We show a simple mechanism by which changes in the rate of horizontal
stirring
by mesoscale ocean eddies can trigger or suppress plankton blooms and
lead to an abrupt change
in the average plankton density.
We consider a single species phytoplankton model with logistic growth and
grazing and
spatially non-uniform carrying capacity. The local dynamics has multiple
steady states for
some values of the carrying capacity that can lead to localized blooms as
fluid moves
across regions with different properties. We show that for this model
even small changes in
the ratio of biological timescales relative to the flow timescales can
greatly
enhance or reduce the global phytoplankton productivity. Thus this may be
a possible mechanism
by which changes in horizontal mixing can trigger plankton blooms or
cause regime shifts
in some oceanic regions. Comparison between the spatially distributed
model and Lagrangian
simulations considering temporal fluctuations along fluid trajectories,
demonstrates that small scale
transport processes also play an important role in the development of
plankton blooms having
significant influence on global biomass.
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