The role of virus infection in a simple phytoplankton zooplankton system.

Many planktonic species show spectacular bursts ("blooms") in population density. Though viral infections are known to cause behavioural and other changes in phytoplankton and other aquatic species, yet their role in regulating the phytoplankton population is still far from being understood. To study the role of viral diseases in the planktonic species, we model the phytoplankton-zooplankton system as a prey-predator system. Here the prey (phytoplankton) species is infected with a viral disease that divides the prey population into susceptible and infected classes, with the infected prey being more vulnerable to predation by the predator (zooplankton). The dynamical behaviour of the system is investigated from the point of view of stability and persistence both analytically and numerically. The model shows that infection can be sustained only above a threshold of force of infection, and, there exists a range in the infection rate where this system shows "bloom"-like stable limit cycle oscillations. The time series of natural "blooms" with different types of irregular oscillations can arise in this model simply from a biologically realistic feature, i.e., by the random variation of the epidemiological parameter (rate of infection) in the infected prey population. The difference in mean strength of infection alone can lead to the different types of patterns observed in natural planktonic blooms.