A two-timescale model of plankton-oxygen dynamics predicts formation of Oxygen Minimum Zones and global anoxia

TL;DR

This paper presents a coupled plankton-oxygen model with different timescales that explains the formation of Oxygen Minimum Zones and potential progression to global ocean anoxia, using analytical and numerical methods.

Contribution

It introduces a two-timescale conceptual model of plankton-oxygen dynamics that accounts for trophic effects and explains OMZ formation and expansion.

Findings

Canard cycle blowup leads to plankton extinction and oxygen depletion.

Initial perturbations can trigger OMZ formation and growth.

Large timescale separation can cause global ocean anoxia.

Abstract

Decline of the dissolved oxygen in the ocean is a growing concern, as it may eventually lead to global anoxia, an elevated mortality of marine fauna and even a mass extinction. Deoxygenation of the ocean often results in the formation of Oxygen Minimum Zones (OMZ): large domains where the abundance of oxygen is much lower than that in the surrounding ocean environment. Factors and processes resulting in the OMZ formation remain controversial. We consider a conceptual model of coupled plankton-oxygen dynamics that, apart from the plankton growth and the oxygen production by phytoplankton, also accounts for the difference in the timescales for phyto and zooplankton (making it a "slow-fast system") and for the implicit effect of upper trophic levels. The model is investigated using a combination of analytical techniques and numerical simulations. We show that the system does not allow for persistent relaxation oscillations; instead, the blowup of the canard cycle results in plankton extinction and oxygen depletion. For the spatially explicit model, an initial non-uniform perturbation can lead to the formation of an OMZ, which then grows in size and spreads over space. For a sufficiently large timescale separation, the spread of the OMZ can result in global anoxia.