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.