The role of atmospheric nitrogen fixation by log mat biofilm in the Spirit Lake ecosystem, Mount St. Helens, WA

Spirit Lake, on the flanks of Mount St. Helens, was dramatically altered as a result of the eruption in 1980, and over the past decades the lake ecosystem has recovered in a rapidly evolving volcanic landscape. While Spirit Lake is similar in many ways to other alpine oligotrophic lakes, it is unique because approximately 20% of the lake's surface remains covered with floating log mats from trees felled during the eruption. The undersides of the logs provide a substrate for benthic primary production (biofilm) that is primarily comprised of algae, diatoms, and cyanobacteria. Due to the sheer amount of log-based surface area available to biofilm, it is an important and novel component in the lake energy and nutrient cycles. Biofilm also grows abundantly on aquatic macrophytes in shallow areas of the lake. Recent nutrient and hydrologic budgets for Spirit Lake revealed a deficit in the nitrogen input into the lake ecosystem. Since several common taxa of cyanobacteria are nitrogen-fixers, we posed the question; do benthic cyanobacteria in the log mat biofilm fix atmospheric N₂, and thus provide a source of nitrogen to the lake ecosystem?

We collected ecosystem samples (log mat biofilm, macrophytes, phytoplankton, lake water) from Spirit Lake in 2017-2019. We used organic elemental and isotopic data collected from the ecosystem samples to investigate the flow of energy and nutrients in the lake. Specifically, we established that potential sources of inorganic nitrogen (lake water NO₃^- and atmospheric N₂) were isotopically distinct. When we compared the δ¹⁵N values of primary producers within the lake to these source values, we found that phytoplankton were similar to NO₃^-, while the macrophyte and log mat biofilm samples were variable, with the contribution of atmospheric N₂ ranging from 0 to 100%. The macrophytes themselves are not nitrogen-fixers, but the samples were highly colonized by biofilm, that likely included cyanobacteria. The variability in log mat biofilm δ¹⁵N values may reflect the spatially and temporally heterogeneous abundance of cyanobacteria in each sample. To further research the role of N₂ fixation by cyanobacteria we will use ¹⁵N₂ incubation assays and acetylene reduction assays to determine the activity of nitrogenase in biofilm samples.