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First published online May 1, 2006
Journal of Experimental Biology 209, 1894-1903 (2006)
Published by The Company of Biologists 2006
doi: 10.1242/jeb.02225

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The hydrodynamic effects of shape and size change during reconfiguration of a flexible macroalga

Michael L. Boller^* and Emily Carrington

Department of Biological Sciences, University of Rhode Island, Kingston, RI 02881, USA

^* Author for correspondence at present address: Hopkins Marine Station of Stanford University, Pacific Grove, CA 93950, USA (e-mail: boller{at}stanford.edu)

Accepted 21 March 2006

Rocky intertidal organisms experience large hydrodynamic forces due to high water velocities created by breaking waves. Flexible organisms, like macroalgae, often experience lower drag than rigid organisms because their shape and size change as velocity increases. This phenomenon, known as reconfiguration, has been previously quantified as Vogel's E, a measure of the relationship between velocity and drag. While this method is very useful for comparing reconfiguration among organisms it does not address the mechanisms of reconfiguration, and its application to predicting drag is problematic. The purpose of this study was twofold: (1) to examine the mechanisms of reconfiguration by quantifying the change in shape and size of a macroalga in flow and (2) to build a mechanistic model of drag for reconfiguring organisms. Drag, frontal area and shape of the intertidal alga Chondrus crispus were measured simultaneously in a recirculating flume at water velocities from 0 to 2 m s^–1. Reconfiguration was due to two separate mechanisms: whole-alga realignment (deflection of the stipe) at low velocities (<0.2 m s^–1) and compaction of the crown (reduction in frontal area and change in shape) at higher velocities. Change in frontal area contributed more to drag reduction than change in drag coefficient. Drag coefficient and frontal area both decrease exponentially with increasing water velocity, and a mechanistic model of drag was developed with explicit functions to describe these changes. The model not only provides mechanistic parameters with which to compare reconfiguration among individuals and species, but also allows for more reliable predictions of drag at high, ecologically relevant water velocities.

Key words: biomechanics, ecology, seaweed, Chondrus crispus

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