Selective grazing of zooplankton on phytoplankton defines rapid algal succession and blooms in oceans

Abstract

In a model plankton ecosystem containing phytoplankton and zooplankton, the later usually avoids toxic phytoplankton and prefers nontoxic species by modulating its grazing rate. The toxins released by toxic phytoplankton can lead to a decreased grazing rate of zooplankton on both phytoplankton, and the presence of nontoxic phytoplankton in the system also reduces the grazing pressure of zooplankton on the toxic phytoplankton. In this study, a set of functions were given that model the selective grazing of zooplankton on various phytoplankton. Preference coefficient and avoidance coefficient were introduced in the selective grazing functions to measure the level of zooplankton preference of nontoxic phytoplankton and avoidance of toxic phytoplankton, respectively. A dynamics model was established in a nontoxic phytoplankton-toxic phytoplankton-zooplankton system based on the grazing functions of zooplankton to study the existence and stability of an interior equilibrium and Hopf-bifurcation. Our results show that zooplankton selective grazing promotes both zooplankton and toxic phytoplankton biomass, and contributes to maintain the coexistence of all species. Low grazing selectivity, i.e., low values of preference coefficient and avoidance coefficient, decreases the zooplankton population and increases the opportunity for phytoplankton blooms. A moderate level of grazing selectivity helps keep the system in balance. With high grazing selectivity on phytoplankton, toxic phytoplankton becomes the dominant species, resulting in a rapid algal succession and eventual toxic phytoplankton blooms. When the preference coefficient and avoidance coefficient in the dynamics model crossed critical values respectively, the ecosystem enters into a Hopf-bifurcation around the interior equilibrium, inducing oscillations of these populations.