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The composition of ecological communities differs due to a combination of different processes, which includes selection by local environmental conditions, dispersal from the regional species pool and random events. Additionally, historical processes such as past dispersal events may leave their imprint on communities as well, resulting in historically contingent communities. However, in most ecological studies the existence and the effect of historical processes remained hidden, even though they could be important predictors of contemporary variations in ecological communities.

This thesis focuses on how historical processes could influence aquatic microbial metacommunities by investigating when and where history matters, and which factors may regulate historical contingency.

Using null model approaches, evidence for historical contingency was found in natural ecosystems, more specifically rock pool metacommunities, and appeared to be more likely to influence bacterial than microeukaryotic communities.

The thesis further used an outdoor mesocosm experiment to test how ecosystem-sized induced differences in environmental fluctuations influenced community assembly processes along a disturbance gradient. This study did, however, not provide strong and clear evidence for the importance of historical contingency.

In the face of climate change, results from a laboratory experiment showed that historical contingencies might be strengthened with warming. Specifically, warming increased the resistance of local communities against invasion by decreasing the establishment success of migrant species. Hence, temperature-dependent historical contingency was found in aquatic bacterial communities, although its persistence differed between local communities and the degree of invasion they were exposed to.

Taken together, this thesis suggests that historical processes can leave their imprint on aquatic microbial communities, even though their importance is highly context dependent. Future studies, should therefore consider historical contingency, or in other words, the legacy of the past as a potentially important mechanism that can contribute to the spatial diversity of microbial communities.