Irregular harmful algal blooms triggered by feedback between toxin production and zooplankton feeding

• There is increasing evidence of blooms of toxic phytoplankton throughout the globe. • The reasons behind the irregular occurrence of toxic blooms are not well understood. • Strength of toxicity on zooplankton regulates timing, frequency, and severity of HABs . • Concentrations of zooplankton and toxic-phytoplankton indicate the timing of HABs. • This complements our understanding of the mechanisms of toxic bloom occurrence. Blooms of toxic or harmful phytoplankton (known as ‘harmful algal blooms’ or ‘HABs’) pose a significant and increasing threat to human health and fisheries throughout the globe. The most interesting nature of these HABs is their irregularity, in terms of both magnitude and frequency of occurrence, which makes the prediction of HABs difficult. Although such irregularities are very common in nature, the reasons behind them are not well understood. Here we study a food web model consisting of nutrients, toxic phytoplankton, non-toxic phytoplankton, and grazer zooplankton. The toxic substance produced by the toxic phytoplankton provides an advantage to both phytoplankton species by suppressing the grazer population. We find that the seasonal variation in nutrient concentration shapes the phytoplankton community, while the toxic effect results in different seasonal successional patterns. Additionally, our results provide a new mechanism for the irregularity in HAB formation. Examining the impact of toxins on the timing, frequency, and magnitude of toxic blooms, we demonstrate that the bloom dynamics of toxic phytoplankton become irregular and more severe in the presence of high toxic effects, while the non-toxic phytoplankton show annual blooms. We also find that the concentrations of zooplankton and non-toxic phytoplankton at the beginning of the year can be indicative of the timing and severity of HABs; the most severe HABs occur during summer and are related to very low zooplankton together with certain specific concentrations of non-toxic phytoplankton. This improved level of understanding of the factors regulating the timing, frequency, and severity of HABs can help to predict and determine strategies to mitigate the impact of HABs on ecosystems and society.