  Plankton blooms are complex nonlinear phenomena whose occurrence can be
described by the two-timescale (fast-slow) phytoplankton-zooplankton model
intrpduced by Truscott and Brindley 1994. In their work, they observed that a
sufficiently fast rise of the water temperature causes a critical transition
from a low phytoplankton concentration to a single outburst: a so-called
plankton bloom. However, the dynamical mechanism responsible for the observed
transition has not been identified to the present day. Using techniques from
geometric singular perturbation theory, we uncover the formerly overlooked
rate-sensitive quasithreshold which is given by special trajectories called
canards. The transition from low to high concentrations occurs when this
rate-sensitive quasithreshold moves past the current state of the plankton
system at some narrow critical range of warming rates. In this way, we identify
rate-induced tipping as the underlying dynamical mechanism. Our findings
explain the previously reported transitions to a single plankton bloom, and
allow us to predict a new type of transition to a sequence of blooms for higher
rates of warming. This could provide a possible mechanism of the observed
increased frequency of harmful algal blooms.
