Diel Dynamics of Freshwater Bacterial Communities at Beaches in Lake Erie and Lake St. Clair, Windsor, Ontario.

Bacteria play a key role in freshwater biogeochemical cycling as well as water safety, but short-term trends in freshwater bacterial community composition and dynamics are not yet well characterized. We sampled four public beaches in southern Ontario, Canada; in June, July, and August (2016) over a 24-h (diel) cycle at 2-h intervals. Using high-throughput sequencing of 16S rRNA gene, we found substantial bi-hourly and day/night variation in the bacterial communities with considerable fluctuation in the relative abundance of Actinobacteria and Proteobacteria phyla. Moreover, relative abundance of Enterobacteriaceae (associated with potential health risk) was significantly high at night in some dial cycles. Diversity was significantly high at night across most of the diel sampling events. qPCR assays showed a substantial bi-hourly variation of Escherichia coli levels with a significant high level of E. coli at night hours in comparison with day hours and the lowest levels at noon and during the afternoon hours. Taken together, these findings highlighted a considerable short-term temporal variation of bacterial communities which helps better understanding of freshwater bacterial dynamics and their ecology. E. coli monitoring showed that multiple samples in different hours will provide more accurate picture of freshwater safety and human health risk. Graphical abstract.