Kelp are ecologically important organisms because they provide food and habitat to many other creatures. On wave-swept rocky shores, kelp must withstand the force of moving water from waves and currents, while also growing to large sizes to outcompete neighboring macroalgae for light and space. If the forces from moving water (e.g. drag) exceed the strength of the kelp, the kelp can be damaged or dislodged from the shoreline, effectively removing the kelp from the ecosystem. Here, I use the intertidal kelp Egregia menziesii, one of the largest kelp found in the rocky intertidal zone along the west coast of North America, to investigate how the kelp’s growth and structure are affected by, and alter its interaction with, moving water in waves, as well as the animals that use the kelp for food and habitat.In Chapter One, I examine how the gas-filled bladders (pneumatocysts) of kelp help keep the fronds of E. menziesii afloat in wave-driven flow. Field surveys revealed that this species shows great variation in pneumatocyst size, number, and location on fronds. In laboratory towing-tank studies, I found that drag on pneumatocysts was reduced when they were bent over by flowing water. The drag due to pneumatocysts was small compared to the drag on a whole frond. At flow speeds up to 0.58 m s-1, the buoyant force exerted by a pneumatocyst was greater than the drag it experienced. In wave-tank experiments using models of fronds with pneumatocysts at different positions, the pneumatocysts were most effective at lifting fronds high in the water column when they were located at the distal tips of the fronds, both in small and large waves. However, if fronds had pneumatocysts that were not at the tip, an increase in the peak velocities of waves led to an increase in the heights of the fronds in the water column. In the field, pneumatocysts did not affect the back-and-forth horizontal motion of E. menziesii exposed to waves, but fronds with pneumatocysts were higher in the water column than fronds with no pneumatocysts, even when the number of pneumatocysts on a frond was low. My results indicate that pneumatocysts can exhibit great variability in size, number, and location with only a small effect on hydrodynamic forces on a kelp, that pneumatocysts at frond tips are most effective at holding kelp high in the water column, but that only a few pneumatocysts at any location along a frond can enhance the frond's height in waves.In Chapter Two, I examine the consequences of long, flexible kelp fronds being knotted (a single frond tied around itself) and tangled (multiple fronds intertwined) as they move back and forth with ocean waves. I found that knots increased the drag forces on fronds while also making those fronds break under small forces than unknotted fronds. Tangled fronds of E. menziesii provided spatially complex habitats that hosted more and larger herbivores than did untangled fronds, and that herbivore damage, coupled with the higher hydrodynamic forces and mechanical stresses experienced by knotted fronds, made them more vulnerable to being broken by waves. Breakage of knotted fronds can enhance the survival of kelp by pruning individuals to smaller size, thereby reducing their risk of being dislodged by large waves during winter storms. Although earlier studies have documented that the physical environment can affect ecological processes, I found that this is a ‘two-way street’ where ecological interactions in turn can alter how organisms perform in the physical environment. By analyzing the organismal-level, biomechanical mechanisms underlying the ecological interactions between kelp, herbivores, and the physical environment as all three changed with the seasons, I found that reconfiguration and damage caused by both physical and biological processes affect both kelp survivorship and community structure.In Chapter Three, I examine how the morphology of E. menziesii, and the types of epifauna on the kelp, change over seasons at sites exposed to heavy wave action (“exposed”) or to moderate wave action (“moderate”) in northern California. Sessile suspension-feeding animals (mussels, barnacles) were more prevalent on kelp at exposed sites, whereas motile herbivorous epifauna (amphipods, limpets, isopods, crabs) were found on more of the kelp at moderate sites. Kelp at both wave exposures showed similar seasonal patterns of morphological change, growing longer fronds from spring through autumn and decreasing in size during winter while maintaining the same number of fronds. The variation in frond lengths on a kelp decreased in summer when the proportion of intact fronds was high, but increased as more fronds broke during autumn and winter. Although patterns of change were similar, some aspects of kelp morphology differed between exposed and moderate sites. During spring, kelp at exposed sites had fewer fronds than those moderate sites. During spring through autumn, the kelp at exposed sites had longer fronds, a greater proportion of intact fronds, and less variation in frond length than those at moderate sites. Frond breakage at wounds inflicted by herbivores, which infest a greater proportion of the kelp at moderate sites than at exposed sites, may be responsible for these morphological differences. In winter, large waves prune kelp to similar morphologies at exposed and moderate sites.