Mechanical Consequences of Size in Wave‐Swept Algae

The intertidal zone of wave—swept rocky shores is characterized by high velocities and exceedingly rapid accelerations. The resulting hydrodynamic forces (drag, lift, and the accelerational force) have been hypothesized both to set an upper limit to the size to which wave—swept organisms can grow and to establish an optimal size at which reproductive output is maximized. This proposition has been applied previously to intertidal animals that grow isometrically, in which case the accelerational force is the primary scaling factor that constrains size. In contrast, it has been thought that the size of wave—swept algae is limited by the interaction of drag alone with these plants' allometric pattern of growth. Here we report empirical measurements of drag and accelerational force in three common species of intertidal algae (Gigartina leptorhynchos, Pelvetiopsis limitata, and Iridaea flaccida). The drag coefficients for these species decrease with increased water velocity, as is typical for flexible organisms. For two of these species, this decline in drag coefficient is dramatic, leading to small drag forces with concomitant low drag—induced mortality at plant sizes near those observed in the field. However, all three species have surprisingly large inertia coefficients, suggesting that these plants experience large accelerational forces in surf—zone flows. Preliminary calculations show that these accelerational forces combine with drag to act as a size—dependent agent of mortality, constraining the size of these algae. This study further models the interplay between size—dependent survivorship and reproductive ability to predict the size at which reproductive output peaks. This "optimal size" depends on the strength distribution and morphology of the algal species and on the flow regime characteristic of a particular site. This study shows that the optimal size predicted for G. leptorhynchos, calculated using velocities and accelerations typical of the moderately protected location where this species was collected, closely matches its observed mean size. Similarly, the predicted optimal sizes of P. limitata and I. flaccida at the exposed site where these plants were sampled also match their mean observed sizes. These data, although preliminary, suggest that mechanical factors (in particular the accelerational force) may be important in limiting the size of intertidal macroalgae and that attention solely to biological constraints may be inappropriate.