A mathematical model describing the dynamics of toxin producing phytoplanktonzooplankton interaction with instantaneous nutrient recycling is proposed. We have explored the dynamics of plankton ecosystem with multiple delays; one due to gestation period in the growth of phytoplankton population and second due to the delay in toxin liberated by TPP. It is established that a sequence of Hopf bifurcations occurs at the interior equilibrium as the delay increases through its critical value. The direction of the Hopf bifurcation and the stability of the bifurcating periodic solutions are determined using the theory of normal form and center manifold. Meanwhile, effect of toxin on the stability of delayed plankton system is also established numerically. Finally, numerical simulations are carried out to support and supplement the analytical findings. less affected by human activities, and often cause economic losses, such as those in aquaculture and tourism. In the last decades of the past century, a global increase of toxin producing phytoplankton (TPP) blooms has been observed. Number of HABs, economic losses from them, the types of resources affected and number of toxic species have also increased dramatically in recent years [1][2][3]. The adverse effect of HABs is clear, but the control of such a problem is under investigation. HABs is known to have a negative impact on the zooplankton and on the species feeding on them such as large fish in the ocean which ultimately affects the human food chain. The different modes and mechanisms by which harmful phytoplankton species can cause mortality, physiological impairment, or other negative in situ effects have been discussed in [2]. The results presented by Webb et al. [8], Gakkhar and Naji [9], Henson and coworkers [10] and Upadhyay et al. [11] have also contributed to the clarification of the mechanism of HAB's. Several researchers have tried to explain the dynamics of plankton blooms by focusing on different factors such as nutrient upwelling [4,5], spatial patchiness [6] and species diversity [7]. The impact of zooplankton grazing on HAB is an important aspect of the plankton ecology. If zooplankton community's grazing impact on initial stages of a HAB is sufficiently high, then a bloom does not develop [12]. A number of field studies [13-15] as well as laboratory experiments have established that toxicity has a great impact on phytoplankton-zooplankton interaction dynamics. Some studies reveal that many copepods experience decreased grazing and fecundity in the presence of HAB species [16]. It is also well known that zooplankton population tries to avoid the areas where the concentration of phytoplankton is very large. The reason may be either dense concentration of phytoplankton or the effect of toxic substances released by them. Buskey and Hyatt [17] have shown in their field study that macroand meso-zooplankton populations are reduced during the blooms of chrysophyte Aureococcus anophagefferens. Many researchers have proposed different deterministic models on HAB...