Figure 6: Lake Dynamics for b=0.7
two equilibria. The third root, i.e. the highest value of x where the curve
intersects the x-axis, gives the basins of attraction: above this point, the
high equilibrium will result and below this point the low equilibrium. This
means that if the lake flips to the eutrophic state and reaches that level
of phosphorous stock, reducing levels of phosphorous loading will not be
sufficient to bring the lake back to an oligotrophic state. In that case, only
a change in the parameter b can restore the lake because the hysteresis
is irreversible. Altering b means resorting to such costly measures as
biomanipulation and oxygenation as discussed earlier in the chapter. (See
Figure 7)
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