Abstract
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 approximately 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.
Highlights
The distribution and abundance of rocky shore intertidal organisms are partly determined by damage and dislodgement caused by the hydrodynamic forces created by breaking waves (Dayton, 1971; Menge, 1976; Paine and Levin, 1981; Denny, 1995; Carrington, 2002)
This study examined reconfiguration of the red alga Chondrus crispus, a dominant space holder in the New England rocky intertidal zone, to determine the mechanisms of drag reduction via flexibility
The reconfiguration of macroalgae is an unavoidable consequence of their flexibility and an essential component for their strategy for survival in the wave-swept rocky intertidal zone (Denny and Gaylord, 2002; Harder et al, 2004)
Summary
The distribution and abundance of rocky shore intertidal organisms are partly determined by damage and dislodgement caused by the hydrodynamic forces created by breaking waves (Dayton, 1971; Menge, 1976; Paine and Levin, 1981; Denny, 1995; Carrington, 2002). These forces include the well-known lift and drag, and the impingement force, associated with the impact of a wave on an emerged object (Gaylord, 2000). The impingement force may often exceed drag in the intertidal zone (Gaylord, 2000), but has not been extensively studied and is not addressed here
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