Although the relationships between trophic conditions and viral dynamics have been widely explored in different pelagic environments, there have been few attempts at independent estimates of both viral production and decay. In this study, we investigated factors controlling the balance between viral production and decay along a trophic gradient in the north Adriatic basin, providing independent estimates of these variables and determining the relative importance of nanoflagellate grazing and viral life strategies. Increasing trophic conditions induced an increase of bacterioplankton growth rates and of the burst sizes. As a result, eutrophic waters displayed highest rates of viral production, which considerably exceeded observed rates of viral decay (up to 2.9 ؋ 10 9 VLP liter ؊1 h ؊1 ). Viral decay was also higher in eutrophic waters, where it accounted for ca. 40% of viral production, and dropped significantly to 1.3 to 10.7% in oligotrophic waters. These results suggest that viral production and decay rates may not necessarily be balanced in the short term, resulting in a net increase of viruses in the system. In eutrophic waters nanoflagellate grazing, dissolved-colloidal substances, and lysogenic infection were responsible together for the removal of ca. 66% of viral production versus 17% in oligotrophic waters. Our results suggest that different causative agents are primarily responsible for the removal of viruses from the water column in different trophic conditions. Factors other than those considered in the past might shed light on processes responsible for the removal and/or decay of viral particles from the water column.Viruses have been recognized as responsible for a relevant fraction of bacterial mortality (see references 5, 36, and 41 and references therein). Viral lysis, by causing the release of new viruses and host cell contents, can lead to a significant increase of dissolved organic carbon (DOC) in the environment, which in turn can affect bacterial community structure (30) and have a large impact on bacterial carbon cycling (17, 24).Virus-induced bacterial mortality has been shown to be strongly dependent on local trophic conditions (20,33,34). Several authors have found that eutrophic environments support a higher standing stock of bacteria and consequently of bacteriophages than oligotrophic systems. Trophic conditions can influence the production of new viral particles by changing the metabolism and size of the host cells (3, 9, 23).Since the maintenance of viral assemblages and the hostvirus relationships are controlled by the decay and replenishment (production) of viral particles, estimates of viral turnover times are crucial to evaluate the potential of viruses to change in space and time (20). In order to test the steady-state assumption, often utilized as a basic assumption in viral ecology studies (4), a correct balance of viral production versus viral decay rates has to be evaluated. A correct analysis of viral dynamics should require an independent measurement of both viral produ...

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