Microbial and mass characteristics following deglaciation and exposure of watersheds in West Greenland

Deglaciation alters chemical compositions of streams and lakes of the arctic tundra. As the Greenland Ice Sheet retreated, watersheds became isolated from glacial meltwaters, with source water derived from precipitation and permafrost and active layer meltwater.  The changes in source water, as well as increased weathering and ecological succession after exposure, alter chemical properties of the streams and lakes. These changes should allow for the development of a more complex aquatic microbial community. However, changes in microbial communities and links to changes in chemical and physical properties of streams and lakes are not fully understood. In this study, we sampled four distinct watersheds in western Greenland, including inlets and outlets of lakes, from start of the melt season in May to around mid-August. Two near-ice watersheds include a glacier melt-water stream and a non-glacially sourced stream that has been exposed for ~7 ky. Two coastal watersheds sampled  ~170 km west of the near-ice watersheds also drain no glacial meltwater and have been exposed for ~11 ky. Sampling was designed to evaluate the net flux of dissolved organic matter, cell abundance and cellular biomass through the system. Differences in these parameters at the inlet and outlet of lakes evaluate how the lakes affect processing of the cells and nutrients. Using epifluorescence microscopy, cells were counted, and the images were used to estimate the approximate biomass of the system. Cell counts and chlorophyll-a were collected from the stream to measure relative primary production and cellular abundance. The measurements for the near-ice watersheds show increased chlorophyll concentration and cell abundance at the outlets and decreases sharply midseason where it then levels off, while the furthest watershed from the ice is much more stable and increases through the melt season. The inlets for each of the watersheds show different patterns as the season progresses. Consistent with previous studies, the lowest cell concentrations occur in the glacial meltwater watershed; however, the cells were larger on average than the other watersheds. At the most upstream site for the glacially fed watershed and the furthest watershed from the ice, the average cell size increased from 0.58 μm² to 2.04 μm². Given that the field sites sampled represent the transition from being connected to the glacier to distantly isolated from the glacial meltwaters, these trends could indicate the change in how microbes interact with organic matter within streams as the glacier retreats. The distinction in microbial communities between the watersheds indicate that along with weathering and ecological success these communities respond to changing chemical and physical characteristics of watersheds following exposure after ice sheets retreat.