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As climate change increases the magnitude and variability of precipitation, aquatic systems are especially impacted by the enhanced inputs of nutrients and coloured dissolved organic matter (cDOM) via runoff. Here, we investigated how runoff regimes differing in magnitude and frequency influence bacterial abundance and community composition. For that mesocosm experiments were conducted in three different Swedish lakes during spring and summer. Results showed more deviation in bacterial growth and composition from control conditions under frequent, low-intensity runoff compared to single, high-intensity events. The lake with the highest cDOM concentration appeared less impacted by runoff, with bacterial communities strongly structured by deterministic processes. Furthermore, variation in bacterial community composition was primarily associated with carbon-related variables during spring and nutrient-related variables inputs during summer. Cyanobacteria exhibited higher relative abundances under frequent additions than under single high-intensity events, highlighting the importance of runoff frequency for drinking water management. Overall, these findings demonstrate that runoff influences both bacterial growth and community composition, with responses strongly dependent on season and lake characteristics. 

Legacy effects that arise when past environmental conditions influence present community responses are increasingly recognized as important drivers of microbial dynamics. However, how repeated disturbances and the intervals between them shape community assembly, and whether they lead to predictable trajectories or more contingent outcomes, remains poorly understood. Here, we used a controlled chemostat experiment to examine how two alternative histories of repeated minor salinity pulses shaped bacterial community responses to a subsequent salinity pulse. Using a high-resolution time series of 26 sampling points over 50 days, we detected systematic treatment effects and increasing divergence among within-treatment replicates over time. Communities with disturbance histories responded differently to a subsequent major pulse than communities without disturbance history: previous exposure altered both the magnitude and direction of richness change, and even promoted richness when recovery time was brief. Moreover, exposure to a subsequent salinity disturbance maintained elevated levels of inferred stochasticity in communities with disturbance histories following the major pulse. Together, these findings demonstrate that disturbance sequence and recovery interval critically shape microbial stability and influence the balance between stochastic and deterministic assembly processes.

Both deterministic (e.g. species-environment interactions) and stochastic processes (e.g. random birth and death events) shape communities, but it remains poorly understood, which environmental conditions promote stochasticity. Here, we investigated interactive effects of nutrient availability and community size on stochasticity in order to predict how eutrophication and biomass loss shift the balance between predictable and random community dynamics. For this, we used freshwater bacterial communities in a microcosm experiment, where communities were diluted to varying sizes and exposed to low, intermediate, and high nutrient concentrations. Stochasticity was estimated with null modelling and as beta-diversity among replicate communities. At low nutrient concentrations, deterministic processes dominated, especially in smaller communities, which had the lowest diversity and abundance. Whereas, higher nutrient concentrations increased stochasticity. In contrast to theoretical predictions, this was particularly the case in larger communities with the highest diversity and abundance, likely due to stochastic initial growth. The findings underline how nutrient availability and community size jointly influence stochastic assembly processes, with important consequences for bacterial diversity and ecosystem functioning under environmental change. This study shows that nutrient availability and community size jointly determine whether freshwater bacterial communities are shaped more by deterministic or stochastic processes, with low nutrients favouring deterministic assembly and high nutrients promoting stochasticity, especially in larger, more diverse communities.

Stochasticity, or random changes in community composition over time, can generate unpredictable community outcomes, particularly in bacterial communities which are dominated by rare taxa. As nutrient and temperature rise under global change, they can promote growth of taxa and might enhance stochastic community assembly processes. So far, abiotic factors, like nutrients, have mostly been tested on bacterial communities in isolation, even though complex natural systems also impose biotic factors, such as grazing pressure. Therefore, we aimed to study how grazing pressure mediates bacterial community divergence and stochasticity in response to environmental pulses under dispersal-limited conditions. In a microcosm experiment, freshwater bacteria were exposed to heterotrophic nanoflagellate grazers and short-term nutrient, temperature and combined pulses. Grazing appeared to be the dominant driver of bacterial community assembly, which promoted community dissimilarity and determinism, while the effects of the nutrient and temperature pulses were neglectable. These results indicate that top-down control can mediate bacterial community responses to environmental changes.