Fig. 3. Example model predictions of dynamic tuning in macroalgae subjected to
unbroken waves showing how inertial effects can dominate applied force. In the
case of the large surface-canopy kelp Nereocystis luetkeana (solid
line and the ordinate on the left), peak tensile force (including forces due
to the momentum the plant acquires as it moves) may occur when the waves
arrive at a frequency equal to half the natural frequency of the seaweed, and
these forces may greatly exceed the tensile force from drag alone. (The
natural frequency is the frequency at which the plant would tend to oscillate
if nudged slightly in still water.) Similarly, in an erect understory kelp,
Eisenia arborea (dashed line and the ordinate on the right), total
bending moments (i.e. moments including forces due to the momentum the plant
acquires as it moves) may at times substantially exceed bending moments that
would arise from drag alone. In this situation, the tuning is most apparent
when the dominant frequency of the waves is the same as the natural frequency
of the alga. Note that these model predictions assume a zero longshore
current; if one is present, it may partially offset the exceptionally large
tension ratios seen for N. luetkeana. In both curves, the magnitude
of the orbital velocities produced by the waves is held constant across
frequency. Denny et al. (1998)
provide full details on this topic.
