Meeting Abstract
Bull kelp (Nereocystis luetkeana) are an important source of primary production and three-dimensional habitat in nearshore subtidal areas in the NE Pacific, including the Salish Sea. These kelp must be adapted to persist in in wave-swept environments subject to strong tidal currents throughout their life cycle. Previous data collected subtidally near San Juan Island indicate that juvenile mortality of bull kelp is very high. While material properties of adult bull kelp have been studied previously, less is known about those of juveniles. We used SCUBA to collect juvenile bull kelp (stipe length less than 40 cm) at several subtidal sites in the San Juan Channel and investigated material properties in the lab using a Materials Testing System (MTS). For each individual, we measured stipe length, stipe width under the bulb, stipe width above the holdfast, stipe width at the visually narrowest point, and bulb width; we also noted any damage to the stipe. We used a custom 3D printed cradle to hold the bulb and freeze-clamped the holdfast before putting the stipe under tension until failure. For each trial we recorded maximum force, force at failure, and calculated Young’s Modulus. We found morphology to be an important determinant of juvenile bull kelp stipe failure and that cross-sectional area may play a role in determining where stipes fail. Additionally, juvenile bull kelp stipes showed great extensibility prior to failure. Data on the material properties of juvenile bull kelp is important to help us understand the potential causes of high juvenile bull kelp mortality and “beheaded” individuals observed in the field. Ultimately, this work will help inform future conservation, restoration, and management efforts.
