A whole lake experiment confirms a small centric diatom species as an indicator of changing lake thermal structure
In many lakes across the Northern Hemisphere, paleolimnological records have revealed that the relative abundances of the small centric diatom, Discostella stelligera, changed over the past century, with these widespread shifts attributed to climate change. Specifically, small scale experiments and current spatial distribution patterns suggested that this species is more abundant when lake mixing depths are shallower, but a direct test of this hypothesis at the whole lake scale was lacking. We conducted a whole lake mixing manipulation in a remote arctic lake that normally has relatively shallow thermal stratification and abundant D. stelligera populations during the summer. We employed a "Before After Control Impact" design using an experimental lake and a control lake. Lake thermal structure and diatom populations were monitored in both lakes in summer 2013 without manipulating either lake, and again in summer 2014 when the experimental lake was manipulated to achieve deeper mixing depths. The abundance of D. stelligera declined during the manipulated period of deeper mixing in 2014, while it increased during the same time frame in the control lake. The same pattern was not observed for the four other diatom taxa found in both lakes in both years. Our results confirm the use of D. stelligera as an indicator of changing lake thermal structure, and suggest that the broader application of this tool to lake sediment records will yield greater insight into longer term variability in the response of lake ecosystems to climate.