HABITAT SHIFTS AND THE INFLUENCE OF A PERSISTENT FLOATING LOG MAT ON ECOSYSTEM RECOVERY AT SPIRIT LAKE, MT. ST. HELENS

The recovery of Spirit Lake after the 1980 eruption of Mt. St. Helens has been impacted by the unique habitat created when trees blasted into the lake created a floating log mat that now covers approximately 20% of the lake surface. The eruption and subsequent landslide also fundamentally re-shaped the lake basin, which is now larger and shallower than pre-eruption. Using collections from an artificial substrate attached to the floating log mats, plankton tows, sediment traps, and sediment cores, we investigate the relative abundance of various algal groups, including nitrogen-fixing cyanobacteria, that may be significant to nutrient dynamics in this oligotrophic lake. Sediment cores suggest that the lake evolved from a plankton-dominant system to one with substantial benthic growth as the water clarity increased. Biofilm on the logs is largely composed of green algae and diatoms; however, significant growth of filamentous cyanobacteria has been detected in the periphyton communities that grow on submerged vegetation and is present in the log biofilm throughout the summer at a low abundance. Given the significant area that the logs occupy in the lake, the biofilm growth may have a significant impact on nutrient dynamics in the lake over time.