Biofilms are hubs of microbial activity in streams. These complex matrices of algae, fungi, and bacteria—sometimes called “microbial skin” for their critical role in processing nutrients—adhere to streambeds in slippery mats. Despite their importance in biogeochemical processing in streams globally, researchers know very little about how stream biofilms in some parts of the world, such as the Arctic, respond to shifts in nutrient availability, a critical knowledge gap as climate change rapidly reshapes that part of the globe.
Indeed, the Arctic region is warming significantly faster than the global average. As permafrost thaws, microbes in the formerly frozen soil become more active, breaking down organic matter and releasing more nutrients into the environment, which can be carried away in seasonal meltwater. Here Pastor et al. study how changes in nutrient (e.g., nitrogen and phosphorus) availability in streams fed by thawing permafrost and seasonal snowmelt influence biofilm growth.
The team looked at two streams in Greenland’s Zackenberg Valley, a region of the High Arctic where summertime temperatures typically top out at about 4.5°C (40°F), and chose six sampling locations representing a broad range of hydrological conditions and nutrient concentrations. To measure biomass accrual, the researchers used glass disks—either untreated or preloaded with nutrients—as substrates to mimic surfaces in streams where biofilms would accumulate and placed these artificial substrates into the streams in the late summer, at the peak of seasonal thaws. Although many previous studies on stream biofilms measured biofilm growth by tracking only algal chlorophyll, the methods here also account for fungal and bacterial communities in biofilms. The team also monitored other stream conditions such as water temperature, flow velocity, and nutrient concentrations.
Overall, the researchers found that nitrogen concentrations, which are typically low in High Arctic streams, had a strong influence on biofilm growth. Biomass levels were highest at upstream sampling sites, where nitrate levels were up to 10 times higher than at downstream locations. The results suggest that climate change impacts in the Arctic, including the downslope flow of nutrients from soils, are likely to affect biofilm growth in the region’s streams. (Journal of Geophysical Research: Biogeosciences, https://doi.org/10.1029/2019JG005557, 2020)
—Kate Wheeling, Science Writer