by Angelica Lucchetto
Impacts of Floating Woody Debris on Algae Communities: A Comparison Between Spirit Lake and Coldwater Lake, Mount St. Helens Introduction The May 18, 1980 eruption of Mount St. Helens devastated the surrounding landscape, eliminating all vegetation in the blast zone. A series of volcanic explosions and pyroclastic flows altered the hydrology of the blast zone by obstructing the outlet of Spirit Lake, raising the lake’s surface elevation by 64 meters (USGS, “Lakes and Drainages Around Mount St. Helens”). The debris avalanche also blocked the outlet of Coldwater Creek, forming Coldwater Lake as it is today. Effectively, the current geologic configurations of these two lakes were formed from the eruption of Mount St. Helens, opening exciting research opportunities to compare the succession of their ecosystems over time. Our research compares the composition of algal communities within and between Spirit and Coldwater lakes. We specifically focused on the role of Spirit Lake’s floating log mat in creating unique conditions that shape the lake’s algal community not present in Coldwater Lake. Because primary productivity of both lakes was eliminated by the eruption, algal communities were essentially reset and then re-established over time. Larson et al. (2006) enumerated the phytoplankton species present in Spirit Lake between 1983 and 2005, demonstrating community recovery and change
Because primary productivity of both lakes was eliminated by the eruption, algal communities were essentially reset and then reestablished over time. 6 fieldnotesjournal.org
over that time period. We extended that work by analyzing plankton tow and benthic periphyton samples collected from rocks and logs along the shoreline of both lakes, as well as from logs floating on Spirit Lake’s surface. We used these samples to determine the relative abundances of the phytoplankton taxa present in Spirit Lake in 2018 and 2019 and in Coldwater Lake in 2020. By comparing our data to previously reported findings, this study contributes to our current understanding of multi-decadal community change. In addition, our findings establish data specifically on periphyton (algae and other microorganisms that cling to substrate), an algal community that had not previously been investigated in either lake. The presence of a floating log mat on Spirit Lake’s surface is the key differentiating feature between the lakes. This floating log mat is comprised of trees blasted from surrounding hills into the lake during the 1980 eruption. The logs now float on the lake’s surface, moving in a large mass with the lake’s currents. Although logs were present on the surfaces of both lakes in the months following the eruption (Figure 1), today only Spirit Lake has a significant floating log mat (covering ~20% of the lake’s surface area). Previous research by Gawel et al. (2018) found that the area of Spirit Lake where the log mat resides most often, the East Arm, has higher concentrations of nitrogen and phosphorus compared to other areas of the lake. They hypothesized that the log mat may be a significant contributor to these elevated nutrient levels. One explanation for high nitrogen concentrations in log mat-frequented areas is the presence of nitrogen-fixing cyanobacteria in the periphyton on the underside of the logs. Because Coldwater Lake does not have a persistent log mat, we were able to compare the log mat’s relative importance to the lakes’ periphyton communities.