THE ROLE OF LOG MAT BIOFILM IN THE SPIRIT LAKE ECOSYSTEM AFTER THE ERUPTION OF MOUNT ST. HELENS

Spirit Lake, on the flanks of Mount St. Helens, was dramatically altered as a result of the eruption in 1980, and over the past 38 years 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 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 part of the lake energy and nutrient cycles. Initial data sets of invertebrate abundance and diversity suggest that log mats host a robust food web, that is likely supported by biofilm productivity. As well, a range of biogeochemical data collected from various parts of the lake ecosystem (water samples, primary producers, suspended sediment traps, lake bottom sediments) show that biofilm is a distinct and identifiable source of organic material. Here we use carbon and nitrogen elemental and isotopic data collected from ecosystem samples (including sediment cores) to investigate both spatial patterns in biofilm-derived organic material and temporal changes in amount and source of organic material in sediments. Areas of the lake that are most frequently occupied by the floating log mats (determined by prevailing winds) have the highest nutrient and carbon load, likely related to higher biofilm biomass. This appears to be a relatively recent pattern, as older sediments have very low organic content. Combined, these spatial and temporal data sets provide insight into how the post-eruption Spirit Lake ecosystem continues to evolve, and the dynamic role of log mat resources.