Chattopadhyay et al. [Biosystems (2003), 68, pp. 5-17] proposed and analyzed an N – P model in the presence of viral infection on phytoplankton population. They studied the dynamics under the constant nutrient input. The present paper deals with the problem with seasonal variability on nutrient input. We use a general periodic function for nutrient input. We observe the dynamics of the system by considering (i) the infected phytoplankton consumes nutrient and (ii) the infected phytoplankton is not in a state to consume nutrient. Conditions for the persistence and extinction of populations are worked out. Our numerical experiments show that if the infected phytoplankton does not take nutrient then susceptible phytoplankton coexists with the infected ones. But if the infected phytoplankton consumes nutrient then there is a chance for extinction of susceptible phytoplankton for high rate of infection. We also observe that periodic nutrient input enforces the system to enter into chaotic region.