Monterey Bay Aquarium Research Institute
Marine Botany

Drag on Egregia menziesii  


There are four main forces acting on algae as it experiences wave stress. These are drag , lift , buoyancy, and an accelerational force . Drag and the accelerational force (both in the direction of wave motion) are probably the biggest two.  I clocked waves at Hopkins Marine Station to be going as fast as 17 miles per hour.  That is a great deal of force to put on the algae (somewhere greater than 400 meters per second squared as the accelerational force).  Most algae are restricted in size due to drag limitations. The bigger you are, the greater drag you feel.  Most algae cannot get any bigger than a certain size before the forces acted upon them by the waves are too great for the plant to stay intact.  Egregia menziesii, however, seems to get past this, it is very much larger than other algae in the same waves.  
Measurements made in Mark Denny's lab at Hopkins Marine Station (see reference) showed that intact specimens of Egregia menziesii can withstand forces up to 145 Newtons before breaking.  I, myself, measure one sample which broke at 80 Newtons.  Apparently, however, algae will rarely see forces greater than 12 Newtons in the intertidal.

Why is Egregia so strong?  The idea is that the plant grows with such great strength in order to withstand damage done by, say, the limpets which heavily graze upon it.  Also, great strength is needed to compensate for large increases in drag caused by the alga getting entangled in itself and becoming bulky rather than more "streamline".

Egregia menziesii tends to curl up on itself a great deal within the waves.  Stretched to its full length, most of the force that would be put on a frond would occur at the base.  However, with all the tangling it does with itself, large forces are felt all along the length of the rachis.  This can be imagined if you think of the "whip-cracking" motion that would occur as the frond uncurls itself with the moving wave.

What Friedland and Denny figured out, however, is that it is not necessarily the strength of the material of Egregia menziesii that allows it to get so big in the intertidal, but more that it has mechanisms to reduce its drag.  Egregia menziesii, according to the reference, experiences 3.7 to10.6 times less drag than other intertidal algae.

This lack of drag must somehow be due to the shape of the alga.  The long, thin, flat band surely has interesting effects on drag forces, as well as the size and shape of the blades.  Just on the Monterey Peninsula, I have seen incredible variation in blade size and shape, from very long and thin to very short and fat.  It seems that Egregia in more exposed areas tends to have the longer blades than those growing on more sheltered rocks, but I have no data to back that statement up.  It would be a very interesting study for someone to carry out.

Reference for this page:

Friedland, Michelle T., and Mark W. Denny. "Surviving hydrodynamic forces in a wave-swept environment: Consequences of morphology in the feather boa kelp, Egregia menziesii (Turner)." Jour. Exp. Marine Biology and Ecology v. 190 (1995) 109-133

Copyright 1999 Sarah Present. Contact spresent@stanfordalumni.org for any non-educational use.
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