Meeting Abstract
Kelp in wave-swept coastal areas can be damaged if the mechanical hydrodynamic forces they experience exceed the strength of their tissues, especially if the tissues are already wounded by herbivory. Although wave action and herbivory change seasonally, little is known about how the strength, extensibility (how far it stretches before breaking), and stiffness of of kelp tissues are affected by their age, the season, and herbivore damage. We used fronds of Egregia menziesii, a large and abundant intertidal kelp along the coast of western North America, to examine how the mechanical properties of kelp tissue are affected by these factors. Frond tissue became stiffer and stronger as it aged but did not change extensibility. Over a two-year study period, we observed that frond tissue was strongest and stiffest in winter, and weakest and least stiff in summer, but showed no seasonal changes in extensibility. Shifts in frond strength were proportional to seasonal increases in wave action, and inversely proportional to the frond’s growth rates. Our study period included an El Niño winter and a non-El Niño winter. Frond tissue was stronger during the non-El Niño winter than during the El Niño winter. Mechanical properties of the frond tissues did not change in response to herbivore damage, although as damaged tissue aged, they became stronger at the same rate as undamaged tissue sections of the frond. In the summer, when waves are small and herbivores are prevalent, the fast growth rates and weak mechanical properties permit the kelp to grow to a large size. In the late fall, growth rates slow and fronds tend to break. Then in winter, the kelp’s smaller size, slower growth rates, and stronger mechanical properties allow the kelp to better survive the large hydrodynamic forces of winter storms.
