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In recent years, the importance of historical contingency has been increasingly recognized in microbial communities. During community coalescence, immigration history, and dispersal history can become decisive for the developing community. For example, an early arriving pioneer can inhibit the immigration success of a late invader by resource consumption/alteration, also known as priority effects. Alternatively, the signal of past dispersal in the resident community can be long-lasting and contribute more to the communities’ composition than contemporary dispersal. The overall aim of this thesis was to investigate the potential importance of arrival timing and dispersal timing in complex natural lake bacterial communities. This was done by examining the role of priority effects in experiments as well as the role of dispersal, including past dispersal, in natural lakes. Priority effects were difficult to detect on a whole community level but were found in high nutrient levels and in the absence of grazing. In the lakes, the internal production or internal dispersal was the most important assembly mechanism. However, external sources, including dispersal from the groundwater and the main inlet, were also important. Past dispersal, at times, contributed more to the lake bacterial community composition (BCC) than contemporary dispersal. Further, the results showed that past dispersal can leave a long-lasting signal in lake BCC, which mainly resulted from the dispersal of inactive cells. In conclusion, this thesis highlights the potential importance of temporal dynamics in complex freshwater bacterial communities and emphasizes the need to incorporate arrival and dispersal timing in future community coalescence studies.