In March, around the time that snow packs were deepest, I studied snow along an elevation gradient from the lake, through the birch forest, and up Saana. The picture above is the deepest pit I dug, around 1.7 meters, just at the upper edge of the birch forest on the way up Saana.
I measured snow temperature (that’s a thermometer stuck into the snow) and collected snow from the top, middle and bottom of each snow pit. The snow is a wonderful insulator, and temperatures at the bottom of the snow pack were just below zero, while it was as cold as -11 C or so at the surface, especially higher up on Saana.
Down in the forest, the snow was thicker and more fluffy. Up on Saana it is windscoured and dense, and provides less insulation…
…but is still beautiful (this is view from near the top of Saana)
So, anyway, I spent a wonderful week getting to know the snow in its natural habitat. But the fun didn’t stop there. I took samples back to the University of Jyväskylä and studied their microbiology and chemistry. I knew that bacteria can enter the snowpack from snowfall and wind, but seeing how relatively pleasant it is down at the base of the snowpack, I wondered if bacteria might be thriving there during the winter. So I hypothesized that the snow surface would be a random assortment of bacteria deposited by chance, while the base of the snow would host an actual community of active microbes, selected for their ability to grow in snow on whatever food sources there might be.
My hosts were kind enough to allow me use of this flow cytometer, which is basically a microscope with plumbing. It counts cells and gives you information related to their size and other properties, depending on how you stain the cells.
Watch out, it’s data! There were generally more cells at the base of the snowpack (green bars), especially in the birch forest (BF) and forest-alpine transition zone (Tr) where the snow was deepest. The middle depth of the snow packs (red) had the lowest populations, except at the highest site (AH), where snow is thin and blown around like mad.
I also grew bugs from my samples (you can usually only get about 1 % of the total bacteria from the environment to grow, but it’s still useful to try). I let these guys grow at 5 degrees C for 6 weeks before counting them. Left to right shows plates from the snow surface, base of the snowpack and lake ice. Consistent with the flow cytometer data, only the top and bottom samples produced colonies. The middle of the snowpack is basically a desert. The lake ice sample on the right has a bunch of brightly colored colonies. It looks like the lake ice community is probably more like the lake than the snow on top of the lake. The snow microbes could all grow at either refrigerator or room temperatures, but the lake ice isolates could only grow at low temperatures. The flow cytometer also showed that the ice samples look very different from the snow samples.
And, yes! The overall community at the surface of the snow was very different from the one at the base of the snowpack. The data above are from sequences of DNA extracted from filters that had about 100 mL of melted snow water forced through them to capture the cells.
The top of the snowpack was more diverse, with about twice as many kinds of bacteria as in the bottom. The top looked a lot like a soil sample, with Acidobacteria, Actinobacteria and Alphaproteobacteria, which makes sense if these cells were mainly just blowing in from the surrounding environment. But at the base of the snow pack there are a ton of Gammaproteobacteria. Presumably these guys are the ones that can grow under the snow, diluting out the other types that drifted in randomly.
Last question, what are they eating? Based on the spectrum of UV light the snow samples absorb, there is protein and some other compounds with aromatic rings. Again, in the middle of the snowpack (red line) there is very little to eat. The bottom sample (green) has the highest concentration of goodies, and the clearest protein-like pattern, with peaks at 205 and 280 nm.
So what does it all mean? Here’s my story: organic material and microbial cells are transported to the surface of the snow. Most of these cells just sit there in an inactive state. Down at the base of the snowpack the temperature is high and constant enough to support a higher level of microbial growth. These bugs eat the dead cells and organic matter that is deposited with the snow, converting material with a lower UV absorbance into protein.
So the snow is not just a barren expanse of frozen water crystals, it is a blanket that protects and feeds a specialized community of bacteria during the winter.
Thanks for the opportunity to Know the Snow!