Abstract
AbstractWe present field measurements of the movement of the giant kelp Macrocystis pyrifera under wave forcing. We resolve the depth and frequency‐dependent responses along the stipe and find different and counterintuitive patterns of response at the infragravity and swell wave forcing frequencies. At swell frequencies, tilting of the stipe is largest toward the holdfast, whereas at infragravity frequencies, the stipe tilting is largest closer to the water surface. It is postulated that the stretching of blades and subsequent pull on the stipe is, in part, responsible for these patterns. This conclusion is supported by results of manipulative experiments, which show a more along‐stipe uniform response after removal of blades from the kelp. The length of the kelp also exerts a strong control on the relative magnitudes of movements in the different frequency bands, with the swell band becoming more important relative to the infragravity band for shorter length kelp. These results indicate that kelp will differentially dissipate energy over both frequencies and varying depths within the water column. The variety of movement responses over differing wave forcing frequencies may also imply that there exist differing rates of breakage for kelp exposed to hydrodynamics stressors of multiple frequencies.
Highlights
We present field measurements of the movement of the giant kelp Macrocystis pyrifera under wave forcing
We present field observations of the motion of the giant kelp Macrocystis pyrifera under non-breaking swell and infragravity waves
A somewhat counterintuitive result was observed: the lower sensors appeared to respond more strongly to the energy in the swell band (Fig. 7b) than the sensors attached high up the kelp and close to the surface, whereas the opposite effect was observed in the infragravity energy band (Fig. 7a), differences between sensors were subtle over most of the stipe length. This pattern was most clearly discerned on day 3 which experienced the largest wave forcing, it was observed in all measurements of un-manipulated kelp
Summary
We present field measurements of the movement of the giant kelp Macrocystis pyrifera under wave forcing. A somewhat counterintuitive result was observed: the lower sensors appeared to respond more strongly (i.e., the kelp tilted further from the vertical) to the energy in the swell band (Fig. 7b) than the sensors attached high up the kelp and close to the surface, whereas the opposite effect was observed in the infragravity energy band (Fig. 7a), differences (i.e., tilts) between sensors were subtle over most of the stipe length.
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