Wave damping by giant kelp, Macrocystis pyrifera

Area of Science:

• Marine ecology
• Coastal geomorphology
• Oceanography

Background:

• Coastal ecosystems play a role in shoreline protection by attenuating wave energy, especially with increasing storm events.
• Canopy-forming kelps, such as giant kelp (Macrocystis pyrifera), are hypothesized to provide this ecosystem service.
• Previous studies faced challenges in isolating kelp's effect from other factors like seafloor bathymetry.

Purpose of the Study:

• To unambiguously resolve the capacity of giant kelp to dampen waves using a unique site with a kelp transition.
• To quantify the wave energy flux decline attributable to kelp presence.

Main Methods:

• Measurements of waves were taken within and outside giant kelp forests at Marguerite Reef, Palos Verdes, CA.
• The study site transitioned from a bare state to a fully formed kelp forest (8 stipes m-2) within a restoration project.
• Wave energy flux decline was quantified as a function of incident wave conditions as waves entered the reef habitat.

Main Results:

• The kelp forest measurably, but modestly, damped wave energy.
• Seabed interactions reduced wave energy flux by an average of 12% over 180 m.
• The kelp forest induced an additional 7% decrease in wave energy flux, with greater effects on longer-period, smaller-height waves.

Conclusions:

• Giant kelp (Macrocystis pyrifera) forests possess a limited yet measurable capacity to enhance shoreline protection from nearshore waves.
• The substantial impact of kelp forests on wave-induced coastal erosion may need re-evaluation, considering both extant and restored forests.