Forward hysteresis and Hopf bifurcation in an NPZD model with application to harmful algal blooms

TL;DR

This paper develops an advanced NPZD ecological model incorporating harmful algal bloom dynamics, analyzing stability, bifurcations, and ecological thresholds to understand ecosystem resilience and vulnerability.

Contribution

It introduces a novel NPZD model with forward hysteresis and Hopf bifurcation analysis, extending threshold analysis to ecological disturbances and seasonality effects.

Findings

Ecosystems favoring phytoplankton are vulnerable to HABs and zooplankton extinction.

Healthy ecosystems are highly sensitive to nutrient depletion.

The model reveals bi-stability and complex bifurcation behaviors under different conditions.

Abstract

Nutrient-Phytoplankton-Zooplankton-Detritus (NPZD) models, describing the interactions between phytoplankton, zooplankton systems, and their ecosystem, are used to predict their ecological and evolutionary population dynamics. These organisms form the base two trophic levels of aquatic ecosystems. Hence understanding their population dynamics and how disturbances can affect these systems is crucial. Here, starting from a base NPZ modeling framework, we incorporate the harmful effects of phytoplankton overpopulation on zooplankton - representing a crucial next step in harmful algal bloom (HAB) modeling - and split the nutrient compartment to formulate an NPZD model. We then mathematically analyze the NPZ system upon which this new model is based, including local and global stability of equilibria, Hopf bifurcation condition, and forward hysteresis, where the bi-stability occurs with multiple attractors. Finally, we extend the threshold analysis to the NPZD model, which displays both forward hysteresis with bi-stability and Hopf bifurcation under different parameter regimes, and examine ecological implications after incorporating seasonality and ecological disturbances. Ultimately, we quantify ecosystem health in terms of the relative values of the robust persistence thresholds for phytoplankton and zooplankton and find (i) ecosystems sufficiently favoring phytoplankton, as quantified by the relative values of the plankton persistence numbers, are vulnerable to both HABs and (local) zooplankton extinction (ii) even healthy ecosystems are extremely sensitive to nutrient depletion over relatively short time scales.