Maintenance of Dynamic Strain Similarity and Environmental Stress Factor in Different Flow Habitats: Thallus Allometry and Material Properties of a Giant Kelp

We have focused on the giant kelp Nereocystis luetkeana to examine the mechanical scaling of benthic marine organisms loaded in tension by hydrodynamic forces. If we consider simply the allometry of the kelp's morphological characters, we conclude that their stipes are underscaled relative to the blade area they support (i.e. that the kelp do not maintain stress or elastic similarity as they grow). However, a closer look at the characteristics of these kelp in the field reveals (1) that they have different blade shapes (and hence drag coefficients) and stipe material properties in different hydrodynamic environments, and (2) that they show a decrease in drag coefficient as they become larger. One consequence of these adjustments of blade and stipe morphology is that the maximum stresses in N. luetkeana stipes, when the kelp are pulled by peak tidal currents in their respective habitats, are similar for kelp of different sizes and for kelp from different sites. Hence, sessile organisms such as these kelp can, via their growth responses in different mechanical environments, show a phenomenon analogous to dynamic strain similarity. In addition, N. luetkeana also maintain a constant environmental stress factor, the ratio of the stress required to break a component of an organism (in this case the stipe) at some stage in its life to the maximum stress normally encountered in the habitat by that component during that stage (in this case, stress due to the drag on a kelp exposed to the peak tidal currents typical of the site at which it lives), both between habitats and as they grow.