At Helgoland Roads a bacterial succession after the spring algae bloom is regularly observed and could be linked to substrate availability1,2. While the increase in abundance of particular bacterial taxa is well studied, the mortality factors of individual clades are not thoroughly explored yet. In general there are two categories of control regimes that influence population dynamics: bottom-up and top-down control. Bottom-up is the supply of substrates. Low substrate availability can restrict the growth and maintenance of bacterial populations3. Top-down is the control of population dynamics by grazing or lysis. Grazing by protists, mainly heterotrophic nanoflagellates, is one of the two known main sources of mortality of marine bacteria4,5. The other main mortality factor is lysis by phages, which is poorly understood in the environment for marine bacterial key groups.