Surviving hydrodynamic forces in a wave-swept environment: Consequences of morphology in the feather boa kelp, Egregia menziesii (Turner)

Organisms on wave-swept intertidal shores often must withstand water velocities of 10 to 20 m/s and accompanying accelerations of 400 m/s2. Because drag and accelerational forces increase with area and volume respectively, the bigger an individual is, the larger the forces on it become. This size-force relationship suggests that large intertidal algae must be particularly well designed if they are to survive. The feather boa kelp, Egregia menziesii (Turner), grows to a size that is unusually large for its wave-exposed habitat, leading us to inquire into the mechanisms that allow it to attain this stature. A hydro-mechanical computer model is used to predict the magnitude of the forces on E. menziesii in various wave conditions and for various plant sizes. Based on measurements of the strength of E. menziesii, an intact, average-sized plant's survivorship in the range of predicted wave forces is estimated to be nearly 100%. However, entanglement with other plants and damage from herbivory can substantially lower an individual's chances of surviving a severe storm. We speculate that E. menziesii's straplike shape may be an adaptation for accommodating the unpredictable distribution of tensions along the plant's stipe and for reducing drag.