The waters of the oceans are in constant motion. The causes of that motion are many, beginning with the great ocean currents, tidal currents, waves, and other forces, and ranging down to the small-scale circulation patterns caused by local density changes (Vogel 1994; Thurman and Trujillo 2004). Hydrodynamic force is a direct environmental factor, but water motion also affects other factors, including nutrient availability, light penetration, and temperature and salinity changes. The forces embodied in waves are difficult to comprehend, unless one has been dangerously close to them; because of the density of water, a wave or current exerts much more force than do the winds. “Imagine a human foraging for food and searching for a mate in a hurricane and you will have only an inkling of the physical constraints imposed on wave-swept life” (Patterson 1989b, p. 1374). The energy amassed from a great expanse of air–ocean interactions is expended on the shoreline as waves break (Leigh et al. 1987). Equally difficult to visualize are the microscopic layers of water next to seaweed surfaces where the seaweeds’ cells interact with water. Too much water motion imposes drag forces that can rip seaweeds from the rocks, but this also clears patches of “new” space for recruitment. Too little water motion and nutrient concentration gradients form at the seaweed surface which can restrict nutrient uptake, but the same gradients are used by seaweeds to sense how fast the surrounding seawater is moving and thereby cue gamete or spore release.

Studies of seaweed form and function in wave-exposed and wave-protected sites have provided insights into the trade-offs apparent in some species that allow them to maximize resource acquisition in slow flows and minimize drag forces in fast flows. The following texts and reviews provide the necessary background on fluid mechanics: Denny (1988, 1993, 2006); Vogel (1994); Denny and Wethey (2001). “Marine ecomechanics” is an emerging field that uses a “physical framework” to understand the responses of marine organisms on scales from cells to ecosystems (Denny and Helmuth 2009; Denny and Gaylord 2010). We begin this chapter by describing the hydrodynamic environments in which seaweeds grow, and then discuss the mechanisms by which seaweeds can enhance resource acquisition in slow flows and withstand hydrodynamic forces in wave-exposed sites. We finish with a discussion on the effects of wave action and sediments on seaweed communities.