Fig. 2. Example time series of forces applied to seaweeds in the field. (A) Bending
moments (i.e. forces applied with a given lever arm) imposed on the base of
the stipe of an upright understory kelp Pterygophora californica
exposed to unbroken waves. Note the relatively regular character of the
recording. (B) Tensile force in a stipe of Nereocystis luetkeana
exposed to unbroken waves. The slightly more complicated trajectory of force
(which varies around that induced by the plant's buoyancy alone) probably
arises from the seaweed's interaction with the water's surface as it is swept
back and forth by drag. (C) A shorter time series of force applied to an
emergent intertidal alga, Pelvetia compressa. The far more
complicated fluid motions associated with wave impingement and breaking
produce a rapidly evolving force record, particularly at the instant of wave
arrival. Note that the magnitude of forces imposed on this small surfzone
plant (frond length 15 cm) rival those acting on the far larger (7 m length)
subtidal bull kelp (compare B and C). This is a direct consequence of the
greater severity of flow in intertidal regions.
