Avoiding toxic prey may promote harmful algal blooms

• Harmful algal blooms (HAB) have a very negative impact on freshwater ecosystems as well as on human health.
• Toxin-producing species are poor competitors for resources. Nevertheless, toxic blooms are often observed in nature.
• We employ reaction–diffusion equations to model HAB considering an inhibitory effect between toxic prey and predators.
• We introduce a function coupling toxicity and predation rate, describing the predator's sensitivity to toxicity.
• The presence of toxic species may trigger spatial blooms if predators behave selectively and toxicity is at a medium value.

Blooms of freshwater cyanobacteria are a worldwide spread environmental issue. Despite toxin producing planktonic species are generally expected to be poor competitors for resources, dense blooms of toxic cyanobacteria, such as Microcystis, do often occur in nature. Experimental results suggest that the formation of such blooms is promoted by the predatory activity of zooplankton. In fact, such predator grazes on both the nontoxic and toxic species despite the toxicity of the latter actually inhibits its growth. We model this phenomenon through a Lotka–Volterra reaction–diffusion system. Our goal is to investigate the coupled role of toxicity and zooplankton's predation in the persistence of the toxic prey and to study the mechanisms behind the formation of spatially local toxic blooms. It is known that the classical Lotka-Volterra system consisting of one prey and one predator never exhibits pattern formation. In this paper, we show that the introduction of a toxic prey may destabilize the spatially homogeneous coexistence and trigger spatial pattern formation. We also show that local blooms more likely occur when predators avoid the toxic prey and when the strength of the toxicity is of an intermediate level.