Aquatic bacteria, lakes and water column overturn: A model microbial system for disturbance ecology

Microbes and their processes are essential for life. However, a theoretical backbone for microbial ecology is lacking. To address this, I tested principles from disturbance ecology using an in situ microbial system. The components of this system were lakes, planktonic bacteria, and lake mixing as the disturbance. Lake mixing disrupts physical and chemical conditions known to structure lake bacterial communities in space and time. The research progressed from seasonal observations of lake bacterial communities to a natural experiment in a typhoon-influenced lake, and finally to controlled mixing manipulations. In a six-year survey of Lake Mendota, surface bacterial communities followed a predictable seasonal trajectory that was repeated across years. From this, I hypothesized that spring mixing acted as a resetting event. In a second weekly survey of bacterial communities from three lakes across a gradient of mixing regimes, surface (epilimnion) and bottom (hypolimnion) communities were compositionally distinct and also had remarkably different dynamics. A natural experiment followed these observations: I contrasted the responses of surface and bottom bacterial communities after episodic, typhoon-initiated mixing and seasonal, winter mixing in a small Taiwanese lake. I quantified mixing intensity and demonstrated a linear relationship between surface and bottom community similarity and the intensity, which suggested community responses were predictable. Next, I conducted two in situ mixing manipulations. The first was a mesocosm manipulation of hypothesized mixing-associated environmental drivers, and followed by quantification of the resistance and resilience of the surface and bottom communities. These results suggested that bacterial communities from either layer were not resistant but quickly resilient to disturbance. Finally, to determine bacterial community responses after a novel, extreme disturbance, I mixed a whole lake at the peak of stratification. Again, the communities were not resistant but surprisingly resilient to the disturbance, despite the novelty of the post-mixing environment. This reveals an incredible robustness of the lake bacterial communities to unpredictable perturbation. From this research, I have shown that principles of large-scale ecology are applicable for understanding patterns of microbial community disturbance response, and have therefore contributed to developing a disturbance paradigm inclusive of micro- and macro-scale organisms.