Modulation of wave forces on kelp canopies by alongshore currents

The predominant view of the canopy-forming kelp’s mechanical response to water motion is that they sway passively under waves such that they are only rarely stretched out in flow, which reduces relative fluid velocities and decreases the applied force. Such a view is an appropriate first-order approximation but becomes conceptually problematic in the face of the net surface velocities (Stokes drift) that arise under waves of all but infinitesimal height, since such flows can tug organisms into fully extended positions, allowing forces to act unabated. Focusing on Nereocystis luetkeana, the bull kelp, this study examines quantitatively the capacity of alongshore currents to mitigate the consequences of Stokes drift by maintaining canopy-forming macroalgae in ‘‘neutral’’ positions with regard to the onshore–offshore orbits of the waves. Results indicate that alongshore currents can indeed substantially reduce the forces imposed on canopy-forming kelps, as well as decrease the levels of wave damping that result from the interaction of these organisms with the passing fluid. Kelp forests provide essential habitat and food for hundreds of species of marine invertebrates and fish living in temperate nearshore waters (Foster and Schiel 1985). The forests’ proximity to the shore also makes them vulnerable to hydrodynamic forces imposed by surface gravity waves as these waves shoal into shallow water. Indeed, particularly during severe winter storms, large numbers of canopy-forming macroalgae such as Macrocystis pyrifera and Nereocystis luetkeana can be dislodged or destroyed by high-amplitude seas and swell (e.g., Seymour et al. 1989; Dayton et al. 1992). The ecological importance of these organisms, and their susceptibility to flow-driven disturbance in the face of a changing wave climate (Bacon and Carter 1991; Grevemeyer et al. 2000), suggests that efforts to understand the plants’ mechanical relationship to water motion are both valuable and timely. The traditional view of the behavior of canopy-forming kelps in flow has been that they move passively with the fluid over substantial portions of each oscillatory wave cycle (i.e., they ‘‘go with the flow’’), which results in a decrease in the speed of water relative to their fronds, thereby minimizing drag (Koehl 1984, 1986, 1999). In this scenario, it is only the most exceptional wave conditions that result in 1 Corresponding author ([email protected]).