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The mechanics of wave-swept algae

Mark Denny^1,* and Brian Gaylord²

¹ Hopkins Marine Station of Stanford University, Pacific Grove, CA 93950, USA
² Department of Ecology, Evolution, and Marine Biology, University of California, Santa Barbara, CA 93106, USA

View larger version (12K): [in a new window]   Fig. 1. Water velocities produced by unbroken surface gravity waves. In each case, velocity has been normalized to wave height so that these curves apply equally to waves of all heights. (A) Flows at the surface. (B) Flows just outside the benthic boundary layer at the seabed. T is wave period, in seconds. The depth referred to on the abscissa is the depth of the water column in the absence of waves.

View larger version (20K): [in a new window]   Fig. 2. Example time series of forces applied to seaweeds in the field. (A) Bending moments (i.e. forces applied with a given lever arm) imposed on the base of the stipe of an upright understory kelp Pterygophora californica exposed to unbroken waves. Note the relatively regular character of the recording. (B) Tensile force in a stipe of Nereocystis luetkeana exposed to unbroken waves. The slightly more complicated trajectory of force (which varies around that induced by the plant's buoyancy alone) probably arises from the seaweed's interaction with the water's surface as it is swept back and forth by drag. (C) A shorter time series of force applied to an emergent intertidal alga, Pelvetia compressa. The far more complicated fluid motions associated with wave impingement and breaking produce a rapidly evolving force record, particularly at the instant of wave arrival. Note that the magnitude of forces imposed on this small surfzone plant (frond length 15 cm) rival those acting on the far larger (7 m length) subtidal bull kelp (compare B and C). This is a direct consequence of the greater severity of flow in intertidal regions.

View larger version (25K): [in a new window]   Fig. 3. Example model predictions of dynamic tuning in macroalgae subjected to unbroken waves showing how inertial effects can dominate applied force. In the case of the large surface-canopy kelp Nereocystis luetkeana (solid line and the ordinate on the left), peak tensile force (including forces due to the momentum the plant acquires as it moves) may occur when the waves arrive at a frequency equal to half the natural frequency of the seaweed, and these forces may greatly exceed the tensile force from drag alone. (The natural frequency is the frequency at which the plant would tend to oscillate if nudged slightly in still water.) Similarly, in an erect understory kelp, Eisenia arborea (dashed line and the ordinate on the right), total bending moments (i.e. moments including forces due to the momentum the plant acquires as it moves) may at times substantially exceed bending moments that would arise from drag alone. In this situation, the tuning is most apparent when the dominant frequency of the waves is the same as the natural frequency of the alga. Note that these model predictions assume a zero longshore current; if one is present, it may partially offset the exceptionally large tension ratios seen for N. luetkeana. In both curves, the magnitude of the orbital velocities produced by the waves is held constant across frequency. Denny et al. (1998) provide full details on this topic.