1. Viruses have been identified as an important component of plankton communities but their temporal dynamics remain poorly characterized, especially in littoral zones of lakes. Seasonal and diel dynamics of planktonic viruses and their main hosts (i.e. bacteria) were therefore assessed along four parallel transects of a lake, extending from within a littoral reed stand across the littoral shelf to the pelagic zone.
2. Viral abundance ranged from 1.9 to 9.7 × 107 virus-like particles (VLP) mL−1 with distinct seasonal peaks in spring at all locations. Seasonal variation in bacterial abundance varied more than 10-fold (0.85–12 × 106 cells mL−1) and was synchronized with changes in viral abundance. As a result, the ratio of viruses to bacteria (average of 12.5) varied much less over the year.
3. Bacterial production (0.11–0.98 μg C L−1 h−1) and growth rates (0.06–0.85 day−1) also varied seasonally but, in contrast to abundances, high production rates in spring were maintained in mid-summer, particularly in the reed stand, where growth rates were highest.
4. Variation among locations was small in most measured parameters, probably due to extensive exchange between pelagic and littoral water masses. Nevertheless, average bacterial growth and production rates in all seasons were consistently highest in the reed stand.
5. In the August of an exceptionally warm summer, viral and bacterial abundance varied little over a diel cycle (1.23–1.75 × 108 VLP mL−1 and 0.78–1.90 × 107 cells mL−1, respectively) in spite of a trend towards higher values at night. Diel changes in bacterial production (0.23–0.69 μg C L−1 h−1) and growth rate (0.02–0.06 day−1) were more pronounced, with higher values also occurring during the night, possibly indicating that bacterio- and phytoplankton production were not tightly coupled during our diel study.
6. Overall these results indicate that seasonal changes of virio- and bacterioplankton were mainly driven by high microbial activity in spring and were more pronounced than either diel changes on a hot summer day or spatial differences at a scale of 10–100 m.