viruses include phages, which attack only bacteria and blue-green algae. viruses are usually identified
by their protein shell or the disease they cause. little is known about their role in limnology other than that cyanophages are associated with the decline of blue-green algae growing as periphyton or phytoplankton. the use of cyanophages as an algal nuisance-control technique has been attempted, but blue-green algae may build up a resistance to the virus just as humans develop immunity to influenza virus.