Interannual dynamics and phenology of bacterial communities in a eutrophic lake

We investigated patterns of intraand interannual change in pelagic bacterial community composition (BCC, assessed using automated ribosomal intergenic spacer analysis) over six years in eutrophic Lake Mendota, Wisconsin. A regular phenology was repeated across years, implying that freshwater bacterial communities are more predictable in their dynamics than previously thought. Seasonal events, such as water column mixing and trends in water temperature, were most strongly related to BCC variation. Communities became progressively less similar across years between the months of May and September, when the lake was thermally stratified. Dissolved oxygen and nitrate + nitrite concentrations were highly correlated to BCC change within and across seasons. The relationship between BCC and seasonal drivers suggests that trajectories of community change observed over long time series will reflect large-scale climate variation. Bacteria underpin ecosystem function in freshwater lakes by mediating most biogeochemical cycling (Cotner and Biddanda 2002). However, surprisingly little is known of what drives bacterial community composition (BCC) and diversity in lakes. Consequently, we lack information that may improve our ability to predict rates of biogeochemical transformation in lakes and the responses of freshwater ecosystems to environmental changes that alter BCC. Cultivation-independent molecular techniques have enabled recent investigation of freshwater bacterial species distribution across space and time. Such studies have been conducted at several different spatial distributions: vertical (Urbach et al. 2001) and horizontal (Yannarell and Triplett 2004) within a lake, among lakes within a region (Yannarell and Triplett 2005), among lakes of the same trophic status (Lindstrom 2001) and of different trophic status (Lindstrom 2000; Yannarell et al. 2003), and across continents (Lindstrom et al. 2005). Many short-term temporal studies have documented seasonal BCC variation within a lake or between lakes (Lindstrom 2001; Crump et al. 2003; Kent et 1 Current address: Department of Natural Resources and Environmental Sciences, University of Illinois at Urbana– Champaign, Urbana, Illinois 61801. 2 Corresponding author: 1415 Engineering Drive, Madison, Wisconsin 53706. Phone: 1-608-263-3137; fax: 1-608-262-5199 ([email protected]).