Bull kelp (Nereocystis luetkeana), the only canopy-forming kelp in the Salish Sea, provides primary production in the nearshore subtidal environment and serves as important habitat for economically and ecologically important species. An annual species, each year juvenile bull kelp sporophytes must grow from the hydrodynamically more benign benthos to the water column, where they experience substantial drag at the surface. Because of the differences in morphology and ecology across life stages, and the fact that previous work has focused mainly on adult bull kelp, we tested whether morphology and structural properties change with stipe length, investigating scaling of both juvenile (stipe length <40 cm) and mature (stipe length > 40cm) kelp, and testing how juvenile stipes fail. Juvenile bull kelp grow proportionally (isometric growth) when young, but lengthen more quickly than would be predicted by bulb size (negative allometry) at maturity. Based on our data, the predicted breakpoint between isometric and allometric growth occurred at about 33 cm, likely ~ one to two weeks of growth. Cross sectional area of the stipe, Force to failure, Work to failure, and stiffness (Young’s Modulus) all grow more slowly than would be predicted based on length, while Maximum Stress and Toughness increase more quickly than predicted. There is no change in extensibility over the size range we tested, suggesting that this material property does not change with stipe length. The differences in biomechanics between juvenile and adult kelp are likely a response to the varied hydrodynamic environments experienced during the annual lifecycle, which highlights the importance of studying organisms across life stages.

Length measurements for scaling measured using caliphers annd rulers/tape measures

Cross-sectional area measured in ImageJ

Force/displacement data collected from MTS Synergie 100 (MTS Systems Corp.), and used, with cross-sectional information, to calculate stress/strain data, including Toughness.