algae on snowpack is access to liquid water. Metabolically available water is needed for photosynthesis and is the main transportation method for much of the nutrients found on the snowpack. Red-snow algae gets around this roadblock by changing the surface albedo of the snow pack surrounding it. Snow is extremely reflective, over 90% of visible radiation is reflected by clean snow. The darker pigment of red-algae decreases surface albedo and melts the surrounding snowpack. As snow pack melts red algae becomes more abundant, further reducing surface albedo. This process compounds through the summer months, peaking in late August to mid September. Come October new snow begins to cover the high elevation snowpack, the algae is covered, and it's melt properties are halted for winter. Come May temperatures increase and sunlight hours lengthen, the red-algae resurfaces and begins its seasonal growth again.

Wild places like the Harding Icefield are ripe with opportunity for scientific investigation, but harsh terrain and unpredictable weather do not make that investigation an easy endeavor. To survey rugged landscapes such as The Harding icefield satellite imaging is incredibly useful.

To observe the long-term trends of algae abundance on The Harding Icefield we gathered spectral data from the Land-sat 8 mission for 34 individual dates ranging from 2013-2019. Using established calculations we isolated pixels containing snow and ice, these pixels represent the extent of the Harding Icefield and it's surrounding snowpack.

Using the isolated snow and ice pixels we then calculated algae abundance for each pixel. This two part calculation was established in a field study which measured the spectral reflectance of various concentrations of red algae. The first step is a band calculation: (red-green)/(red+green). Any pixels which give values equal to or less than zero are assumed to be clear of red algae, and removed from the equation. Part two takes any remaining pixels and multiplies them by 1400, this gives an estimation of algae abundance for each pixel. This equation yields a strong linear relationship to field measures, R=0.93. Images created from this equation are shown on the left.

We then charted red algae abundance data over a 7 year time line. We found that there is a significant upward trend in red algae growth for the observed dates. This finding indicates that populations of red algae have been increasing for the last 7 years on The Harding Icefield. This population increase could be due to a variety of reasons. Perhaps red algae is not seeing the normal die-off rates in winter due to more mild weather conditions. This could mean that larger blooms are emerging in May/June and populations are getting a head start on seasonal growth. Additionally, there might be an increase in nutrient levels on The Harding Icefield from increased pollen load, larger wildfires, or more frequent dusting events. Higher nutrient levels could spike red algae populations given the icefield's normal highly oligotrophic environment. Whatever the reasoning behind this upward trend in abundance, this increase is not a slight shift and we believe it will continue to steadily increase in the coming years.

Red algae populations on The Harding Icefield have been increasing for the past 7 years, but what, if any, effect has that had on the Icefield itself? To understand the cumulative effect of red-algae on The Harding Icefield we used a model of melt rate by algae abundance. This model was again established by a field study on The Harding Icefield. This model uses algae abundance values to calculate cubic cm of water equivalent per day (cm^2 w.e.d). The equation for this model is: m = 2.33 + 0.132 *√A. Where m = cm w.e.d. and A = algae abundance. This model's linear relationship is less strong, R=0.34. This is likely because this model does not account for weather, temperature, or other albedo reducing factors; however, this is the only established algae melt model supported by field data. The images created from this model are shown to the left. Each image shows melt for an individual day at a similar seasonal time for each year, 2013-2019.

Visually, it appears that the increase in algae abundance has in turn resulted in increased melt from 2013 to 2019. However, when this data is tested statistically the results reveal a less significant relationship. We graphed average melt per day due to red algae from 2013 - 2019, this did not reveal a signifiant relationship. Although the general trend of melt due to algae is continuing upward, it is not a significant finding. There are many factors not accounted for by this model that can have a significant effect on melt rate. We suggest that a model which could account for: daily temperature shifts, effects of precipitation, and seasonal shifts in sunlight intensity could reveal a more accurate and potentially more significant result. Longterm trends aside, the seasonal effect red algae is having on the Harding Icefield is immense. On any given summer day red algae is causing approximately 3.5 gigaliters of melt, that's 1400 olympic swimming pools, per day. Organisms such as red algae highlight the value and power of the microbial world.

Red algae and other glacier microbes are found on high latitude ice sheets and in high elevation snowpack across the world. Their effects on albedo, hydrology, and alpine ecosystem balance are in action everywhere they are found. These microbes evolved under natural selection to demand liquid water in frozen environments. This demand has effectively begun a snow ball effect which will result in increasing melt with increasing abundance. Dispersal patterns and warming trends indicate that areas of flat snow covered topography will see the fastest increase in red algae populations, places such as Antartica and the Greenland ice sheet are extremely vulnerable.

Additionally, we are seeing increased rates of airborne debris being deposited on these high latitude ice sheets. Ash from burning biomass and dust from agricultural ventures represent some of the biggest anthropogenic sources of debris. This debris is rich in important nutrients such as phosphorus and nitrogen.

Rising global temperatures and increasing nutrient input will result in faster growth of red algae and thus increase global ice sheet melt. Any climate model that does not take into account the melt effects of glacier microbes will severely underestimate global warming rates and sea level rise.