  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.
