MECHANICAL CONSEQUENCES OF SIZE IN WAVE-SWEPT ALGAE

The intertidal zone of wave-swept rocky shores is characterized by hig h velocities and exceedingly rapid accelerations. The resulting hydrod ynamic forces (drag, lift, and the accelerational force) have been hyp othesized both to set an upper limit to the size to which wave-swept o rganisms can grow and to establish an optimal size at which reproducti ve output is maximized. This proposition has been applied previously t o intertidal animals that grow isometrically, in which case the accele rational force is the primary scaling factor that constrains size. In contrast, it has been thought that the size of wave-swept algae is lim ited by the interaction of drag alone with these plants' allometric pa ttern of growth. Here we report on empirical measurements of drag and accelerational force in three common species of intertidal algae (Giga rtina leptorhynchos, Pelvetiopsis limitata, and Iridaea flaccida). The drag coefficients for these species decrease with increased water vel ocity, as is typical for flexible organisms. For two of these species, this decline in drag coefficient is dramatic, leading to small drag f orces with concomitant low drag-induced mortality at plant sizes near those observed in the field. However, all three species have surprisin gly large inertia coefficients, suggesting that these plants experienc e large accelerational forces in surf-zone flows. Preliminary calculat ions show that these accelerational forces combine with drag to act as a size-dependent agent of mortality, constraining the size of these a lgae. This study further models the interplay between size-dependent s urvivorship and reproductive ability to predict the size al which repr oductive output peaks. This ''optimal size'' depends on the strength d istribution and morphology of the algal species and on the flow regime characteristic of a particular site. This study shows that the optima l 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. Simila rly, the predicted optimal sizes of P. limitata and I. flaccida at the exposed site where these plants were sampled also match their mean ob served sizes. These data, although preliminary, suggest that mechanica l 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.