The abundance of viral-like particles in marine ecosystems ranges from <104>^–1 to >10⁸ ml^–1. Their distribution in time and space parallels that of other biological parameters such as bacterial abundance and chlorophyll a. There is a lack of consensus between methods used to assess viral activity, i.e., rate of change in viral abundance (increase or decrease). The highest rates, 10–100 days^–1, are observed in experiments with short sampling intervals (0.2–2 h), while lower rates, on the order of 1 day^–1, are observed in experiments with longer sampling intervals (days). Few studies have been carried out, but viruses appear, at least in some cases, to have a significant impact on carbon and nutrient flow in microbial food webs. Viruses have also been demonstrated to exert a species specific control of both bacteria and phytoplankton populations in natural waters.