Hydrodynamics of marine macroalgae: Biotic and physical determinants of drag

The drag generated by breaking waves is one of the primary determinants of the distribution and abundance of organisms in the wave-swept rocky intertidal. Drag on macroalgae is modified by three main biological factors: morphology, reconfiguration and the surrounding canopy. These studies investigated the interplay among these factors by defining a mechanistic model of reconfiguration, examining the effects of morphological and material variation on reconfiguration, and measuring the effects of the canopy on hydrodynamic forces in situ.; In Manuscript I, reconfiguration in Chondrus crispus was examined by direct measurement of changes in size, shape, and drag of individuals in a flume. This new technique was used to examine the mechanisms of reconfiguration over a wide range of water velocities and develop a new mechanistic model of drag. Reconfiguration at low velocity was caused by changes in both area and shape; high velocity reconfiguration was due to changes in area alone. A mechanistic drag model was developed that provides parameters with which to compare reconfiguration among individuals and species, and allows for predictions of drag at high water velocities.; In Manuscript II, the interactions among morphology, solid mechanics, and drag were examined. Reconfiguration, morphology and material stiffness were characterized in ten species of intertidal macroalgae in the laboratory. Hydrodynamic performance, morphological and structural properties all varied among species. The relationships between performance and structural properties were velocity dependent; performance was more often correlated to structural properties at low velocities. Functional form groups were identified by hydrodynamic performance that may be useful for higher level ecological analyses.; In Manuscript III, miniature autonomous force sensors were developed to measure hydrodynamic forces imposed on Chondrus crispus in the field to examine the effects of canopy, morphology and sea state on force. The canopy reduced force on individuals 15 to 65% compared to the same alga in a solitary position. Morphology also influenced force, such that small planar morphologies were insensitive to the effects of breaking waves. These results suggested that large individuals depend on the ameliorating effects of the canopy to persist in intertidal habitats.