Importance of Interactions and Feedbacks for Experimental Microbial Aquatic Communities

Freshwater ecosystems face increasing anthropogenic pressures despite their importance to provide goods and services to the human population. Therefore, being able to predict consequences of environmental disturbances on the individuals and populations in freshwater communities, and the ecosystem processes they underpin, is a primary challenge for ecologists. These consequences depend on features of the community and on the nature of disturbances (press, pulse or ramping), the type of environmental change (e.g., temperature, nutrient availability, mortality), how multiple environmental changes interact, and also on feedbacks between organisms and their environment. I characterised five major interconnected challenges to investigate in the context of global change biology research: 1) the stability of ecosystem properties, 2) the presence of multiple environmental disturbances, 3) the possibility to up-scale environmental changes across levels of biological organisation, 4) implications on the predictability of disturbances, 5) and the organism-environment feedback. I addressed these challenges by experimentation with microbial aquatic communities in laboratory conditions. These experiments provided a useful and interesting tool to investigate ecological processes, mechanisms and theories. I conducted two main experiments. The first experiment consisted of exposing a relatively complex microbial community to four common environmental disturbances in a full-factorial experimental design. Results indicated that increasing the number of disturbances reduced the stability of an ecosystem variable, namely dissolved oxygen concentration. Additionally, one particular disturbance (organic matter addition) had a dominant impact across levels of biological organisations, and interactions among the four disturbances were rare and weak. Nevertheless, predictions of the responses to the environmental disturbances were more accurate when interactions among them were accounted for. This highlights that the same experiment can give different conclusions about the importance of environmental effects depending on if one seeks explanation or to make accurate predictions. The second experiment consisted of investigating the effects of feedbacks between organisms and their environment. Specifically, I tested a general theory that stronger feedback increases the likelihood of multistability, and increases the strength of nonlinearity and hysteresis in response to environmental change. The strength of feedback was experimentally manipulated by controlling rate of gas exchange inside the experimental ecosystems with gas outside (i.e., in the atmosphere). As predicted, stronger organisms-environment feedback increased nonlinearity and the hysteresis observed with a gradual change of environment, but contrary to predictions there was no increase in likelihood of multistability. In summary, I provided empirical evidence of the consequences of multiple environmental disturbances and the consequences of organism-environment feedback strength in a microbial aquatic community. These consequences suggest that interactions among environmental disturbances are important for predicting ecosystem dynamics and that further experimental studies, particularly when coupled with system-specific mathematical models, are required to better understand the dynamical consequences of environment-organism feedback strength