The present study demonstrates for the first time a clear seasonal variability in the abundance of virobenthos that largely followed the variations in bacterial abundance. Following the spring bloom settlement, the benthic metabolic activity intensified and viral and bacterial abundance in the surface sediment increased 4.0- and 2.6-fold while the increase in a deeper anoxic sediment layer (6–10 cm) was 5.0- and 2.5-fold, respectively. The data strongly indicated a relationship between the pelagic–benthic coupling of organic material, the benthic metabolic activity, and the abundance and activity of bacteria and viruses in marine sediments on a seasonal basis. These suggestions were supported by parallel measurements of viral production rates in short-term laboratory incubations of surface and deep sediments: in these, viral production was significantly enhanced by addition of organic carbon and increasing temperature, with maximum values obtained in organic carbon-enriched incubations in July (19.7×106 and 38.4×106 virus-like particles cm−3 h−1 in surface and deep incubations, respectively). However, the relative enhancement was most explicit during periods of low in situ sedimentation. Conservative estimates indicated that viral lysis caused bacterial mortality rates of 2.8–27.4×105 cells cm−3 h−1, corresponding to 2–61% day−1 of bacterial standing stock during incubations (average 12.3±13.9%, n=18). An estimation of virus-generated dissolved organic carbon production using theoretical values of bacterial cell carbon content (20–50 fg cell−1), showed that viral lysates could sustain 2–38% of the measured bacterial respiration. The data suggest that viruses may have considerable impact on benthic microbial mortality and carbon cycling.