Lake bacterioplankton dynamics over diurnal timescales LORENA M. GRUBISIC*, STEFAN BERTILSSON*, ALEXANDER EILER*, FRIEDERIKE HEINRICH*, ANDREAS BRUTEMARK † , ‡ , LAURA ALONSO-S  AEZ* , § , ANDERS F. ANDERSSON* , ¶ , STEPHAN GANTNER* , **, LASSE RIEMANN †† AND SARA BEIER* , ‡‡ *Limnology and Science for Life Laboratory, Department of Ecology and Genetics, Uppsala University, Uppsala, Sweden † Tv€ arminne Zoological Station, Hanko, Finland ‡ ARONIA Coastal Zone Research Team, Novia University of Applied Sciences and  Abo Akademi University, Eken€ as, Finland § Marine Research Division, AZTI, Sukarrieta, Spain ¶ School of Biotechnology, KTH Royal Institute of Technology and Science for Life Laboratory, Stockholm, Sweden **Department of Educational Sciences in Biology, Leibniz Institute for Science and Mathematics Education, Kiel, Germany †† Marine Biological Section, Department of Biology, University of Copenhagen, Helsingør, Denmark ‡‡ Leibniz Institute for Baltic Sea Research Warnem€ unde (IOW), Rostock, Germany SUMMARY 1. Planktonic bacterial community dynamics over short timescales can be of great importance for food webs and ecosystem functioning but are rarely described when microbial community and composition are assessed. To study the significance of such dynamics we sampled the surface water at the deepest point of a mesotrophic lake (Lake Erken, Sweden) every third hour over two days. 2. By combining 454 pyrosequencing of 16S rRNA genes with bromodeoxyuridine immunocapturing of DNA, replicating populations were identified and compared to the community retrieved from total DNA samples. This comparison revealed a significant difference between the actively replicating and total community. 3. The high-frequency diurnal sampling was compared to a year-long survey conducted in the same lake in order to compare the diurnal and seasonal variation in bacterioplankton community composition. At the diurnal-scale, the variation was significantly higher in the replicating than in the total community. However, variation in both active and total diurnal community was significantly lower than the variation in the seasonal total community. 4. Our analysis revealed pronounced short-term dynamics of individual bacterial populations uncoupled from the diurnal light cycle. For example, the proliferating fraction of the most abundant bacterial tribe (LD12) followed a cyclic pattern that covaried with viral abundance. This implies that environmental factors other than light may act as important drivers of microbial community composition, at least in mesotrophic Lake Erken. Keywords: community, microbes, molecular, pelagic, population Introduction Bacteria play central roles in aquatic biogeochemical cycles where they hold a unique position in the transfor- mation of organic substrates and nutrient remineraliza- tion in microbial food webs (Cotner & Biddanda, 2002; Bertilsson & Jones, 2003). In recent years, much has been learned about the taxonomic identity of typical freshwater bacterioplankton and their roles in the ecosystem (Newton et al., 2011). Extensive surveys have shown that aquatic bacterial communities follow season- ally reoccurring patterns in taxonomic composition and contain both persistent and more ephemeral bacterial populations (Kent et al., 2004; Fuhrman et al., 2006; Shade et al., 2007; Andersson, Riemann & Bertilsson, 2010; Eiler, Heinrich & Bertilsson, 2012; Lindh et al., Correspondence: Sara Beier, Department of Biological Oceanography, Leibniz Institute for Baltic Sea Research, Seestraße 15, 18119 Rostock, Germany, E-mail: sara.beier@io-warnemuende.de © 2016 John Wiley & Sons Ltd 191 Freshwater Biology (2017) 62, 191–204 doi:10.1111/fwb.12861